A Journey Around Our Cosmic Neighbourhood: The Solar System and Its Secrets

Webb, Stephen

doi:10.1007/978-3-031-52437-0_1

Stephen Webb²

Part of the book series: Springer Praxis Books ((POPULAR))

22 Accesses

Abstract

For most of history, humans have been terrestrial creatures. Our forebears spent their lives on the rocky parts of Earth’s surface underneath the protective blanket of an atmosphere. Now, though, we know how to get above the atmosphere. We can reach into space.

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1.1 Quiz

1.
What is the Kármán line?
- □ The orbital path taken by Apollo 11 to reach the Moon
- □ A generally agreed boundary between Earth and space
- □ The curve adopted under gravity by the material of a (hypothetical) space elevator
- □ A straight line which, when orbits align, can be drawn through any four bodies in the solar system
2.
The Sun accounts for how much of the mass of the solar system?
- □ More than 99.8%
- □ Approximately 95%
- □ Between 92–94%
- □ Less than 89.9%
3.
The Sun has a mean diameter of 1,391,400 km, but its polar diameter is smaller than its equatorial diameter. How much smaller?
- □ 10 km
- □ 1000 km
- □ 5000 km
- □ 10,000 km
4.
After hydrogen and helium, which is the most abundant element in the Sun?
- □ Carbon
- □ Iron
- □ Neon
- □ Oxygen
5.
How does the average density of the Sun compare with that of Earth?
- □ It is about 75% as dense as Earth
- □ It is about 50% as dense as Earth
- □ It is about 25% as dense as Earth
- □ It is about 10% as dense as Earth
6.
What is the surface temperature of the Sun?
- □ 10,000,000 K
- □ 148,500 K
- □ 12,286 K
- □ 5778 K
7.
When was the first known observation of a sunspot made?
- □ 4th century BCE
- □ 1st century BCE
- □ 2nd century CE
- □ 12th century CE
8.
During a span of 30 years one might expect roughly 50,000 sunspots to be visible. In the period 1672–1699, how many sunspots did astronomers observe?
- □ 100,000
- □ 45,000
- □ 5000
- □ 50
9.
The Moon shines because it reflects light from the Sun. Who was the first to propose this idea?
- □ Anaxagoras
- □ Anaximander
- □ Arhytas
- □ Aristarchus
10.
From Earth we can see only 50% of the Moon’s surface at any one time. Overall, what percentage of the Moon’s surface is visible from Earth?
- □ 50%
- □ 54%
- □ 59%
- □ 63%
11.
Today, we can measure the distance to the Moon to what level of precision?
- □ Kilometer-level precision
- □ Meter-level precision
- □ Millimeter-level precision
- □ Nanometer-level precision
12.
At arm’s length, roughly what angle does the width of your index finger span?
- □ 10°
- □ 5°
- □ 2°
- □ 1°
13.
What is the name for a straight-line configuration of three celestial bodies in a gravitational system?
- □ Crwth
- □ Syzygy
- □ Trochilus
- □ Tsktsk
14.
On 16 July 2186, observers in northern Guyana will experience a total solar eclipse. This eclipse will have the longest duration in recorded history. How long will totality endure for those observers?
- □ 2 minutes 32 seconds
- □ 4 minutes 59 seconds
- □ 7 minutes 29 seconds
- □ 9 minutes 46 seconds
15.
A solar eclipse once interrupted a battle. Since we know when the eclipse happened, this is the earliest historical event we can date to a particular day. When was the battle?
- □ 2 December 776 BCE
- □ 28 May 585 BCE
- □ 14 March 444 BCE
- □ 25 February 257 BCE
16.
Our word ‘planet’ comes from the Greek word planētes. What does planētes mean?
- □ Large star
- □ Sky god
- □ Unwavering light
- □ Wanderer
17.
What characteristic of the Sun, Moon and planets gave rise to the ordering of the days of the week?
- □ Their apparent size
- □ Their colour
- □ Their distance from Earth
- □ The strength of the god associated with them
18.
Who was the first person to argue that Earth orbits the Sun?
- □ Aristarchus of Samos
- □ Hipparchus of Nicaea
- □ Nicolaus Copernicus
- □ Pythagoras of Samos
19.
How are the words ‘eclipse’ and ‘ecliptic’ related etymologically?
- □ The two words are etymologically unrelated
- □ They both come from Latin and Greek root words meaning ‘to be hidden’
- □ They both come from Latin and Greek root words meaning ‘a flat plane’
- □ They both come from Latin and Greek root words meaning ‘circular’
20.
The French astronomer Delambre wrote several books about astronomical history. Which scientist did Delambre call the ‘father of astronomy’?
- □ Aristarchus of Samos
- □ Eratosthenes of Cyrene
- □ Hipparchus of Nicaea
- □ Thales of Miletus
21.
The Greek scientist Eratosthenes had a nickname. What was it?
- □ Alpha
- □ Beta
- □ Gamma
- □ Delta
22.
We know little about the life of the Greek (or Egyptian) astronomer Claudius Ptolemy, except that he flourished between the years 127–141 CE. How do we know this?
- □ The dates are mentioned in passing in a biography of the Greek physician Galen, Ptolemy’s near-contemporary
- □ These are the earliest and latest dates of observations made by Ptolemy
- □ Ptolemy is cited in a surviving letter from the Roman emperor Marcus Aurelius
- □ The years are carved into stone column at an observatory in Alexandria
23.
The poet Omar Khayyam measured the length of the year. What value did arrive at?
- □ 400 days
- □ 365.24219858156 days
- □ 364.99 days
- □ 116π days
24.
Brahmagupta, an Indian scientist, made several important contributions to mathematics. Which of the following developments did he pioneer?
- □ Elliptic functions
- □ Probability
- □ Rules to compute with zero
- □ Transcendental numbers
25.
Copernicus is best known for the formulation of a heliocentric model of the universe, but he was active in other fields too. In 1519, he formulated an important economic principle. What is that principle now called?
- □ Gibrat’s law
- □ Goodhard’s law
- □ Gresham’s law
- □ Parkinson’s law
26.
The Danish astronomer Tycho Brahe had an artificial brass what?
- □ Left big toe
- □ Nose
- □ Right ear
- □ Right index finger
27.
In 1620, how was Johannes Kepler able to help his mother?
- □ He defended her against a charge of witchcraft
- □ He helped her produce a horoscope for the Holy Roman Emperor
- □ He gave her the royalties from his science fiction novel Somnium (The Dream)
- □ He offered her a place in his household, after she fell into destitution
28.
In science, which year is sometimes called ‘The Year of Wonders’?
- □ 1114
- □ 1412
- □ 1573
- □ 1666
29.
Edmond Halley’s most important contribution to science was in helping to fund the publication of Newton’s Principia, the most influential book in science. The Royal Society was unable to publish Principia because it had blown its publishing budget. On what?
- □ Publication of the book De History Piscium (Of the History of Fish)
- □ Investment in the tulip craze
- □ Late entry into a seventeenth century version of what would come to be known as a pyramid scheme
- □ A refurbishment of Robert Hooke’s laboratory in London
30.
In 2012, the International Astronomical Union defined what distance to be precisely 149,597,870.7 km?
- □ Astronomical unit
- □ Average Earth–Venus distance
- □ Light-second
- □ Radius of the solar system
31.
Knowledge of the Earth–Sun distance is crucial for the definition of which distance unit commonly used in astronomy?
- □ Light-second
- □ Light-year
- □ Parsec
- □ Redshift
32.
Who coined the term ‘parsec’ for the distance unit commonly used in astronomy?
- □ George Ellery Hale
- □ Edwin Powell Hubble
- □ Harlow Shapley
- □ Herbert Hall Turner
33.
Which planet has the greatest orbital eccentricity (in other words, which planetary orbit is most elliptical)?
- □ Saturn
- □ Jupiter
- □ Venus
- □ Mercury
34.
For which planet did Egyptian astronomers have two names, Set and Horus?
- □ Mercury
- □ Venus
- □ Earth
- □ Mars
35.
Which planet has the longest day?
- □ Mercury
- □ Venus
- □ Mars
- □ Jupiter
36.
Which planet is hottest?
- □ Mercury
- □ Venus
- □ Earth
- □ Mars
37.
In 1639, who made the first recorded observation of a transit of Venus?
- □ Bonaventura Francesco Cavalieri
- □ Galileo Galilei
- □ Jeremiah Horrocks
- □ Martin van den Hove
38.
Which is the densest planet in the solar system?
- □ Mercury
- □ Venus
- □ Earth
- □ Mars
39.
Polaris is the current pole star. What was the previous pole star?
- □ Antares
- □ Kochab
- □ Pherkad
- □ Thuban
40.
Who or what is Lilith?
- □ The first monkey to complete one orbit of Earth
- □ A hypothetical second moon of Earth
- □ The name given to the lens of the largest ever refracting telescope
- □ A small moon of Mars
41.
Contemplation of what is reputed to have caused Isaac Newton’s head to ache?
- □ An apple falling on his head
- □ Binomial theory generalisation to non-integer exponents
- □ Curve of fastest descent
- □ The motion of the Moon
42.
What are Kordylewski clouds?
- □ Patches of black pixels that appear on CCD images
- □ Blurring of images of nearby astronomical objects that appear on pre-1945 photographic plates
- □ Clouds that inhibit infrared observations from ground-based telescopes
- □ Concentrations of dust at the L4 and L5 Lagrangian points of the Earth–Moon system
43.
The highest mountain in the solar system is on which planet?
- □ Mars
- □ Earth
- □ Venus
- □ Mercury
44.
What causes Mars to appear red?
- □ Crustal rocks on Mars contain large amounts of red jasper
- □ Dust scattering in the Martian atmosphere
- □ Iron oxide (rust) on the Martian surface
- □ Optical effects in Earth’s atmosphere
45.
Which planet has the largest magnetosphere?
- □ Earth
- □ Mercury
- □ Neptune
- □ Jupiter
46.
The biggest mean ocean tidal range on Earth is 11.7 m. By how much does Io’s crust rise and fall through the action of tides?
- □ 0.1 m
- □ 1 m
- □ 10 m
- □ 100 m
47.
Who first demonstrated that light travels at a finite speed?
- □ Bernard Le Bovier de Fontenelle
- □ Christiaan Huygens
- □ Jean Picard
- □ Ole Rømer
48.
How long is a light-year?
- □ 9,461,000,000 km
- □ 365,250,000,000 seconds
- □ 9,461,000,000,000 km
- □ 299,792,458,000,000 seconds
49.
Which is the least dense planet?
- □ Jupiter
- □ Saturn
- □ Uranus
- □ Neptune
50.
What part of Galileo’s body can you see if you visit the eponymous museum in Florence?
- □ Heart
- □ Brain
- □ Big toe
- □ Middle finger
51.
Which Greek myth did Galileo invoke after observing Saturn through a telescope?
- □ Cronus eating his own children
- □ Hephaestus being banned from Olympus
- □ Hermes moving between world and underworld
- □ Niobe being transformed into a rock
52.
The Roche limit is the distance from an object at which an orbiting body, held together only by gravitational self-attraction, is pulled apart by tidal forces. Where is the Roche limit for the Sun for a rigid body with the density of Earth? (In other words, in theory, how close would Earth have to be to the Sun for it to be tidally disrupted.)
- □ The Roche limit is inside the Sun
- □ The Roche limit lies at the surface of the Sun
- □ The Roche limit is twice the Sun’s radius
- □ The Roche limit is at 0.2 AU
53.
Which object was initially called George?
- □ Saturn
- □ Uranus
- □ Neptune
- □ Pluto
54.
Which of the following did William Herschel NOT do?
- □ Composed Allegro in G Minor (keyboard work)
- □ Discovered Mimas and Enceladus, moons of Saturn
- □ Established that coral is not a plant but a marine invertebrate
- □ First observation of Martian ice caps
55.
Who was the first paid female astronomer?
- □ Mary Somerville
- □ Caroline Lucretia Herschel
- □ Henrietta Swan Leavitt
- □ Jill Cornell Tartar
56.
Which planet is invisible to the naked eye?
- □ Mercury
- □ Saturn
- □ Uranus
- □ Neptune
57.
The ‘Happy face’ crater on Mars is officially named after an astronomer involved in the discovery of Neptune. Which astronomer?
- □ Johann Gottfried Galle
- □ Urbain Jean Joseph LeVerrier
- □ John Couch Adams
- □ James Challis
58.
Regarding the colours of Uranus and Neptune, which of the following claims is correct?
- □ Both are shades of blue, but Uranus is a dark blue and Neptune is a lighter blue
- □ Both are shades of blue, but Uranus is a light blue and Neptune is a darker blue
- □ Both are the same shade of blue
- □ Both are a shade of ivory
59.
On average, which is the closest planet to every other planet?
- □ Earth
- □ Mars
- □ Mercury
- □ No single planet is the closest to every other planet
60.
On which celestial body would you find Sputnik Planitia?
- □ Moon
- □ Mars
- □ Venus
- □ Pluto
61.
How many natural satellites does Pluto possess?
- □ 2
- □ 3
- □ 4
- □ 5
62.
After the dwarf planet Pluto, and its satellite Charon, which was the first trans-Neptunian object to be discovered?
- □ 15760 Albion
- □ 15810 Arwan
- □ 19521 Chaos
- □ 20000 Varuna
63.
Which is the ninth most massive object in orbit around the Sun?
- □ Ceres
- □ Eris
- □ Pluto
- □ Sedna
64.
In 2019, NASA’s New Horizon spacecraft flew past a Kuiper Belt object that looked rather like a snowman. What is the official name of the object?
- □ Arrokoth
- □ Frosty
- □ Thorin
- □ Ultima Thule
65.
The Titius–Bode law successfully predicted the orbits of which two objects in the solar system, before its predictions started to fail?
- □ Ceres and Neptune
- □ Ceres and Uranus
- □ Neptune and Pluto
- □ Uranus and Neptune
66.
The word ‘asteroid’ means ‘star-like’. Who coined the word ‘asteroid’?
- □ William Herschel
- □ Caroline Herschel
- □ Giuseppe Piazzi
- □ Johann Bode
67.
After whom is minor planet 8749 (discovered in 1998) named?
- □ The Beatles
- □ Brian May
- □ Queen
- □ Patrick Moore
68.
The three main types of asteroid, based upon composition, are C (chondrite), S (stony), and M (metallic). Which is the commonest type of asteroid?
- □ C
- □ S
- □ M
- □ C, S, and M are equally common
69.
One way of classifying asteroids is to use their location. What class of asteroid is to be found in specific parts of Jupiter’s orbit?
- □ Achaeans
- □ Hellenes
- □ Spartans
- □ Trojans
70.
A few asteroids orbit entirely within Earth’s orbit. (At aphelion they are closer to the Sun than Earth is at perihelion.). Following the convention that an asteroid class should be named after the first confirmed member of the class, what is this type of asteroid called?
- □ Amor asteroid
- □ Apollo asteroid
- □ Aten asteroid
- □ Atira asteroid
71.
In 2022, astronomers confirmed the discovery of an asteroid whose orbit lies entirely interior to Venus. Following the convention that an asteroid class should be named after the first confirmed member of the class, what will this type of asteroid be called?
- □ 'Aló'chaxnim asteroid
- □ Amor asteroid
- □ Apollo asteroid
- □ Aten asteroid
72.
Analysis of material samples taken from the asteroid 162173 Ryugu confirmed the presence of what?
- □ Amino acids
- □ Diamonds
- □ Highly magnetic particles
- □ The past presence of water ice
73.
More than 10,000 Apollo asteroids—Earth-crossing objects—are known. How many of them are classified as potentially hazardous objects?
- □ More than 8000
- □ 5500–6500
- □ 1500–2500
- □ Fewer than 500
74.
In all of history, how many confirmed cases have there been of a meteorite striking someone?
- □ 0
- □ 1
- □ 17
- □ 32
75.
Which event is regarded as the greatest meteor shower recorded in modern times?
- □ The Capricornids shower of 1647
- □ The Geminids shower of 1792
- □ The Leonids shower of 1833
- □ The Perseids shower of 1993
76.
How did Fred Whipple describe comets?
- □ Cruddy pellets
- □ Dirty snowballs
- □ Icy spillikins
- □ Watery wisps
77.
Halley’s Comet is probably the most famous of all comets. When was the first certain observation of this object made?
- □ 467 BCE
- □ 240 BCE
- □ 164 BCE
- □ 12 BCE
78.
Who is generally credited as being the greatest visual discoverer of comets?
- □ Johann Elert Bode
- □ Iwahashi Zenbei
- □ Jean-Louis Pons
- □ Herbert Dingle
79.
How many Oort Cloud objects have astronomers observed and identified?
- □ None
- □ 50–100
- □ 1000–10,000
- □ More than one million
80.
Which science fiction author foresaw the potential of geostationary satellites for mass communications?
- □ Robert A. Heinlein
- □ Philip K. Dick
- □ Thomas M. Disch
- □ Arthur C. Clarke
81.
Which craft made the first successful soft landing on another planet?
- □ Ranger 6
- □ Surveyor 2
- □ Venera 7
- □ Viking 1
82.
How many nations have successfully operated landers on the Martian surface (a successful operation being defined here as one that lasted longer than 2 min)?
- □ 1
- □ 2
- □ 3
- □ 4
83.
In 2020, scientists analysing data from the Messenger spacecraft, which flew past Venus on its way to Mercury, made a crude estimate of what?
- □ The chemical composition of the protostellar cloud from which the solar system formed
- □ The eccentricity of Mercury’s orbit
- □ The lifetime of a free neutron
- □ The temperature of the Sun’s corona
84.
Which spacecraft has achieved the fastest speed (relative to the Sun)?
- □ Apollo 9
- □ Helios 2
- □ Parker Solar Probe
- □ Voyager 1
85.
Of what size are the antenna on Voyager 1 and the receiving antenna on Earth?
- □ 4.2 m diameter antenna on Voyager, 55 m antenna on Earth
- □ 10 m diameter antenna on Voyager, 150 m antenna on Earth
- □ 0.5 m diameter antenna on Voyager, 93 m antenna on Earth
- □ 3.7 m diameter antenna on Voyager, 34 m antenna on Earth
86.
Using the travel times of past probes as a guide, which of the following gives the order of the duration of journeys to the planets, quickest journey first? (Assume the probe must first orbit the planet.)
- □ Mars, Venus, Jupiter, Mercury, Saturn, Uranus, Pluto, Neptune
- □ Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto
- □ Mercury, Mars, Venus, Jupiter, Saturn, Uranus, Neptune, Pluto
- □ Venus, Mars, Mercury, Jupiter, Saturn, Uranus, Pluto, Neptune
87.
How many space craft launched before 2020 are predicted to leave the solar system?
- □ 2
- □ 3
- □ 4
- □ 5
88.
Where was the hypothetical planet Vulcan supposed to be found?
- □ Inside the orbit of Mercury
- □ Between the orbits of Mars and Jupiter (in the Asteroid Belt)
- □ Just outside the orbit of Pluto
- □ Between the Kuiper Belt and the Oort Cloud
89.
In 2014, observations by the WISE spacecraft ruled out the existence of which hypothetical object?
- □ Adrasteia
- □ Kali
- □ Nemesis
- □ Rhamnusia
90.
What is the fate of the Sun?
- □ Its outer layers are ejected to form a planetary nebula, leaving a white dwarf at the centre
- □ It undergoes a supernova explosion, leaving behind a neutron star
- □ It cools to become a red dwarf
- □ It becomes a classical nova

