Distant planet may be emitting gas that's 'only produced by life'

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• Space telescope aims to confirm major finding on distant planet K2-18b
• World is more than eight times the mass of Earth and 120 light-years away

Astronomers could finally show that alien life does exist on a distant planet following observations today.

Planet K2-18b – which is more than twice as big as Earth and 120 light-years away – sits within the habitable zone of its star in the Leo constellation.

Scientists said last year they thought they'd detected dimethyl sulphide gas in its atmosphere – a compound that is 'only produced by life'.

Now, to confirm the finding, the James Webb Space Telescope will undertake hours of observations of the planet on Friday.

However, space fans will have to wait several months for the results to be authenticated and published.

Planet K2-18b: Key facts

Discovered: 2015

Star: K2-18

Orbital duration: 33 days

Constellation: Leo

Mass: 8.6 times that of Earth

Radius: 2.6 times that of Earth

Investigations of planet K2-18b are being led by Dr Nikku Madhusudhan, an astrophysicist at the University of Cambridge.

He's called it a 'hycean' world – a relatively new term he coined for a rocky planet with a hydrogen-rich atmosphere and oceans of water.

'If we do detect DMS [on K2-18b] it does put it basically at the top for potential signs of habitability,' he told the Times.

K2-18b – more than eight times the mass of Earth and over twice as big – was discovered in 2015.

But it was only in 2019 that the presence of water vapour in K2-18b's atmosphere was reported.

Then, last year, the James Webb telescope detected carbon dioxide and methane in its atmosphere, as well as a shortage of ammonia.

Dr Madhusudhan called this a 'very profound moment' because it supports the theory that there's a water ocean underneath a hydrogen-rich atmosphere.

But it was the presence of something else that got astronomers even more excited.

Initial Webb observations provided a possible detection of a molecule called dimethyl sulfide (DMS), which on Earth is 'only produced by life'.

The bulk of the DMS in Earth’s atmosphere is emitted from phytoplankton in marine environments – suggesting a similar form of life on the distant planet.

In fact, scientists have been unable to think of any natural geological or chemical process that could create DMS without living organisms.

Dr Madhusudhan said the finding was a shock but because they were initial observations he could only say with 50 per cent confidence there is DMS on K2-18b.

'It was a real shock, I had sleepless nights for a week,' he added.

'That week, I didn’t even muster the courage to break it to my own team.'

Today, eight hours of observations by the James Webb telescope should provide a definitive answer – although the expert will have to spend months poring over the data to get it.

NASA's $10 billion (£7.4 billion) observatory is able to analyse the chemical make-up of a distant planet by capturing the light from its host star after it has passed through the planet's atmosphere on its way to Earth.

Gases in the atmosphere absorb some of the starlight but each leave tell-tale signatures in the spectrum of light that astronomers can then unpick.

Although Hycean worlds are predicted to be covered in water, researchers say it is also possible that K2-18 b's hypothesised ocean is too hot to be habitable or be liquid.

In fact, a studypublished earlier this year suggested the ocean may be lava.

K2-18b is known as a 'super Earth' because it is bigger than our planet but smaller than Neptune.

K2-18b's large size – with a radius 2.6 times the radius of Earth – means that the planet's interior likely contains a large mantle of high-pressure ice, like Neptune, but with a thinner hydrogen-rich atmosphere and an ocean surface.

'Although this kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy,' said researcher Subhajit Sarkar of Cardiff University.

'We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, and this allowed us to work out the molecules that exist in its atmosphere.'