A distant ocean-covered world, teeming with life.

That’s the most probable reason for a recent finding by researchers, indicating they have identified the most encouraging indications so far of life beyond our own solar system.

Using data from the James Webb Space Telescope (JWST), the astronomers, led by the University of Cambridge, have identified the chemical fingerprints dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) – molecules that indicate life.

Here on Earth, these molecules are only produced by living organisms – primarily microbial life such as marine phytoplankton.

The molecules have been detected in the atmosphere of exoplanet K2-18b, which is located around 124 light-years from Earth in the constellation of Leo.

It is orbiting a red dwarf star in what is known as the ‘habitable zone’ – considered the most promising location to find life-supporting planets.

K2-18b is 2.6 times larger and 8.6 times as massive than Earth, and experts believe it is likely covered in an ocean – making it what they call a ‘Hycean world’.

The planet’s temperature is similar to Earth’s but it is orbiting so close to its star that a year there lasts just 33 days.

Earlier observations identified methane and carbon dioxide in its atmosphere – the first time that carbon-based molecules had been discovered on an exoplanet in the habitable zone.

Now, analysis of new data has unearthed the compounds which – as far as scientists are aware – are only produced by living organisms.

The concentrations of DMS and DMDS in K2-18b’s atmosphere are very different than on Earth, where they are generally below one part per billion by volume.

On K2-18b, they are estimated to be thousands of times stronger – over 10 parts per million.

Professor Nikku Madhusudhan, from Cambridge’s Institute of Astronomy, led the research.

‘Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds,’ he said.

‘And now we’ve observed it, in line with what was predicted.

‘Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have.’

He said that while the results are exciting, it’s vital to obtain more data before claiming that life has been found on another world.

And while he is cautiously optimistic, there could be previously unknown chemical processes at work on K2-18b that may account for the findings.

Further observations from the JWST are needed to help confirm their results are not due to chance.

His team are also hoping to conduct further experiments to determine whether DMS and DMDS can be produced non-biologically at the level currently detected.

‘Decades from now, we may look back at this point in time and recognise it was when the living universe came within reach,’ Professor Madhusudhan said.

‘This could be the tipping point, where suddenly the fundamental question of whether we’re alone in the universe is one we’re capable of answering.’

To determine the chemical composition of the atmospheres of faraway planets, astronomers analyse the light from its parent star as the planet transits, or passes in front of the star.

As K2-18b transits, JWST can detect a drop in stellar brightness, and a tiny fraction of starlight passes through the planet’s atmosphere before reaching Earth.

The absorption of some of the starlight in the planet’s atmosphere leaves imprints in the stellar spectrum that astronomers can piece together to determine the constituent gases of the exoplanet’s atmosphere.

Last year, JWST detected weak hints of ‘something else happening’ on K2-18b alongside the discovery of methane and carbon dioxide.

‘We didn’t know for sure whether the signal we saw last time was due to DMS, but just the hint of it was exciting enough for us to have another look with JWST using a different instrument,’ Professor Madhusudhan explained.

The earlier, tentative, inference of DMS was made using JWST’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments, which together cover the near-infrared (0.8-5 micron) range of wavelengths.

The new, independent observation used JWST’s MIRI (Mid-Infrared Instrument) in the mid-infrared (6-12 micron) range.

‘This is an independent line of evidence, using a different instrument than we did before and a different wavelength range of light, where there is no overlap with the previous observations,’ Professor Madhusudhan said.

‘The signal came through strong and clear.’

Co-author Måns Holmberg, a researcher at the Space Telescope Science Institute in Baltimore, USA, added: ‘It was an incredible realisation seeing the results emerge and remain consistent throughout the extensive independent analyses and robustness tests.’

DMS and DMDS are molecules from the same chemical family, and both are predicted to be biosignatures.

Both molecules have overlapping spectral features in the observed wavelength range, although further observations will help differentiate between the two molecules.