1.2 Solutions

1.	B	46.	D
2.	A	47.	D
3.	A	48.	C
4.	D	49.	B
5.	C	50.	D
6.	D	51.	A
7.	A	52.	A
8.	D	53.	B
9.	A	54.	D
10.	C	55.	B
11.	C	56.	D
12.	D	57.	A
13.	B	58.	B
14.	C	59.	C
15.	B	60.	D
16.	D	61.	D
17.	C	62.	A
18.	A	63.	B
19.	B	64.	A
20.	C	65.	B
21.	B	66.	A
22.	B	67.	A
23.	B	68.	A
24.	C	69.	D
25.	C	70.	D
26.	B	71.	A
27.	A	72.	A
28.	D	73.	C
29.	A	74.	B
30.	A	75.	C
31.	C	76.	B
32.	D	77.	B
33.	D	78.	C
34.	A	79.	A
35.	B	80.	D
36.	B	81.	C
37.	C	82.	B
38.	C	83.	C
39.	B	84.	C
40.	B	85.	D
41.	D	86.	A
42.	D	87.	D
43.	A	88.	A
44.	C	89.	C
45.	D	90.	A

1.3 Guide: The Solar System

For most of history, humans have been terrestrial creatures. Our forebears spent their lives on the rocky parts of Earth’s surface underneath the protective blanket of an atmosphere. Now, though, we know how to get above the atmosphere. We can reach into space.

1.3.1 The Definition of Outer Space

No clear boundary exists between atmosphere and space. Even the International Space Station experiences atmospheric drag. In that sense, space does not ‘begin’ at any particular altitude. There is, nevertheless, a definition of space that most countries adopt.

1.
The Kármán line, named after the Hungarian-born engineer Theodore von Kármán (1881–1963), is an arbitrary boundary—but as a practical definition of where outer space begins it enjoys acceptance by most international bodies, including the UN. The Kármán line is the altitude beyond which traditional aircraft can no longer fly, the air being too thin to provide lift. To travel above the Kármán line requires some other propulsion mechanism; it requires a spacecraft. Fly above an altitude of 100 km (60 miles), which is roughly the height of the Kármán line, and you earn your astronaut wings. Getting to this altitude requires a lot of energy. Keep going, though, and you escape Earth’s gravity well; you then need expend relatively little energy to explore the solar system. (As the famous SF writer Bob Heinlein once remarked: ‘If you can get your ship into orbit, you’re halfway to anywhere.’) Only a handful of billionaires can currently afford a trip into space. Fortunately, we can explore the entire solar system in our imagination.

1.3.2 Sun

Earth’s atmosphere blocks lethal radiation from space but it is transparent to visible light. This means we can see into the heavens. Even our earliest ancestors would have seen the Moon, the planets, and the stars at night. And of course they would have seen—and felt—the brightest light of all: the Sun.

2.
Any intelligent extraterrestrial who visits our small corner of the cosmos might reasonably conclude that it consists of the Sun and a small amount of rubble. The Sun accounts for about 99.86% of the mass in our solar system.
3.
The Sun’s disk can appear noticeably flattened at sunrise and sunset. Atmospheric conditions change with height, and so when the Sun is on the horizon light rays from the top and bottom of the Sun’s disk can bend by different amounts. But this is a purely visual effect caused by atmospheric refraction. The Sun is one of the most perfectly spherical objects we—and perhaps any intelligent extraterrestrials who are visiting—can observe: its polar diameter is just 10 km smaller than its equatorial diameter of 1.39 million km.
4.
As we shall see in more detail in later chapters, a star’s composition changes gradually over time. In the current phase of the Sun’s life cycle, most of its mass is in the form of hydrogen (which accounts for 74.9% of its mass) and helium (23.8%). The next most abundant of the solar elements is oxygen (accounting for 0.77% of the Sun’s mass) followed by carbon (0.29%), iron (0.16%), and neon (0.12%).
5.
Internally, the Sun consists of a number of different zones. Each zone has a different density. The core, where fusion reactions take place, is dense—about 12 times denser than planet Earth. The Sun’s outer zones are much less dense. If one calculates the Sun’s average density, in other words if we divide its total mass by its total volume, then we find that the Sun is only about 25% as dense as Earth.
6.
Although the Sun can blind us with its brightness, it is—in physics parlance—an excellent approximation to a ‘black body’. A black body is an ideal object that absorbs all electromagnetic radiation falling on it, regardless of wavelength or angle of incidence. A black body not only absorbs electromagnetic radiation, it emits radiation in a particular manner over the spectrum. Physicists can describe the form of black body radiation with some simple equations, the only variable involved being the temperature of the object. By measuring the amount of radiation emitted by the Sun at each wavelength, astronomers can determine its effective black body temperature. It turns out the Sun has an effective surface temperature of 5778 K, a result consistent with other methods of estimating the temperature. This is hot, of course, but the temperatures of some physical processes here on Earth can be higher: lightning, for example, can generate higher temperature than the surface of the Sun. Temperatures in the Sun’s core, where nuclear fusion takes place, are much higher: about 15 million K. The temperature of the Sun’s corona, or outermost layer, is also extremely high. But the temperature of the Sun’s photosphere—the visible surface, the object that blinds us when we stare at it—is 5778 K.
7.
For the most part, the Sun’s disk shines bright and clear. But not always. Shi Shen, a Chinese astronomer who flourished in the fourth century BCE, was the first person to observe and record sunspots. Shi Shen proposed a charming if incorrect explanation for sunspots: he thought a solar eclipse somehow started from a sunspot and grew in size. Although his account was wrong, Shi Shen correctly identified sunspots as a solar phenomenon rather than as some sort of local meteorological effect. We now know that a sunspot (Fig. 1.1) is an area on the Sun’s surface where the magnetic field is particularly strong, a situation that causes a slight cooling. The cooler area (although still hot!) appears black in comparison with the rest of the surface. Sunspots are important because the tangling of magnetic fields near them can generate solar flares—eruptions that spew high-energy particles into space, which in turn can affect us here on Earth.
8.
Had Shi Shen enjoyed the use of a telescope he might have discovered the solar cycle: a regular variation, with a period of 11 years, in the number of sunspots. Astronomers have observed the cycle for centuries. At a peak, one sees many sunspots; 11 years later one again sees a peak; but during the minimum one sees fewer sunspots. During the period 1645–1715, a timespan that covered more than six solar cycles, sunspots were rare: the cycle reached a minimum in 1645 and then did not recover. Over those seven decades, about 50 sunspots were recorded; in some years, no sunspots were seen at all. This period is called the Maunder Minimum, after the American husband-and-wife team of solar astronomers, Edward Walter Maunder (1851–1928) and Annie Russell Maunder (1868–1947). The Maunder Minimum occurred within the period of the Little Ice Age, a time of low temperatures, at least as recorded in Europe. A popular hypothesis claims that reduced solar activity might have contributed to the cooling experienced during those years. Perhaps. One difficulty with this hypothesis, though, is that the Little Ice Age was already underway by the time the Maunder Minimum began. More work is needed to tease out whether a causal relationship exists between sunspot activity and climate.

