Artist rendering of the view on a Hycean world.
Media outlets widely reported last fall that a possible sign of life had been found on another planet. The evidence came from the James Webb Space Telescope, which identified the presence of a molecule called dimethyl sulfide on an exoplanet called K2-18b.
On Earth, DMS is produced by phytoplankton in the oceans.
Evidence Of Life?
Did NASA discover evidence of life elsewhere in the cosmos? Not likely, argues a new study published this week in Astrophysical Journal Letters. It claims that there is no DMS around K2-18b and that the detection published last fall was incorrect.
This negative take on the initial discovery hasn't prevented news outlets from running the same stories from the fall about NASA finding proof of life. False headlines abound from journalists who haven't even reviewed the previous study, let alone the latest one.
Here's what's actually going on.
Intriguing Exoplanet
K2-18b is an intriguing exoplanet—a planet that orbits a star other than our own. This one is 120 light-years distant and orbits a cool red dwarf star. It's in the constellation Leo in our night sky.
An exoplanet 8.6 times as massive as Earth, it's classed as a "super Earth" and a "sub-Neptune"—meaning it's bigger than Earth but smaller than Neptune. Such planets appear to be the most common type in the Milky Way, but there are no planets like them in our solar system.
K2-18b was discovered by NASA’s Kepler Space Telescope in 2015. In 2019, the Hubble Space Telescope found evidence of water vapor in its atmosphere.
‘Hycean World’
K2-18b is considered the archetype “Hycean” exoplanet. Its abundance of methane and carbon dioxide—and shortage of ammonia—suggests an ocean may be underneath a hydrogen-rich atmosphere.
Not only is DMS considered a biosignature—evidence for past or present life—but K2-18b also has some potentially habitable features, including an Earth-like ocean, atmosphere and roughly the same amount of sunshine.
“This planet gets almost the same amount of solar radiation as Earth. And if the atmosphere is removed as a factor, K2-18b has a temperature close to Earth’s, which is also an ideal situation in which to find life,” said Shang-Min Tsai, the paper's author and a project scientist at the University of California, Riverside.
Filtered Starlight
What planetary scientists know about exoplanets is essentially starlight filtered through their atmosphere. The results reveal molecular signatures. “The DMS signal from the Webb telescope was not very strong and only showed up in certain ways when analyzing the data,” said Tsai. “We wanted to know if we could be sure of what seemed like a hint about DMS.”
Using computer models of the physics and chemistry of DMS and the planet's hydrogen-based atmosphere, the researchers concluded that DMS on K2-18b is unlikely. “The signal strongly overlaps with methane, and we think that picking out DMS from methane is beyond this instrument’s capability,” said Tsai. Although more DMS was detected than is present on Earth, its detection was rated as unreliable unless it accumulated to about 20 times more than is present on Earth.
Wishing you clear skies and wide eyes.