‘Our work is the starting point for all the investigations that are now needed to confirm and understand the implications of these exciting findings,’ said co-author Savvas Constantinou, also from Cambridge’s Institute of Astronomy.

The team say their observations have reached the ‘three-sigma’ level of statistical significance – meaning there is a 0.3 per cent probability that they occurred by chance. 

To reach the accepted classification for scientific discovery, the observations would have to cross the five-sigma threshold, meaning there would be below a 0.00006 per cent probability they occurred by chance.

They said between 16 and 24 hours of follow-up observation time with JWST may help them reach the all-important five-sigma significance.

The discovery was published in the journal The Astrophysical Journal Letters.

Last month, scientists announced that organic molecules of ‘unprecedented size’ had been discovered on Mars, adding further evidence that life may once have existed on the Red Planet.

Experts found long carbon chains, containing up to 12 consecutive atoms, in samples of Martian rock which date back billions of years.

These organic molecules – the longest identified so far – could originate from fatty acids, which are the building blocks of fats and oils and are created on Earth through biological activity.

And scientists said the discovery is of ‘high interest’ in the search for potential signs of life.

KEY DISCOVERIES IN HUMANITY’S SEARCH FOR ALIEN LIFE

Discovery of pulsars

British astronomer Dame Jocelyn Bell Burnell was the first person to discover a pulsar in 1967 when she spotted a radio pulsar.

Since then other types of pulsars that emit X-rays and gamma rays have also been spotted.

Pulsars are essentially rotating, highly magnetised neutron stars but when they were first discovered it was believed they could have come from aliens.

‘Wow!’ radio signal

In 1977, an astronomer looking for alien life in the night sky above Ohio spotted a radio signal so powerful that he excitedly wrote ‘Wow!’ next to his data.

In 1977, an astronomer looking for alien life in the night sky above Ohio spotted a radio signal so powerful that he excitedly wrote ‘Wow!’ next to his data

The 72-second blast, spotted by Dr Jerry Ehman through a radio telescope, came from Sagittarius but matched no known celestial object.

Conspiracy theorists have since claimed that the ‘Wow! signal’, which was 30 times stronger than background radiation, was a message from intelligent extraterrestrials.

Fossilised Martian microbes

In 1996 Nasa and the White House made the explosive announcement that the rock contained traces of Martian bugs.

The meteorite, catalogued as Allen Hills (ALH) 84001, crashed onto the frozen wastes of Antarctica 13,000 years ago and was recovered in 1984. 

Photographs were released showing elongated segmented objects that appeared strikingly lifelike.

Photographs were released showing elongated segmented objects that appeared strikingly lifelike (pictured)

However, the excitement did not last long. Other scientists questioned whether the meteorite samples were contaminated. 

They also argued that heat generated when the rock was blasted into space may have created mineral structures that could be mistaken for microfossils. 

Behaviour of Tabby’s Star in 2005 

The star, otherwise known as KIC 8462852, is located 1,400 light years away and has baffled astronomers since being discovered in 2015.

It dims at a much faster rate than other stars, which some experts have suggested is a sign of aliens harnessing the energy of a star.

The star, otherwise known as KIC 8462852, is located 1,400 light years away and has baffled astonomers since being discovered in 2015 (artist’s impression)

Recent studies have ‘eliminated the possibility of an alien megastructure’, and instead, suggests that a ring of dust could be causing the strange signals.

Exoplanets in the Goldilocks zone in 2017 

In February 2017 astronomers announced they had spotted a star system with planets that could support life just 39 light years away.

Seven Earth-like planets were discovered orbiting nearby dwarf star ‘Trappist-1’, and all of them could have water at their surface, one of the key components of life.

Three of the planets have such good conditions, that scientists say life may have already evolved on them. 

Researchers claim that they will know whether or not there is life on any of the planets within a decade, and said: ‘This is just the beginning.’