A photograph of the sunspot. A dark central region is surrounded by a lighter-colored fibrous structure. — **Fig. 1.1**

1.3.3 Moon

The Sun dominates the daytime sky but the Moon is the brightest object at night. Little wonder, then, that our Moon has fascinated people since the earliest times.

9.
The pre-Socratic philosopher Anaxagoras (c 500–c 428 BCE), who was born in what is now Turkey but as an adult moved to Athens, thought deeply about many natural phenomena. He pondered the nature of meteors, rainbows, eclipses, and the distribution of life in the universe. He also speculated on the nature of the Moon. Although some of his ideas seem odd to modern eyes, his thinking was based on observation. According to Hippolytus of Rome, writing in A Refutation of All Heresies, Anaxagoras taught that ‘the Moon does not shine with its own light, but receives its light from the Sun’. Correct! His materialistic opinions regarding heavenly bodies did not go down well in Athens, though, and led to charges of impiety. He was forced into exile. His grave had the following inscription: ‘Here Anaxagoras, who in his quest of truth scaled heaven itself, is laid to rest’.
10.
We often talk of the ‘dark side of the Moon’, a hemisphere forever invisible to us. But the situation is slightly more complicated than that phrase suggests—and in the distant past, when the Moon was much closer, the situation was entirely different. When two objects orbit each other, as with Earth and Moon, gravitational interactions can change the orbits and the rates at which the objects turn on their axes. If one of the bodies is smaller than the other then, over time, a state can develop in which the smaller body experiences no further change in its rotation rate over a complete orbit. The smaller body is then said to be tidally locked to the larger body. This has happened in the Earth–Moon system: the Moon now turns on its axis in the same time it takes to orbit Earth. The Moon thus only ever shows one side to Earth. The Moon’s orbit, however, is slightly eccentric; this causes it to wobble slightly as seen from Earth. The result: we can see just over the Moon’s east and west limbs, and just over and under its north and south poles. In total, 59% of the Moon’s surface is visible to us. That extra 9% of visible surface lies in the so-called libration zones. The other 41% of the Moon’s surface is never visible from Earth.
11.
How far away is the Moon? Around 150 BCE, Hipparchus of Nicaea (c 190–c 120 BCE) used various observations and arguments to provide the first reasonable estimate of the distance to the Moon in terms of Earth’s radius. Given the technology available to him, he did a remarkable job; Hipparchus himself, however, understood his estimate was precise to no better than a few Earth radii. Now, more than two millennia later, we can do much better. Apollo astronauts left behind retroreflectors—devices that reflect light back to a source with minimal scattering. Scientists at various laboratories on Earth are able to shine lasers onto these retroreflectors and observe the return beam. Since we know the speed of light with great precision, a measurement of the time delay between sending the laser pulse and receiving the echo leads directly to the distance to the Moon. Ignoring its orbital eccentricity, the characteristic distance to the Moon is 384,399 km—and, thanks to those Lunar Laser Ranging experiments, we know this value to a precision of better than a millimeter. It’s like knowing the distance between London and New York to within a hair’s breadth!
12.
The Moon is the closest celestial body, and thus appears large in the sky: it has a large apparent angular diameter. Most celestial bodies appear much smaller on the celestial sphere. To get some feel for angular sizes, and for the astonishing precision with which astronomers now work, stretch out your arm. At arm’s length your fist spans about 10°, your index finger about 1°. A minute of arc, or 1′, is 1/60th of a degree. The average angular diameter of the full Moon is 31′ (or about 0.5°). A second of arc, or 1″, is 1/3600th of a degree. Modern observations often have a precision measured in thousandths of a second of arc! (Note that the angular diameter of the Andromeda galaxy, a celestial object we shall discuss later, is about 3°—or six times wider than the Moon. Andromeda thus covers a surprisingly large patch of sky. The reason we do not see Andromeda at night is not because the galaxy is small but because it is faint.)
13.
A beautiful cosmic accident means Moon and Sun possess almost the same angular size: about 0.5°. This becomes important at times of syzygy—a term deriving from an Ancient Greek word for ‘yoke’ and referring to a straight-line configuration of three celestial bodies in a gravitational system. A syzygy involving Earth, Sun, and a planet can give rise to an occultation (when the Sun passes in front of a planet) or a transit (when the planet passes across the Sun’s disc). A syzygy involving Earth, Moon, and Sun is special. A lunar eclipse is mildly interesting. But, since Moon and Sun have the same apparent size in the sky as seen from Earth, a syzygy can give rise to perhaps the most awe-inspiring event in astronomy: a total solar eclipse (see Fig. 1.2).

A photograph displays the phenomenon of solar eclipse. A series of bright dots represent the gradually decreasing and increasing shape of the sun. — **Fig. 1.2**

14.
If the Moon’s orbit around Earth were circular, and if it were in the plane of the ecliptic (a term whose definition we will come to soon), then we would experience a total solar eclipse every month. Reality is messier than that. A total solar eclipse occurs somewhere on Earth once every 18 months or so, on average; any given location on Earth experiences an eclipse once every 400 years or so, on average. The duration of a total solar eclipse also varies, and the maximum possible duration of an eclipse changes because the Moon’s angular size is gradually decreasing as gravitational interactions cause the Earth–Moon distance to increase. So each solar eclipse is different. At present, and for the next few centuries, the maximum duration is 7 min 32 s. A solar eclipse in the year 2186 will be only three seconds shorter that that maximum; if people are around to observe it, they will experience the longest-lasting eclipse since at least 3000 BCE. It will not be bettered for at least 6000 years.
15.
Whatever its duration, any solar eclipse is sure to capture the imagination of those who observe it. That was certainly the case for the ancients. One of the most intriguing eclipses took place more than 2600 years ago. The Greek historian Herodotus wrote that the warring Medes (an ancient Iranian people) and Lydians (an ancient Anatolian people) interpreted a solar eclipse as an omen, and chose to pause their fighting. The only eclipse matching the description of Herodotus occurred on 28 May 585 BCE. This ancient, interrupted battle is the earliest historical event dated to a particular day. Of even more interest is that Herodotus claimed Thales of Miletus (c 626–548 BCE), the first known person whom we can reasonably class as being a scientist, predicted the eclipse. Other Greek and Roman writers independently backed the claim. But that leaves a mystery: we do not know how Thales could have made the prediction, given the knowledge available to him at the time.

1.3.4 Pre-Copernican Astronomy

The heavens contain more than just Sun and Moon: when we look up into the night sky we see points of light. Ancient astronomers developed theories to explain the movement of these points of light. Those theories were wrong—but they were influential for more than 1500 years and they remain a testament to human ingenuity.

