A potential water world orbiting a faint red star just 120 light-years away just got even more exciting. Study of K2-18b’s atmosphere revealed the presence of carbon dioxide and methane, as scientists had previously predicted for an oceanic planet, or Hycaean world, with a hydrogen-rich atmosphere.

In addition, the observations reveal a tantalizing hint of dimethyl sulfide, a signature of biology that scientists predict may be present on Hyckean worlds. Here on Earth, dimethyl sulfide is produced only by life; and most of it comes from marine phytoplankton.

“Our findings highlight the importance of looking at a variety of habitable environments in the search for life elsewhere,” says astronomer Nikku Madhusudhan from the University of Cambridge. “Traditionally, the search for life on exoplanets has focused mainly on smaller rocky planets, but the larger Hyckeian worlds are much more amenable to atmospheric observations.”

In 2019, it became the first habitable zone on a rocky world (as opposed to gaseous ones like Jupiter and Saturn) to have water in its atmosphere. The habitable zone is the distance from the parent star where liquid water could be stored on the surface; Not so much that it freezes into ice, and not so much that it evaporates. K2-18b is very different from Earth. It is much heavier; approximately 8.6 Earth masses and 2.6 Earth radii; and orbits very close to the red dwarf every 33 days.

However, red dwarf stars are cooler and dimmer than the Sun; This means that K2-18b receives the same stellar radiation as Earth. Like porridge at the Bear, as it should be. Madhusudhan and colleagues evaluated K2-18b in 2020 and found it to be habitable. They also identified K2-18b as a potential Hycaean world in 2021.

“Some of the conditions in the oceans of these worlds may be similar to habitable conditions in Earth’s oceans, meaning similar temperatures and pressures, liquid water, and the presence of energy from the star,” Madhusudhan told ScienceAlert at the time.

“There are many open questions, but at this stage this is just a first guess. The hypothesis is that if microbial aquatic life can occur in these oceans, just as it does on Earth, then some biological signatures may also be common.”

But researchers needed additional data. They needed the James Webb Space Telescope to peer into the exoplanet’s atmosphere for signs. And they finally understood. Using JWST’s NIRSpec and NIRISS near-infrared instruments, they observed K2-18b twice as it orbited between us and its host star. They then examined the difference in starlight caused by traveling through the exoplanet’s atmosphere and changing its composition.

By examining these changes, researchers found evidence of carbon, carbon dioxide and methane molecules. The light spectrum also revealed hints of dimethyl sulfide; but only hints. This means there are not enough observations to determine whether K2-18b is habitable. But we are extremely close.

“Future Webb observations will be able to confirm whether dimethyl sulfide is indeed present in significant quantities in the atmosphere of K2-18b,” says Madhusudhan.

The next step will be to direct JWST’s mid-infrared MIRI to the exoplanet to further investigate the possibility of dimethyl sulfide and search for other possible biosignatures in the exoplanet’s atmosphere. More generally, the results show that JWST can detect biological signatures in exoplanet atmospheres, an important first step towards finding life on another planet.

“Our ultimate goal is to detect life on a habitable exoplanet, which will change our understanding of our place in the universe,” says Madhusudhan. “Our discoveries are a promising step in this quest towards a deeper understanding of the Hykean worlds.” Source