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Most point-like lights in the sky have positions that remain fixed, relative to each other, night after night. Ancient Greeks called these lights asteres, from which we get our word ‘star’. Five asteres—Mercury, Venus, Mars, Jupiter, Saturn—are seen to move, from night to night, relative to the fixed stars. These are ‘wandering stars’ or, as the Greeks called them, planētes asteres. The Greeks often shortened this phrase to planētes, from which we get our word ‘planet’.
17.
Those ancient Greek astronomers worked out some rules to determine the relative distances from Earth of the Sun, Moon, and planets. For example, they reasoned the Moon must be the closest celestial object because it sometimes moves in front of other objects but those objects never move in front it. Another example: they used the rate at which planets move across the sky to estimate relative distance. Mercury moves quickest and was thus deemed to be closer than Saturn, which moves slowest. Putting all the information together provided the following sequence, from closest to most distant: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn. According to ancient astrologers, one of the seven planets (the five wandering stars, plus Moon and Sun) ‘ruled’ each hour of the day. The astrologers assigned Saturn, the furthest planet, the honour of ruling the first hour of the first day; Jupiter, next furthest, ruled the second hour of the first day; Mars ruled the third hour, Sun the fourth hour, and so on to nearby Moon, which ruled the seventh hour. And then the cycle repeated. So the first hour of successive days was ruled by Saturn, Sun, Moon, Mars, Mercury, Jupiter, and Venus. The day was named after the ruler of its first hour—thus the days of the week get their names from an early version of the cosmic distance ladder! It should be obvious how the days Saturday, Sunday, Monday come from Saturn, Sun, and Moon. In English, the other days get their names from the Teutonic equivalents of Latin gods: Woden for Mercury, Thues for Mars, Thor for Jupiter, and Freia for Venus.
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Ancient Greek astronomers typically assumed that the heavenly bodies wheel around Earth, a reasonable enough assumption given how the ground feels stationary beneath our feet. But Aristarchus of Samos (c 310–c 230 BCE) promoted a heliocentric theory—the notion that the Sun rather than Earth is the centre of the universe. Aristarchus developed this idea more than two centuries before the birth of Christ. He also followed the notion of Heraclides of Pontus (c 387–c 312 BCE) in arguing that the apparent daily rotation of the celestial sphere is due to the rotation of Earth on its axis. Furthermore, Aristarchus developed an ingenious geometric method for estimating the relative sizes and distances of Sun and Moon, based on observations made when the Moon is exactly half lit by the Sun. (His estimate: the Sun is 20 times farther from Earth than is the Moon, and is 20 times larger. His method was correct in principle. He got the numbers wrong because the required observations are difficult to make.) So his view of the solar system was remarkably modern. However, although Aristarchus appears to have been recognised by contemporaries as a great thinker, his heliocentric theory never gained popularity and the idea was ignored for about 1700 years.
19.
As Aristarchus suggested, and as we now know, Earth orbits the Sun. The projection on the celestial sphere of the plane of Earth’s orbit around the Sun is called the ecliptic. Observations show that the planets all orbit the Sun in roughly the same plane as Earth: the ecliptic is an echo of the original spinning dust cloud from which our solar system emerged. The planets do not orbit exactly in the ecliptic—the orbit of each planet is slightly inclined to the plane—but the variation is just a few degrees. Similarly, the Moon is not quite on the ecliptic; its orbit is tilted by about 5.2°. If the Moon and planets orbited precisely on the ecliptic then we would see an occultation (in other words, an eclipse) of the planets and the Sun once every month. Instead, as Greek astronomers were aware, a solar eclipse happens when, during a New Moon phase, the Moon crosses the ecliptic. The word ‘eclipse’ ultimately comes from the Greek word for an idea meaning ‘to be hidden’, which makes sense for obvious reasons. The term ‘ecliptic’, referring to the projection of Earth’s orbit on the celestial sphere, comes from the observation that eclipses happen when the Moon crosses the path of Earth’s orbit around the Sun.
20.
In the ancient world, many of the most accurate observations of the motion of the Sun, Moon, and planets were made by Hipparchus, a man whom the French scientist Jean Baptiste Joseph Delambre (1749–1822) described as the greatest astronomer of antiquity. Many would agree with Delambre. As we have already seen, Hipparchus estimated the distance to the Moon. He also compiled a star catalogue, measured the precession of the Earth, and is often credited with the invention of the armillary sphere and the astrolabe. More importantly for our present story, his accurate observations of the motion of celestial objects, combined with his development of the mathematical techniques of trigonometry, enabled him create an effective model of the solar system.
21.
Like Hipparchus, Eratosthenes of Cyrene (c 276 BCE–c 195 BCE) was a remarkable thinker. He: developed a method for identifying prime numbers (the ‘Sieve of Eratosthenes’); calculated Earth’s axial tilt; produced the first world map with meridian lines and parallel lines (like lines of latitude and longitude); explained, for the first time, why the Nile floods each year; wrote a timeline of scientific history from the siege of Troy onwards; served as director of the Library of Alexandria; and, most impressive of all to my mind, calculated Earth’s radius. Yet his nickname was ‘Beta’, the second letter of the Greek alphabet, since he was second best in everything, best in nothing. Perhaps it didn’t help that he lived at the same time as his friend Archimedes of Syracuse (c 287 BCE–c 212 BCE), who was perhaps the greatest scientist of all until Newton came along. Nevertheless, we can see how the work of Eratosthenes—along with that of Hipparchus, Aristarchus, and several other Greek astronomers—could be combined to paint a relatively sophisticated picture of the solar system.
22.
Much of the Greek understanding of astronomy was summarised by Claudius Ptolemy (c 100 CE–c 170 CE) in The Almagest, one of the most influential science books of all time. This book, describing the apparent motions of celestial objects, was the basis for the geocentric model of the Universe. Philosophers accepted this model until the time of Copernicus. (As we have seen, Aristarchus promoted a heliocentric theory but the incorrect geocentric theory won out.) Few details survive about the author of The Almagest, but we know when Ptolemy was active because he records the dates of some of his observations. The earliest observation was dated 26 March 127, the latest 2 February 141.
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During the dark ages of European thought, the Arab world maintained the works of Greek scholarship. But Arab scientists did more than preserve existing knowledge; they were strong researchers. Consider, for example, Omar Khayyam (1048–1131). Khayyam’s ability as a mathematician and astronomer has been overshadowed by his fame as a poet (Edward Fitzgerald’s translation of the Rubaiyat contains the immortal verse beginning ‘The Moving Finger writes, and, having writ, Moves on…’). Khayyam measured the length of the year to be 365.24219858156 days. The accuracy he provided (11 decimal places!) is absurd, but Khayyam was astonishingly correct. The length of the year measured in days will have decreased in the sixth decimal place even over the few decades since a typical reader of this book was born. Back when Fitzgerald was translating the Rubaiyat, some eight centuries after the birth of Khayyam, the length of the year was 365.242196 days. Khayyam did an incredible job!
24.
India also had a strong tradition in mathematics and science while European thought languished. Consider the number zero. Given that we routinely use zero as a placeholder and as a number we manipulate in the same way as other numbers, it seems strange that the notion of zero is relatively recent. The Romans, for example, had no numeral corresponding to 0. Think how cumbersome arithmetic is in the absence of zero, how awkward it is to distinguish between the numbers 2, 20, and 200. And yet the number 0 seems different to other numbers. We can readily understand what the fraction 1/2 or 2/5 might represent, but what does the fraction 1/0 mean? The Indian mathematician Brahmagupta (c 598–c 668) was the first to lay down rules for manipulating the number 0. (Most of his rules were correct. His understanding of what division by zero means is different to our modern understanding.) Brahmagupta brought his mathematical skills to bear on various astronomical questions, too, including the development of methods to calculate the position of Sun, Moon, and planets over time. The work of Indian and Arab scientists added to the store of pre-existing Greek knowledge so, by the time of the Renaissance, Copernicus was in a place to set in motion the Scientific Revolution.

1.3.5 Heliocentric Theory

Geocentric theory held sway until the sixteenth century. Over a period of two centuries the Scientific Revolution, bookended by Copernicus and Newton, changed humanity’s understanding of the heavens. Copernicus put the Sun at the centre of the solar system while Newton showed how the movement of objects in the solar system was governed by laws that he could express in mathematical form. The Scientific Revolution transformed how we view the world.

25.
Nicolaus Copernicus (1473–1543) is renowned for his book De revolutionibus erbium coelestium, publication of which was delayed until the time of his death—he probably suspected that his Sun-centred model of the universe would meet with religious objections. Copernicus was not the first to develop a heliocentric theory; as we have seen, Aristarchus beat him to the idea by the best part of two millennia. Copernicus, however, unlike Aristarchus, proved to be influential. His heliocentric theory gradually came to dominate people’s view of the solar system. And today the ‘Copernican Principle’—the idea there is nothing special about Earth or our place in the universe—remains key in modern astronomy. But Copernicus had several strings to his bow. He was a deep thinker on economics, for example. In his treatise Monetae cudendae ratio he argued that debased coinage drives high-value coinage out of circulation—in other words, bad money drives out good. This is commonly known as Gresham’s law, but is sometimes called the Gresham–Copernicus law. (Few ideas are ever truly new. Just as the Greeks flirted with heliocentricism, an understanding of the Gresham–Copernicus law appears to date back to Greek times.)
26.
Not every astronomer jumped to accept the heliocentric model, and it took time for Copernican ideas to prevail. Consider the Danish astronomer Tycho Brahe (1546–1601), one of the greatest observers of the pre-telescopic era and the last major astronomer to work without the aid of a telescope. He admired Copernicus but could not accept the idea that Earth moved. Brahe melded the Ptolemaic and Copernican systems into his own model of the universe, one in which Sun and Moon orbited Earth and the planets orbited the Sun. (Brahe’s model of the solar system may not have been influential, but his observation of a supernova in 1573 upended philosophical thought: Brahe’s supernova was proof that belief in an unchanging celestial realm, as promoted by Aristotle and generally accepted thereafter, was untenable. For this, and other observations, Brahe is one of the most famous names in astronomy. For ENT surgeons, a more interesting fact about Brahe is that contemporaries sometimes referred to him as ‘the man with the golden nose’. When he was 20, he got into a drunken fight with his cousin Maderup Parsberg. Swords were drawn, and Brahe lost part of his nose. After he recovered, Brahe often wore a prosthesis. His body was exhumed in 2010, and a team from Aarhus University subjected his bones and hair to spectroscopic analysis. The team found high levels of copper and zinc, and concluded that Brahe must have often worn a brass, rather than gold, prosthesis. See Fig. 1.3.)

A monochromatic photograph of Tycho Brahe. — **Fig. 1.3**

27.
The German astronomer Johannes Kepler (1571–1630) worked for a time with Brahe, and eventually inherited Brahe’s immensely detailed observations of planetary motion. Kepler was able to show that the heliocentric model of Copernicus was in excellent agreement with Brahe’s observational data, provided that the planets move in elliptical orbits around the Sun. Kepler eventually defined the three laws of planetary motion, work that provided the foundation for Newton’s development of the theory of gravitation. (Kepler’s astronomical work must have required an ability to reason and to spot inconsistencies, skills that came in useful when he defended his mother Katharina against accusations of witchcraft. Over a period of several years Katharina Kepler had been accused of practicing ‘forbidden arts’—by poisoning a neighbour, killing livestock, turning herself into a cat … the usual—and in 1620 she was in a cell and being threatened with torture. Johannes dropped his work in Linz, Austria and moved his household to Leonberg, Germany to take successful charge of his mother’s defence. The move might have saved Kepler’s own life: while he was in Germany, Catholic authorities executed 27 Protestant believers in Linz. Had he been in Austria, the profoundly religious Kepler might have been one of the victims.)
28.
And so we come to Isaac Newton (1642–1726). See Fig. 1.4. In 1666, the ‘Year of Wonders’, when he was aged just 23, Newton transformed science. He developed calculus. He laid down the laws of motion. He made revolutionary discoveries in optics. And he formulated the law of universal gravitation, a law that explained how the planets must move in elliptical orbits. At once, the heliocentric model of Copernicus as improved by Kepler had a natural and mathematical explanation. There could be no longer be any doubt: Earth and the other planets orbited the Sun.

A painting of Isaac Newton. — **Fig. 1.4**

29.
It is difficult to overstate the importance of Newton’s contribution to human thought. Prior to Newton, people typically described the world in qualitative terms; after, people were much more likely to describe the world in quantitative terms. Prior to Newton, people tended to defer to the knowledge of ancient thinkers such as Aristotle; after, since Newton’s thinking was so clearly superior to that of the ancients, people felt confident to trust in their own observations rather than accept the words in ancient manuscripts. A strong case can therefore be made that Newton’s Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy or, to give it its catchier name, Principia), the book that set out a new approach to investigating and understanding the world, is the most important work in all of science. But the book almost did not reach the printers. The Royal Society had promised to bring it out but their finances were stretched because they had recently published Francis Willughby’s Of the History of Fish. Willughby’s book did not find a wide readership (surprise, surprise) and the project lost a great deal of money. The English astronomer Edmond Halley (1656–1741), who was then the clerk of the Royal Society, recognised the world-changing nature of Principia and funded publication himself. The Royal Society was so tight that, after Halley had secured publication of Newton’s book, they told their clerk they could not pay him his £50 salary. They instead paid him in kind—with unsold copies of Of the History of Fish.
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Over the period from Copernicus to Newton, then, the heliocentric model won out. In such a model, where the Sun is an object around which planets orbit, the Earth–Sun distance—known by the term astronomical unit (AU)—becomes an important quantity for defining the scale of the solar system. Astronomers, however, have come up with different definitions of the AU. The simplest says the astronomical unit is the average of the aphelion (maximum distance of Earth from Sun) and perihelion (minimum distance), these being different because the Earth–Sun distance varies by about 3% over the course of a year. Astronomers determined that, under this definition, 1 AU was 150 million km (slightly less than 93 million miles, or 8.3 light-minutes). But as measurements became increasingly accurate, the value of this important quantity was liable to change. So, in 1976, the International Astronomical Union changed the definition of the AU to be the distance from the Sun at which a particle of zero mass would have a period of one year. The Sun is always losing mass, though, so under this definition the AU changes over time. So, in 2012, the International Astronomical Union decided upon yet another definition: this time they set the astronomical unit to be exactly 149,597,870.7 km. No more changes!
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The AU is a useful unit when discussing distances within the solar system. We can say, for example, that Venus orbits at 0.72 AU from the Sun and Jupiter orbits at 5.2 AU from the Sun. These are convenient numbers to work with. For most distances in astronomy, though, the AU is too small a unit to be practical. For distances to stars, astronomers use the parsec as the base unit; longer distances within the Milky Way galaxy might require the use of kiloparsecs (kpc); the distance to most galaxies requires the use of megaparsecs (Mpc); and quasars are typically at gigaparsecs (Gpc). In this chapter we stay within the realm of the solar system, so we will not need to use such a large unit. Nevertheless, it makes sense to consider the parsec at this point because it is defined in terms of the astronomical unit.
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You are probably familiar with the phenomenon of parallax, the offsetting of a foreground object against the background when you change your perspective. You observe parallax on a small scale whenever you look at the world first with your left eye closed and then with your right eye closed: you see nearby objects shift slightly compared to a distant background. On a larger scale, astronomers have long known from shared observations that, at any given instant, people at widely separated locations on Earth observe the Moon to be among different stars. On an even larger scale, astronomers can see tiny positional changes in a nearby star, relative to background star fields, as Earth orbits the Sun. Imagine you were somewhere out in space, studying the Earth–Sun system. From a certain distance the radius of Earth’s orbit—1 AU, which as we have seen has been defined to be 149,597,870.7 km—would subtend an angle of 1′ (1 second of arc; 1/3600 of a degree). This distance is the parsec (pc). It corresponds to 206,000 AU. The British astronomer Herbert Hall Turner (1861–1930) came up with the word ‘parsec’ in 1913, when he used it to shorten the phrase ‘parallax of one second’.

1.3.6 A Tour of the Eight Planets

Ancient astronomers identified five planets. We now know of two more. If we follow Copernicus, and consider Earth to be just one of several objects that orbits the Sun, we have a solar system of eight planets. Let’s take a brief tour of the solar system, starting with the planet closest to the Sun and working outwards.

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The closest planet to the Sun is Mercury. Mercury, with an orbital eccentricity of 0.2056, has the most elliptical orbit of any of the planets. (Pluto, with an orbital eccentricity of 0.248, used to hold this record. But, as we shall see, Pluto was demoted and is now classed as a dwarf planet.) Mercury’s egg-shaped orbit means that, when it is closest to the Sun, it receives twice as much solar radiation compared to when it is farthest from the Sun. The planet’s eccentric orbit, combined with its slow axial rotation, also gives rise to a strange phenomenon that perhaps astronauts might one day observe in person: at certain locations on Mercury, the Sun rises for a short time, sets, then rises again; at sunset the reverse happens.
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The Sun shines too brightly for us to see the planets during the full glare of day, but at sunrise and sunset we can easily make out Mercury and Venus. Since both planets orbit closer to the Sun than does Earth, we always observe them to be relatively close to the Sun in the sky. The Ancient Egyptian astronomers, and the early Greek astronomers, thought there were four different celestial objects involved here: two ‘morning stars’ and two quite different ‘evening stars’. When Mercury appeared as a ‘morning star’ the Egyptians called it Set—an evil star that tried to fly upwards but was caught by the rising Sun. When it appeared as an ‘evening star’ the Egyptians called it Horus—a follower of the great god Amun-Ra. (For the Ancient Greeks, the two equivalent names for Mercury were Apollo and Hermes.) Even before the time of Aristarchus, however, astronomers had realised that the Egyptian Set/Horus or the Greek Apollo/Hermes were the same object: Mercury (as named by Romans). See Fig. 1.5. Similarly, the ‘morning star’ Phosphor and the ‘evening star’ Hesperus were understood to be the same planet: Venus.

A photograph of the planet Mercury. The Mercury has many circular pits on its surface. Some bright and dark spots are visualized. — **Fig. 1.5**

35.
Venus has the longest day of any planet in the solar system: it completes one rotation on its axis every 243 Earth days. The Venusian year, the time it takes to complete one orbit of the Sun, is 224.7 Earth days. So the Venusian day is longer than its year. (For reference, the planet with the shortest day is Jupiter: it rotates once on its axis every 9 h 55 min 40 s.)
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Although Mercury is the closest planet to the Sun, it is not the hottest planet. Venus is hotter. The daytime surface temperature on Mercury can reach 430 °C; at night, though, the surface temperature can plummet to −180 °C. The reason for this wild temperature fluctuation is that Mercury lacks an atmosphere to trap heat. Venus has a thick atmosphere containing lots of carbon dioxide, which in the distant past led to a runaway greenhouse effect. The Venusian atmosphere traps and circulates heat. Surface temperatures on Venus are about 475 °C: hot enough to melt lead.
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Observations of Venus—and in particular observations of transits of Venus—have played an important role in the development of astronomy: they helped astronomers establish the size of the solar system, and thus the first rung on the cosmic distance ladder. A transit is when a small body crosses the line-of-sight between a large body and the observer. We see a transit of Venus when the planet crosses the Sun’s disk. See Fig. 1.6. Transits of Venus are both regular and rare: they occur in pairs, separated by eight years, and the pairs are separated by durations of 121.5 years and 105.5 years. We had a recent pair of transits in 2004 and 2012. The next pair will happen in 2117 and 2125. Earlier pairs occurred in 1874 and 1882; 1761 and 1769; and 1631 and 1639. Kepler predicted the 1631 event, but no one observed it. The first transit to be recorded was the 1639 event, by the young English astronomer Jeremiah Horrocks (1618–1641). The most recent pair of transits have been useful not for establishing distances within the solar system, which are by now well known, but for helping astronomers better understand what to look for when exoplanets transit and cause a small dip in light from the transited star.

A photograph displays the bright flaming surface of Venus. It visualizes a dark circular spot near its surface. — **Fig. 1.6**

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Earth, the third planet from the Sun, is unique (as far as we know) in being a home for life. A rather less notable way in which our planetary home is special relates to its density. Planetary density varies as one moves along the planet’s radius. Earth, for example, has a crust, a mantel, and a core; the three layers each have a different density. If we know a planet’s radius and its mass, however, then we can easily calculate its average density. Earth, with a mean density of 5.514 g cm⁻³, is the densest planet. (The density of Earth’s inner core is even higher: about 13 g cm⁻³.) Mercury, with a mean density of 5.428 g cm⁻³, is almost as dense as Earth; then comes Venus (5.244 g cm⁻³) and Mars (3.934 g cm⁻³). These are the four rocky, terrestrial planets; the four outer planets are much less dense.
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Another interesting fact about our planet, a fact known to Hipparchus, is that it precesses: its rotational axis slowly shifts, rather like a wobbling spinning top, completing a full cycle in about 25,772 years. The precession is caused by the gravitational pull of Moon and Sun on Earth’s equatorial bulge. At present, Earth’s northern axis points quite closely to Polaris. Indeed, Polaris makes an excellent pole star or guide star: it is within about one degree of the northern pole; it is brightish (being the 45th brightest star); and it has no bright neighbouring stars that might confuse the navigator. But as Earth precesses, and its rotational axis shifts, Polaris will eventually move away from the pole. Equally, Earth’s precession means that in the relatively recent past Polaris was not the closest star to the pole. Between 1500 BCE to 500 CE, the pole star was Kochab. (Kochab was not as close to the pole as Polaris currently is, so it was a less accurate guide.) Before that, Thuban was the pole star. (In modern, light-polluted skies Thuban would be a poor guide star: it is too dim to see easily.) The other factor one must consider when thinking of pole stars is that stars move through space. Polaris is within one degree of the pole today; when it was the pole star in 23,500 BCE it was even closer to the pole; when Earth’s precession makes it the pole star again, in 27,800, it will be farther from the pole.
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Neither Mercury nor Venus possesses a natural satellite. Earth, of course, has one large natural satellite: our Moon. But does it have any other moons? Well, no it does not. But there have been numerous claims to the contrary. In 1918 the eccentric English astrologer Walter Gorn Old (1864–1929), aka Sepharial, proposed the existence of Lilith—a body as massive as the Moon but too dark to see—and used her in his astrological computations (calculations that were clearly nonsensical because Lilith does not exist). Earlier, in 1846, Frédéric Petit (1810–1865) claimed to have observed a second moon; in 1898 Georg Wilhelm Waltemath (1840–1915) claimed to have observed a whole system of small moons; and in 1968 John Pendleton Bagby (1924–2004) claimed that ‘Earth has at least ten close natural moonlets which broke off from a larger parent body’. None of these claims has ever been substantiated.
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We should note here in passing that astronomers have struggled to understand the motion of even a single large Moon; a second moon would have made a complicated picture much worse! Even Newton had to work hard when thinking about the Earth–Moon system. He provided original results about Earth’s shape and precession; he showed how the Moon’s gravitational influence gave rise to tides on Earth; and his lunar theory provided an explanation of the Moon’s motion. All this must have appeared almost magical to his contemporaries. But none of this came easily. He is reputed to have told Halley that lunar theory made his ‘head ache’ and kept him ‘awake so often that [he] would think of it no more’. One Moon is enough.
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Although Earth has only one large natural satellite, several small objects orbit the Sun in a similar orbit to Earth: asteroids called Earth trojans, for example. To understand these objects, you needs to know about Lagrangian points. If a body with a small mass is gravitationally influenced by two much more massive orbiting bodies then there can exist five points of gravitational equilibrium for the small body. These are the Lagrangian points, denoted L1, L2, L3, L4, and L5. The points L4 and L5 occur when the three bodies form the corners of an equilateral triangle. If the ratio of the masses of the two large bodies is greater than about 25 (which is the case for the Sun–Earth and Earth–Moon systems) then the L4 and L5 equilibrium points are stable: push the small body away from the point and it returns to that point. Over time, therefore, if small objects fall into the L4 and L5 points of say the Sun–Earth system then they tend to stay and accumulate in the vicinity. This is what happened with the Earth trojans, which orbit at the L4 and L5 Sun–Earth Lagrangian points. A similar phenomenon can happen at the L4 and L5 Earth–Moon Lagrangian points. In 1956, the Polish astronomer Kazimierz Kordylewski (1903–1981) claimed to have observed accumulations of dust at these Earth–Moon L4 and L5 points. The glow emitted by these concentrations of dust is extremely faint, however, and it was not until 2018 that astronomers confirmed the existence of these Kordylewski clouds.
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Earth apart, Mars is perhaps the best-studied world in the solar system. And it is a fascinating place. For example, the western hemisphere of Mars is home to Olympus Mons, a shield volcano, the largest and highest mountain in the solar system. See Fig. 1.7. The Mars Orbiter Laser Altimeter, a NASA spacecraft that operated in an orbit around Mars between 1997 to 2006, measured the height of Olympus Mons to be over 21.9 km—almost 2.5 times higher than Mount Everest.

A photograph of the dusty surface of Olympus Mons. It displays an irregularly shaped pit on its surface. — **Fig. 1.7**

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The most striking feature of Mars, one known even to ancient observers, is its red colour. The Martian regolith, its surface material, contains a lot of iron oxide. The compound iron(III) oxide, which contains two iron atoms and three oxygen atoms, absorbs light at blue and green wavelengths but reflects light at red wavelengths. This compound gives rust its distinctive appearance; blood its colour; and Mars its reddish hue. (In the human imagination the red planet has long been associated with blood: Mars was the Roman god of war.) Mars and Earth formed from the same material but Mars has more surface iron than Earth because it is less massive: our planet’s stronger gravity caused most of its iron to sink and form a molten core. Why, though, did the surface iron oxidize on Mars? In the absence of oxidation, the iron would have a black sheen and Mars would appear dull. Well, planetary scientists still debate the details but one possible scenario is that intense rainstorms on the young Mars caused rusting of the iron in the regolith.
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Moving out from Mars we reach Jupiter, the largest planet in the solar system. Jupiter is a giant in many ways. Consider its magnetosphere. A magnetosphere is the cavity created in the solar wind—the stream of charged particles spewed out by the Sun’s corona—by a planet’s magnetic field. The existence of Earth’s magnetosphere is crucial for life: without its protection, creatures would be bombarded by lethal high-energy particles from the solar wind. Jupiter’s magnetic field results from the planet’s fast speed of rotation. Unlike Earth, however, the magnetic field is not generated by the planet’s inner core but by interactions in its metallic hydrogen outer core. Just as Jupiter is the largest planet, by size and by mass, its magnetosphere is the biggest and most powerful in the solar system. Indeed, by volume, the Jovian magnetosphere is the largest continuous structure in the solar system (apart from the Sun’s heliosphere itself).
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Jupiter has dozens of moons; 92, at the last count. For my money, the most interesting is Io. The orbit of Io, the third largest Jovian moon, is perturbed by the presence of Ganymede (the largest moon) and Europa (the fourth largest). Io orbits in an elongated ellipse, so sometimes it is close to the giant planet and sometimes it is distant. This means Io is subject to tidal forces that raise and lower its solid surface by as much as 100 m! Compare this with Earth: the biggest difference between high and low tides in the ocean is less than 20 m, and few places on Earth have a mean tidal range as large as 10 m. The tidal action on Io is a remarkably efficient heat generator, so the crust underneath the surface melts. This lava forces itself to the surface wherever it can: hundreds of volcanoes spew lava tens of kilometers above the surface. See Fig. 1.8. Io is a fascinating world.

A photograph of a planet displays the phenomenon of volcanic eruption on its surface. — **Fig. 1.8**

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Io is an important object of study for modern astronomers; the satellite also played an important role in the history of physics. The first person to demonstrate that light does not travel instantaneously was the Danish astronomer Ole Rømer (1644–1710), while working in Paris, and key to this demonstration were his observations of Io. The plane of Io’s orbit around Jupiter is close to the plane of Jupiter’s orbit around the Sun. Io takes 42.5 h to orbit Jupiter and, in each lap, Io spends some time in Jupiter’s shadow: it is eclipsed. An observer on Earth sees Io disappear when it moves into Jupiter’s shadow, and reappear when it moves out of the shadow. Rømer noticed that the time between eclipses was not constant. When Earth and Jupiter were moving closer, as they orbited the Sun, the eclipse interval for Io decreased; when Earth and Jupiter were moving apart, the eclipse interval increased. Rømer believed that Io’s orbital period was constant, so his observations only added up if light travelled at a finite speed. He estimated that light took about 22 min to traverse the diameter of Earth’s orbit around the Sun.
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At this point it makes sense to introduce yet another unit of distance measurement that is often used in astronomy: the light-year. A light-year is the distance that light, moving at the constant speed of 299,792,458 m s⁻¹, travels in one Julian year of 365.25 days. A quick calculation shows that a light-year is 299,792,458 × 60 × 60 × 24 × 365.25 m, or 9,461,000,000,000 km (which works out to be about 6 trillion miles, if you prefer those units). This is too large a unit for comfortable use when discussing distances in the solar system: an astronomical unit is about 0.000,016 light-years, for example, and even the distance from Pluto to the Sun is only about 0.000,754 light-years. As soon as we begin to talk about stellar distances, however, the light-year becomes a convenient unit. In terms of the parsec, we find that 1 pc is about 3.26 ly.
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As we have seen, the solar system contains four small, high-density planets: Mercury, Venus, Earth, and Mars. The other four are large, low-density planets. Jupiter and Saturn are ‘gas giants’; Uranus and Neptune are ‘ice giants’. Of the four ‘giants’, Saturn possesses the lowest density: 0.687 g cm⁻³. This is considerably less than the density of water, which is 1 g cm⁻³. The second least-dense planet is Uranus (1.271 g cm⁻³), followed by Jupiter (1.326 g cm⁻³) and Neptune (1.637 g cm⁻³).
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Saturn, the sixth planet from the Sun, is visible to the naked eye, but the first person to study the planet through a telescope was Galileo di Vicenzo Bonaiuti de’ Galilei (1564–1642)—or simply Galileo, as he typically called himself. Galileo, a truly great scientist, continued the Scientific Revolution started by Copernicus. He investigated basic physics concepts (velocity and acceleration, gravity and inertia) using experimental techniques; he was a skilled engineer, building a thermometer and designing an escapement mechanism for a pendulum clock; and he was the first to observe the skies with a telescope (discovering four satellites of Jupiter, the phases of Venus, and numerous lunar craters). No matter how important his discoveries, papal disfavour meant Galileo was buried in an unremarkable spot. About a century later, his body was reburied in a more prominent place—and, during the move, the antiquarian Anton Francesco Gori took the chance to help himself to Galileo’s middle finger. You can see it, proudly on display, in a glass jar in the Museo Galileo in Florence. (Gori must have taken a few more spare parts because recently the middle finger has been joined by Galileo’s thumb and index finger, plus a tooth.)
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When Galileo looked at Saturn through a telescope in 1610, he observed something surprising: the planet appeared to have two lobes, like ears, either side of a central sphere. When he looked again in 1612, the lobes had disappeared. He wrote ‘Has Saturn swallowed his children?’ His question invoked a story involving Cronus (the Greek god on whom the Romans based their god Saturn). Cronus devoured his children to try and prevent them from overthrowing him. Galileo’s confusion deepened in 1616 when he saw the lobes again, this time larger than in 1610. He never did understand what he was seeing. We now know, of course, that a ring system orbits Saturn. Because the axis of Saturn is tilted at 27° to the ecliptic, the amount of the ring system we can see changes as it orbits the Sun. Every 15 years or so the rings turn edge-on to us and seem to disappear; Galileo observed precisely this phenomenon in 1612.
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Saturn’s dramatic rings, we now know, came from the tidal disruption of a former moon. The French astronomer Édouard Albert Roche (1820–1883) was the first to calculate how close a small object, held together only by gravity, could approach a larger object before being pulled apart by tidal forces. The distance depends on the radius of the larger body and the ratio of the densities of the two bodies. Roche argued that the tidal disruption of a small satellite could have led to the formation of Saturn’s rings: when the satellite approached the limit, tidal forces deformed it; at the limit, the satellite disintegrated; then, as bits of the satellite closer to Saturn orbited more quickly than distant bits, over time the remnants spread out to form a ring. See Fig. 1.9. (Incidentally, an event such as this could not happen in the case of, say, Earth and Sun: a dense object such as Earth would have to be inside the Sun for it to be tidally disrupted. The Roche limit is larger for less dense objects such as comets, and a comet could be tidally disrupted if it got too close the Sun or to one of the planets.)

A photograph of Saturn displays the distribution of small remnants spread out to form a ring. — **Fig. 1.9**

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As we have already noted, the ancients knew that five ‘planets’—Mercury, Venus, Mars, Jupiter, and Saturn—wandered across the celestial sphere relative to the fixed stars. The object Uranus can sometimes be seen with the naked eye, and indeed it was observed many times by astronomers and mistakenly classified as a star. The earliest observation might have been by Hipparchus in 128 BCE. On 13 March 1781, Frederick William Herschel (1738–1822) observed Uranus through a telescope and this led to its recognition as another planet—the first such identification in history. Herschel initially named the planet after King George III. It was not until seven decades later that astronomers agreed on the name that would lead to innumerable bad puns.
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Herschel’s discovery of a new planet would have been sufficient to grant him fame. But he went on to become one of the greatest observational astronomers of all time. His discovery of Mimas and Enceladus (moons of Saturn) and Titania and Oberon (moons of Uranus) were relatively minor highlights of a stellar career, as was his observation that the Martian ice caps change with the Martian seasons. (Herschel was not, however, the first to observe the ice caps themselves; the first definite mention of a Martian ice cap was by Cassini, more than a century earlier.) We shall encounter some of Herschel’s more important discoveries in later chapters. Outside of astronomy, Herschel was a trained musician and he composed a number of works. And, by studying coral under a microscope, he was the first to establish that coral was not a plant. Quite the Renaissance man!
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William’s sister, Caroline Herschel (1750–1848), was a formidable astronomer in her own right. Caroline was born in Hanover and, at the age of 10, contracted typhus. The disease stunted her growth and affected the sight in her left eye, so—as was the way in those days—it was assumed she would never marry. Her mother demanded she therefore train to be a house servant. Caroline was saved from a life of drudgery when she joined her brother William in Bath, to sing in accompaniment to his organ recitals. After William discovered Uranus, however, their fortunes changed. William became the King’s astronomer, and Caroline became William’s assistant. The post attracted a salary of £50, which made Caroline the first paid female astronomer. She was much more than an assistant, however: she discovered eight comets and created an influential catalogue of stars.
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Herschel’s identification of Uranus as a planet required a telescope. Nevertheless, when conditions are just right—the planet is at its closest to the Sun, the Moon is new, and you know where to look—then a person with good eyesight can make out Uranus with the naked eye. The only planet that can never be seen without some visual aid is Neptune. Even when it is at its closest approach to the Sun, Neptune is faint: you need at least a decent pair of binoculars to see it.
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Galileo was the first person to see Neptune, but he did not recognise it as a planet; he thought it was a star. James Challis (1803–1882) is famous for not discovering Neptune: he observed the planet twice in August 1846, but did not appreciate what he had seen. Urbain Jean Joseph Le Verrier (1811–1877) and John Couch Adams (1819–1892) generally share the credit for ‘discovering’ Neptune, since they independently predicted the existence and position of the planet. They did so to explain discrepancies in the orbit of Uranus: rather than discard Newton’s law of gravitation, they postulated a gravitational influence on Uranus from an as-yet-unobserved planet. (Le Verrier gets the lion’s share of the credit, since it was he who convinced the astronomical community of the approach.) Johann Gottfried Galle (1812–1910) is often the forgotten man in this story: in September 1846 he became the first to observe Neptune and know what he was looking at. In a real sense, Galle discovered Neptune. At least he has the last laugh when it comes to being commemorated by craters: Le Verrier, Adams, and Challis all lend their names to unremarkable features on the Moon; Galle has the ‘Happy face’ crater on Mars.
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Compared to the other planets, the two ice giants Uranus and Neptune remain rather unfamiliar worlds. Uranus is a pale blue planet without features. Neptune is a darker blue, and has clouds and dark spots that come and go. The outer atmospheres of both planets are rich in methane, and the methane ice condenses into particles that form a ‘haze’. Uranus has a sluggish atmosphere, and the thick haze layer persists: this gives rise to that featureless, light-blue colour. Neptune is more active. The haze layer experiences churn, and those methane particles are driven deep down into the atmosphere. The thinner haze layer produces a deeper blue colour, and the activity gives rise to dark spots.
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Following this brief tour of the planets of the solar system, from Mercury through to Neptune, you might ask yourself: ‘what is the closest planet to Earth?’. The obvious answer to this question is: ‘Venus’. This answer makes sense. Earth, by definition, orbits at 1 AU from the Sun while Venus orbits at 0.72 AU from the Sun. Arithmetic then tells us that Venus is 0.28 AU from Earth. This is true—but only for a short time. No planet ever gets closer to Earth than Venus, certainly, but for most of the time in its orbit Venus is much further away than 0.28 AU. At times it can be at 1.72 AU. A better approach to the question of planetary distances, an approach investigated by Tom Stockman, Gabriel Monroe, and Samuel Cordner, is to define the average distance between two planets as the average distance between all points along their orbits. These three researchers built a computer model that examined the distance between all eight planets over a period of 10,000 years (assuming circular planetary orbits, all of which are in the plane of the ecliptic; a reasonable assumption for these purposes). Under this definition, it turns out Mercury is the closest planet to Earth: for half the time, Mercury is closer to Earth than any other planet; the rest of the time the closest planet can be either Venus or Mars. On average, Mercury is 1.04 AU away from Earth while Venus is at an average distance of 1.14 AU. It turns out, indeed, that Mercury is not only the closest planet to Earth—it is the closest planet to every other planet in the solar system!

1.3.7 Minor Bodies

In addition to the Sun and planets, the solar system contains many smaller bodies—dwarf planets, asteroids, and comets. They are small in size but vast in number.

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For 76 years, astronomers considered Pluto to be the solar system’s ninth planet. And then, in 2006, for reasons we shall soon discuss, astronomers demoted it to the rank of ‘dwarf planet’. Whichever way we classify it, Pluto remains an intriguing world. One interesting feature is Sputnik Planitia, a basin formed probably 4 billion years ago when Pluto was hit by a Kuiper Belt object. The impact carved out a huge crater, roughly 1000 km by 800 km. Over time, nitrogen ices have accumulated there so that now the basin contains an ice sheet that is at least 4 km thick. The New Horizons space probe investigated Sputnik Planitia and found that its bright, ice-covered surface bore no evidence of cratering, suggesting the surface itself is younger than ten million years old. Although the region is crater free it does contain rocky hills and hundreds of dark ‘sublimation pits’—surface depressions where sunlight turns the ice directly into gas. Pluto continues to fascinate.
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Pluto has five satellites. The first, Charon, was discovered in 1978. Charon is about half the diameter of Pluto and one-eighth the mass, and so the two objects orbit around a common centre of gravity that is about 960 km above the surface of Pluto. See Fig. 1.10. The other four satellites, discovered by the Hubble Space Telescope, are much smaller: Hydra (55 km along its longest axis), Nix (42 km), Kerberos (12 km), and Styx (7 km). The moons all get their names from mythology associated with the underworld, Pluto being the Roman god of that place. Charon was the boatman who ferried souls across the river Styx; Nix, the goddess of darkness, was the mother of Charon; Hydra was a nine-headed serpent that guarded the underworld; and Kerberos was a three-headed dog that prevented the dead from escaping.

A photograph of two dwarf planets Pluto and Charon in front of the Earth. The Charon is about half the diameter of Pluto. — **Fig. 1.10**

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Although Pluto was not officially demoted from the planetary ranks until 2006, doubts about its status had been growing for several years. In 1992, two astronomers using the Mauna Kea Observatory, David Clifford Jewitt (1958–) and Jane Luu (1963–), observed an object orbiting at about 40 AU from the Sun. Neptune orbits at about 30 AU, so this was clearly a trans-Neptunian object (TNO). For certain parts of its orbit, this TNO is even more distant than Pluto and Charon, the first two known TNOs, from the Sun. Jewitt and Luu intended to call the object Smiley, after Le Carre’s spymaster, but that name was already in use for an asteroid. It eventually was given the name Albion, after the primeval man in William Blake’s work. For a short time after the discovery, various news outlets called the object the ‘tenth planet’. But it turned out that Albion is only about 108–167 km in diameter, so in essence it is just a big rock. Astronomers have since discovered about 2500 more TNOs.
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After the eight planets, the TNO Eris, discovered in 2005, is the most massive object orbiting the Sun. Indeed, it was the discovery of Eris that led to a serious discussion within the astronomical community about what constitutes a planet. The community decided neither Pluto nor Eris made the cut. For several years after its discovery, astronomers believed Eris to be larger than Pluto as well as more massive. But in 2010 Eris occulted a faint star, and observations of this event allowed astronomers to determine the diameter of Eris: 2320–2350 km. This makes it slightly smaller than Pluto. It follows, since Eris is smaller and more massive than Pluto, that it is denser; Eris must consist mainly of rocky material. Eris has an eccentric orbit, and has at least one moon, called Dysnomia.
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NASA’s New Horizons launched in 2006, and in 2015 became the first spacecraft to flyby Pluto. After sending back some remarkable images of Pluto and its moons, New Horizons headed further into the depths of the Kuiper Belt—an asteroid belt extending 30–50 AU from the Sun. Four years later it made a flyby of 2014 MU69, the most distant object humanity has explored in this way. Mission scientists nicknamed the object Ultima Thule, from a Latin phrase meaning the farthest point of a journey. Its official name, however, was given by the native Powhatan people of Maryland: Arrokoth, which means ‘sky’ in the Powhatan/Algonquian language. Arrokoth has two lobes—rather like the upper and lower parts of a two-ball snowman—connected by a thin neck. The two lobes probably formed separately in the early development of the solar system, before gently bumping into each other and sticking together. They have stayed like that, untouched, for four billion years.
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Not all dwarf planets are in the outer reaches of the solar system, in that Kuiper Belt beyond the orbit of Neptune. Consider the history behind the Titius–Bode law, a relation named after the German astronomers Johann Daniel Titius (1729–1796) and Johann Elect Bode (1747–1826). Titius generated a simple rule that seemed to account for the relative distances from the Sun of all the then-known planets, and he used the rule to predict the existence of a planet at 2.8 AU from the Sun, between Mars and Jupiter. There was no known planet in that orbit. But in 1801 astronomers discovered the dwarf planet Ceres, orbiting almost exactly where the rule said it should. See Fig. 1.11. It was quite a success for Titius. (Later, Bode tweaked the rule and the revised Titius–Bode law successfully predicted the distance of the new planet Uranus from the Sun. After that success, however, the rule breaks down: Neptune is not where it should be, according the Titius–Bode law.)

A photograph of the surface of the dwarf planet Ceres. The surface has many circular pits. — **Fig. 1.11**

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Ceres was initially thought to be a planet: it wandered across the sky, relative the fixed stars, in the same way that planets move. But even when observed through a telescope the object did not show a planetary disc; neither did similar bodies, which were soon discovered at similar distances from the Sun as Ceres. Since these objects had a star-like appearance, but were clearly not stars, William Herschel proposed the term ‘asteroid’ for them. The International Astronomical Union (IAU) has never formally defined what an asteroid is. In the absence of a definition, we can think of an asteroid as being an irregular-shaped rock, orbiting the Sun, that is not a planet or dwarf planet. (The IAU has at least defined the terms ‘planet’ and ‘dwarf planet’!)
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When the first asteroids were being discovered, they received names from classical mythology. Now, once a new asteroid’s orbit is determined with sufficient precision, its discoverer can suggest a name to an appropriate committee of the IAU. For example, the object 1998 GJ10, discovered by an Australian amateur astronomer, now has the designation 8749 Beatles. It is a main-belt asteroid—one of the many bits of space rubble that orbits the Sun between Mars and Jupiter—with a period of 3.39 years. Unlike the rock band, about which there is probably nothing left to learn, we know relatively little about this rock. Its size is uncertain (it is probably bigger then 4 km, smaller than 9 km, but we do not really know); and you need a decent telescope to even see it.
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Asteroids are often classified in terms of their composition, with C (chondrite), S (stony), and M (metallic) asteroids accounting for more than 90% of all known objects. Of these, those of type C—the carbonaceous or chondrite asteroids—are most common: they account for about 75% of all known objects. Since chondrites consist primarily of carbon, they are dark: they typically reflect only about 3% of the sunlight that falls on them. They orbit, in general, at the far edge of the main asteroid belt.
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We have already discussed the existence of Lagrangian points, gravitationally stable places in an orbit. If a small body (such as an asteroid) orbits the Sun in the same orbit as a larger body (such as a planet) then, if it is about 60° ahead of the main body, it is at Lagrangian point L4. If it is about 60° behind the main body it is at Lagrangian point L5. Objects that occupy these points are called trojans. Most trojan asteroids occupy the orbit of Jupiter—astronomers know of about 12,000, but there could be millions. (Venus has 1 trojan, Earth has 2, Mars 14, Uranus 2 and Neptune 26.) The asteroids at L4 are in the so-called Greek camp, those at L5 are in the Trojan camp.
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An asteroid that crashes into Earth has the potential to cause untold damage. Small wonder, then, that space agencies keep a close eye on asteroids that might come close. In 2003, astronomers confirmed that the stony asteroid 163693 Atira (actually a binary asteroid) orbited entirely with Earth’s orbit: at aphelion it is closer to the Sun than Earth is at perihelion. The asteroid gave its name to the class of such solar system objects. As of January 2024, 28 Atiras are known. (Note that Atiras is not, yet, an official designation for these asteroids; some astronomers call them simply Inner Earth Objects.) At first glance it might seem that, since Atiras do not cross Earth’s orbit, they pose no impact threat. Unfortunately, if an Atira asteroid makes a close approach to Venus then its orbit could be perturbed in such a way that it crosses Earth’s orbit—and thus pose a risk.
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At least one asteroid orbits even closer to the Sun than the Atira class. The asteroid (594913) 'Aló'chaxnim was discovered in 2020; follow-up observations in 2022 showed that it has a diameter of about 2 km; a reddish colour (similar to S-type asteroids in the inner main belt between Mars and Jupiter); and at aphelion is about 0.65 AU from the Sun. Its orbit thus lies entirely within the orbit of Venus. This is the first such body discovered, which is not surprising given the difficulty of observing them: they are small, and because they are close to the Sun in the sky they can only be found in the time between sunset and full dark (or full dark and sunrise). Computer models suggest that 'Aló'chaxnim probably migrated from the main belt of asteroids in the past million years—a cosmic eyeblink—and that gravitational interactions will likely cause it to leave its current orbit within 30 million years. (The name, incidentally, comes from the Luiseño language. The asteroid was discovered with an instrument at the Palomar Observatory, which is situated on the ancestral lands of the Pauma tribe. The team at Palomar asked the indigenous people to name the asteroid, and they chose the term ‘Venus girl’ in their native tongue.)
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Spacecraft technology has advanced to the point where space agencies can land on an asteroid and return with samples. The Japanese spaceship Hyabusa2 landed on the asteroid 162173 Ryugu in June 2018. As the craft approached, we learned that Ryugu is a diamond-shaped object. It looks rather like a sugar cube, spinning on one of its corners. Its surface is young (about 8.9 ± 2.5 million years) and lacking in dust. Hyabusa2 collected 5.4 g of asteroidal material, and returned to Earth in December 2020. In June 2022, after analysing the material Hyabusa2 brought back, scientists announced they had identified 20 different amino acids—basic building blocks of life—from the asteroid.
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We have seen that the Atira asteroids, if perturbed, might cross Earth’s orbit and become a civilisation-destroying threat. In general, a potentially hazardous object (PHO) is an asteroid or comet that has an absolute magnitude of +22 or brighter (which is an indicator that its diameter is likely to be more than 140 m) and with an orbit that can bring it within 20 lunar distances of Earth. Most PHOs are Earth-crossing asteroids belonging to the Apollo class. Astronomers know of more than 10,000 Apollo asteroids; between 1500–2500 of them are classed as potentially hazardous. More than 100 are greater than 1 km in size. The largest Apollo PHO currently known to exist is (53319) 1999. This asteroid is 7 km across. It carries enough punch to be in the ‘civilisation-ending’ category.
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Although a big meteorite can lead to a mass extinction of life, and a mid-size meteorite can leave behind a large impact crater, humans have not been around long enough to experience these dramatic events. But smaller meteorites hit Earth daily. (Most rocky meteoroids, when they encounter our planet, burn up in the atmosphere: they become meteors or ‘shooting stars’. A meteorite is a solid piece of debris from a meteor that survives its passage through Earth’s atmosphere and reaches the surface.) Despite the frequency with which small meteorites hit Earth, there is only one confirmed case of a person being hit by a rock from space. In 1954, in Sylacauga, Alabama, Ann Hodges was struck by a black rock the size of a cricket ball. She was resting on a couch when the rock smashed through the ceiling, bounced off a radio set, and hit her in the thigh. Apart from a bruise, Ann was unharmed.
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Spotting the fleeting light from a ‘shooting star’ is fun. Sometimes, meteor showers provide a more impressive spectacle. During the Great Leonids Meteor Storm of 1833, a time before street lighting came to North America, and so when skies were dark, tens of thousands of shooting stars, perhaps more, streaked across the heavens every hour. (For comparison, the annual summer Perseids shower is unlikely to reward you with anything much more than one meteor per minute.) Some observers of the Leonids Storm thought they were witnessing the end of the world. Volume 1 of A Handbook of Descriptive and Practical Astronomy (Chambers, 1889) includes this eyewitness account from an observer in South Carolina: ‘Upwards of 100 lay prostrate on the ground…with their hands raised, imploring God to save the world and them. The scene was truly awful; for never did rain fall much thicker than the meteors fell towards the Earth; east, west, north, and south, it was the same.’ With light pollution now affecting most countries on Earth, few of us are likely to experience a meteor shower like the Leonids of 1833.
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As well as asteroids, the solar system contains another type of minor body: comets. Figure 1.12 shows a spectacular example of a comet that appeared in our skies in 2020. Comets differ from asteroids in that they typically have highly eccentric orbits. They also differ in their composition. During the 1950s the Harvard astronomer Fred Lawrence Whipple (1906–2004) developed a model of comets. He suggested that comets were composed of ‘icy conglomerates’. This was later referred to as the ‘dirty snowball’ hypothesis, a name that stuck. Whipple’s basic idea appears to be correct: comets do indeed contain ices. The amount of ice in a typical comet is, however, a matter for research; the latest thinking suggests comets contain relatively small amounts. Rather than ‘dirty snowballs’ a better description of comets might be ‘icy dirtballs’.

A photograph of the spectacular example of a comet that appeared in the sky. The light rays are spread in the sky in one direction, from a single source. — **Fig. 1.12**

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Comet Halley, the first comet to be recognised as a periodic visitor to our night skies, is unique: it is the only short-period comet regularly visible without the need for visual aids, and it is the only naked-eye comet people might see twice in their lifetime. It is visible from Earth every 75 or 76 years. I saw it in 1986; if I live to be a good age then I might see its next appearance in 2061. But before Edmond Halley determined its orbit, and understood that the same object was returning every 75 years or so, people had seen the comet without recognising it as a regular visitor. In 12 BCE, Comet Halley was spotted by Chinese Han astronomers and by Roman observers (for whom it portended the death of Agrippa); in 164 BCE, according to Babylonian tablets in the British Museum, astronomers in Mesopotamia saw the comet; and in 240 BCE, a Chinese chronicler noted its appearance in the sky. This latter description is the earliest record that without doubt refers to Comet Halley. In 467 BCE, Ancient Greek astronomers observed a comet that might have been Halley—but we cannot be certain.
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Comets have long been a target for astronomers. Perhaps the greatest discoverer of comets was the French astronomer Jean–Louis Pons (1761–1831). Pons, coming from a poor family, was largely self-taught and contemporary astronomers seem to have looked down on him. Nevertheless, he rose to become Director of the Florence Observatory. He had a remarkable memory for star fields, and used this skill to good effect in recognising an object whose position had changed relative to the fixed stars—a sign that it might be a comet. Pons discovered or co-discovered 37 comets, more than any other individual. Pons, however, is the named discoverer of only 26 comets; the American astronomer Carolyn Jean Spellmann Shoemaker (1929–2021) broke that record in 1991. With modern techniques, many more comets will be discovered. Astronomers believe the LSST Camera at the Vera Rubin Observatory, for example, will discover about 10,000 comets over a 10-year period. Jean–Louis Pons worked with rather more primitive equipment!
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In 1950, the Dutch astronomer Jan Hendrik Oort (1900–1992) theorised that a vast spherical cloud of icy planetesimals surrounds our Sun. This ‘Oort Cloud’ might contain as many as a trillion planetesimals, orbiting the Sun between about 2000 to 200,000 AU. (This is about 0.03 to 3.2 light-years; at last, the light-year becomes a useful unit of distance!) Astronomers believe that long-period comets, those taking longer than about 200 years to orbit the Sun, originate in the Oort Cloud: gravitational perturbations occasionally ‘nudge’ an object from the Oort Cloud reservoir and it falls inwards towards the Sun. But how many Oort Cloud objects have been observed? As of January 2024: none. The inner edge of the Oort Cloud—if the Cloud exists—is so distant that Voyager 1, a fast-moving space probe that is now far past the orbit of Pluto, will not reach the Oort Cloud for another 300 years. The Oort Cloud objects are too small, too distant, too dim for us to see.

1.3.8 Exploring the Solar System

For most of history, astronomers had no option but to observe the bodies in the solar system and try to deduce something about their properties. Now we can put our technology into space. This ability is transforming our knowledge of the cosmos.

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A satellite in a geostationary orbit, or geosynchronous equatorial orbit (GEO), is at a constant altitude of 35,786 km above Earth’s equator and it orbits in the same direction as Earth’s rotation. This means the satellite appears to be motionless to an observer on Earth’s surface. An antenna pointed at the satellite does not have to rotate in order to track, because the satellite appears in a fixed position on the celestial sphere. This fact makes GEO useful for communications, a fact pointed out by the science fiction author Arthur Charles Clarke (1917–2008) in a 1945 essay published in Wireless World. Today, almost a third of working satellites can be found operating in a GEO.
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Moving beyond the Earth environment, we can send craft to visit other bodies in the solar system. The first successful soft landings on an extraterrestrial body were Luna 9 and Surveyor 1 in 1966. These were lunar landings. The first successful soft landing on another planet took place in 1970, when the Soviet Venera 7 craft landed on the surface of Venus and sent back transmissions for 23 min. The Venera 7 observations revealed Venus to be hellishly hot, and quite inimical to human life. (The first craft to crash-land on another planet was probably Venera 3 in 1966. We cannot know for sure because mission control lost contact with the craft just before it entered the Venusian atmosphere. The follow-up missions Venera 4–6 were all crushed in the thick atmosphere of Venus. It was the lessons learned in developing these failed missions that led to the success of Venera 7.)
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As of January 2024, various nations have sent a total of 21 landers to Mars. When one considers that only 10 missions have succeeded, the difficulty of the task becomes clear. (I class the Mars 3 lander in 1971 as a failure, rather than a partial success: the craft was the first to make a soft landing on Mars, but it died only 110 s after reaching the surface.) The Soviet Union was the first country to attempt to land on Mars, but none of its seven missions succeeded; the closest it came was with the aforementioned Mars 3. Following the breakup of the Soviet Union, Russia has twice tried to land on Mars, once by itself and once in conjunction with the European Space Agency. Both attempts failed. In 2003, the UK tried to deploy the Beagle 2 lander; that mission failed. When it comes to exploring Mars, the outstanding nation is the USA: it has sent 10 lander missions to Mars and only one has failed. The first two successes were Viking 1 and Viking 2 in 1976. Curiosity rover and Perseverance rover are both still busy exploring the Martian surface. In 2021, China became the second nation to successfully operate a lander on Mars: Tianwen-1 and its rover Zhurong explored part of Utopia Planitia, close to the landing site of Viking 2.
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Space missions have numerous practical benefits. They can also help address questions in pure science that at first glance have nothing to do with astronomy. Consider, for example, the problem of the neutron lifetime. A neutron, when inside an atomic nucleus, is typically a stable particle. When a neutron is isolated, however, it undergoes beta decay: it decays into a proton, an electron, and an anti-neutrino, with a mean lifetime of just 15 min or so. For this reason, the space environment contains few free neutrons: the neutrons decay. The precise mean lifetime of the neutron is an interesting puzzle. Physicists have two ways of measuring the free neutron lifetime. They can put neutrons in a ‘bottle’, and measure the time it takes for them to decay. Or they can fire neutrons in a ‘beam’ and count how many protons and electrons come from neutron decay. The ‘bottle’ method gives a mean lifetime of 14 min 39 s (with a margin of error of 0.5 s); the ‘beam’ method gives a mean lifetime of 14 min 48 s (with a margin of error of 2 s). Unfortunately, those error margins do not overlap. The difference is small but clearly apparent. Well, scientists now have a third method: they can measure the free neutron lifetime in space. When cosmic rays strike a planetary surface or atmosphere, the collision can create neutrons—some of which travel into space and stay there until they decay. A neutron spectrometer on the spacecraft Messenger detected free neutrons as it flew past Venus. By modelling how many neutrons they expected to detect, for different values of the neutron mean lifetime, scientists were able to calculate a best-fit lifetime of about 13 min. Unfortunately, the margin of error was extremely large and so the experiment does not resolve the puzzle of the neutron lifetime. The experiment does, however, demonstrate that it is at least possible in principle to measure the lifetime of a free neutron in space. Future experiments may shed light on the puzzle.
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Humankind’s exploration of the space environment leads to technological achievements on many fronts. Consider, for example, the Parker Solar Probe. On 20 November 2021, Parker—a NASA craft launched in 2018 to study the Sun’s outer corona—was at perihelion. The craft was just 8,541,744 km from the Sun’s surface, the closest any artificial object had been. Parker was also moving faster than any artificial object: it moved at 163 kms⁻¹ (about 364,660 mph) relative to the Sun. Parker’s elliptical orbit takes it close to the Sun and then back out towards Venus; the Venus flyby causes the orbit to tighten so that at the next perihelion the probe can get even closer to the surface of the Sun. And the closer Parker gets to the Sun, the faster it moves. At the 2025 perihelion, Parker will be moving at 0.064% of lightspeed!
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The Voyager 1 spacecraft is more than 23.3 billion km from Earth. A radio signal, travelling at the speed of light, takes 20 h and 33 min to cover the distance. And yet, using radio, NASA can still receive data from the spacecraft and send it instructions. Voyager possesses only a 23 W radio, so how is it possible to send a signal all this distance back to Earth? (For comparison, a commercial radio station on Earth might transmit using tens of thousands of watts yet its signal can still fade beyond a few miles.) Well, Voyager and NASA both use big directional antennae—the Voyager antenna is 3.7 m in diameter, the Earthbound NASA antenna is 34 m in diameter—and they are pointing right at each other. Communication takes place using a radio frequency in the 8 GHz range at which there is little artificial interference. This setup allows NASA to pick up Voyager’s faint signals; and when NASA transmits back it uses tens of thousands of watts to ensure Voyager can pick up the signal. More than 45 years after launch, the spacecraft can still receive commands from NASA.
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When engineers send a probe to another planet, they must plan everything in advance. This is not a trivial task. The engineers need to know planetary locations at time of launch and at time of arrival, and they must understand how the probe’s orbit will be affected by masses in the solar system. Consider, for example, the difficulties in putting a probe in orbit around Mercury. If the mission is simply to reach Mercury then engineers can plan for a travel-time of 5 months. But if the mission is to orbit Mercury then engineers have a problem: the Sun’s gravitational pull will accelerate the probe to high speeds. For the craft to slow sufficiently it must make multiple passes of the planet, and a 5-month journey turns into a 78-month journey. It turns out the energy needed to get into orbit around Mercury is greater than the energy needed to reach Pluto. So travel times to planets are subject to various factors. Here are some typical times endured by past missions: Mars (7 months; Opportunity); Venus (15 months; Magellan); Jupiter (6 years; Galileo); Mercury (6.5 years; Messenger); Saturn (7 years; Cassini); Uranus (8.5 years; Voyager); Pluto (9.5 years; New Horizons); and Neptune (12 years; Voyager 2).
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Five NASA probes—Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons—are now beyond the orbit of Pluto. All five are heading into interstellar space. The most distant artificial object is Voyager 1, which is 157 AU from the Sun and getting further away all the time. NASA has lost contact with Pioneers 10 and 11, but mission teams are still in touch with the two Voyager craft some 45 years after launch. Depending upon one’s definition of the ‘edge’ of the solar system, Voyagers 1 and 2 are already in interstellar space: humankind has become an interstellar species!

1.3.9 Mysteries of the Solar System

Thanks to advances in telescopes, instrumentation, and space technology, our knowledge of the solar system is better than ever before. But might some mysteries remain?

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Le Verrier, one of the discoverers of Neptune, noticed an anomaly in the precession of Mercury’s orbit around the Sun. In 1859, he suggested the anomaly could be explained through the gravitational interaction on Mercury of a planet orbiting closer to the Sun than Mercury itself. The suggestion was reasonable and, since such a planet would be difficult to identify because of the Sun’s glare, the lack of observational evidence for the planet was easy to explain. The same year, the French amateur astronomer Edmond Modeste Lescarbault (1814–1894) claimed to have spotted a small object transiting the Sun, an object that could have been Le Verrier’s conjectured planet. The planet was called Vulcan. Nobody ever saw it again. Presumably, Lescarbault was fooled by a sunspot. Le Verrier’s anomaly was real, but the cause turned out to be an effect of general relativity rather than an intra-mercurial planet. Vulcan itself does not exist. A population of asteroids, however, hypothesised to orbit within the orbit of Mercury, are called ‘vulcanoids’ after the planet. They might exist—but no one has ever observed a vulcanoid.
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Another hypothetical solar system planet is Planet X, proposed by the American businessman-turned-astronomer Percival Lowell (1855–1916), the man who popularised the idea of canals on Mars. Planet X was supposed to be a large planet beyond the orbit of Neptune that would account for seeming perturbations in the orbits of Uranus and Neptune. (Given the demotion of Pluto from the ranks of planets, perhaps Planet X should be called Planet IX.) Like Vulcan, Planet X does not exist, at least not as Lowell originally proposed it. Recent work suggests the Planet X hypothesis is not needed to explain the orbits of the two ice giants. Furthermore, the NASA space telescope WISE (Wide-field Infrared Survey Explorer) has observed almost the entire sky at infrared wavelengths. One of its science goals was to look for infrared radiation from objects warmer than 70–100 K in the outer solar system. WISE observations were able to rule out the existence of Planet X. (WISE also ruled out the existence of a brown dwarf called Nemesis—an object hypothesised to explain apparent periodicities in mass extinctions on Earth. The idea was that Nemesis, as it passed through the Oort Cloud, would send a shower of comets down into the inner solar system and increase the chance of a catastrophic collision. WISE observations showed that no such brown dwarf exists within 10,000 AU of the Sun.)
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And how will it all end, this story of the solar system? Well, that is less of a mystery. We shall look at this in more detail in later chapters, but we can finish our tour of the solar system with a brief look at the likely demise of the Sun. Astronomers can peer into the future of our star and they tell us that, as the Sun ages, it will continue to shine for another 5 billion years by fusing hydrogen into helium. Eventually, though, about 5 billion years from now, the Sun will contain insufficient hydrogen for the process to continue. At this point the Sun’s outer layers will expand and the Sun will become a red giant. The Sun’s core will become hot enough to fuse helium, but this helium-burning will continue for only a relatively short time. The Sun’s outer layers will be released into space, forming a beautiful planetary nebula, while a white dwarf will be left behind. The white dwarf will be an extremely dense object, possessing about half the mass of the present Sun packed into a sphere the same size as Earth. Over aeons, the white dwarf will cool. Any intelligent extraterrestrial visiting our small corner of the cosmos at that point would see a small, dense object orbited by the handful of planets that had survived the Sun’s red-giant phase. Life would be quite absent.

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Stephen Webb

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Webb, S. (2024). A Journey Around Our Cosmic Neighbourhood: The Solar System and Its Secrets. In: The Quizzer’s Guide to the Cosmos. Springer Praxis Books(). Springer, Cham. https://doi.org/10.1007/978-3-031-52437-0_1

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DOI: https://doi.org/10.1007/978-3-031-52437-0_1
Published: 19 April 2024
Publisher Name: Springer, Cham
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