Does the planet K2-18b show signs of life? | Explained Premium

K2-18b, a potentially habitable exoplanet 124 lightyears away, may contain biomarker DMS, but its habitability remains uncertain.

The story so far: On April 16, an international research team uploaded a paper in which it reported that the distant exoplanet K2-18b may be habitable. The claim was met with cautious excitement by astronomers. While finding places in the universe that could harbour life is a vital quest in the field, experts — including the team that made the finding — are cautious because many similar claims in the past have had to be retracted after closer inspection.

K2-18b is an exoplanet — a planet outside the solar system. It’s located 124 lightyears away from the earth in the constellation Leo, orbiting the star K2-18. It is 5.2-times wider and roughly nine-times more massive than the earth, dimensions that suggest it likely possesses a hydrogen-rich atmosphere. The amount of stellar radiation it receives from its star is comparable to what the earth receives from the sun. It was discovered by the Kepler telescope in 2015. In 2019, the Hubble Space Telescope found that its atmosphere contains signs of water vapour. Four years later, the James Webb Space Telescope (JWST) also detected signs of carbon dioxide and methane. Scientists had previously shown in modelling studies that it’s possible to find methane and carbon dioxide and no ammonia in a hydrogen-rich atmosphere if (but not only if) there is a liquid water ocean on a planet’s surface.

According to the new work, K2-18b might be a Hycean world: covered with an ocean overlaid by a hydrogen-rich atmosphere. The JWST is designed to study such planets better than other telescopes can, although K2-18b itself isn’t confirmed to be Hycean. Some computer models suggest it may also have a stratosphere and some carbon oxides and cyanide in the middle atmosphere. Researchers have also said its surface conditions may be close to the runaway greenhouse threshold — when the concentration of greenhouse gases in the atmosphere prevents any heat from escaping the surface. This is why Venus is a hellscape.

 Also read: How scientists are exploring what extraterrestrial life could look like

In the new paper, the team reported the presence of two compounds — either dimethyl sulphide (DMS) or dimethyl disulphide (DMDS) — in K2-18b’s atmosphere. Scientists have suggested that DMS could be a biomarker, a sign of life, on exoplanets. DMS on earth is made mostly in the oceans. Phytoplankton produce a molecule called dimethylsulphoniopropionate. When they die, enzymes break up the molecule to release DMS. It’s also emitted when bacteria break up plant matter. In 2015, researchers reported that 76% of soil bacteria contain a gene that allows them to produce DMS.

Experts have said the only way to find extraterrestrial life is for them to directly detect it. This is why NASA launched its Clipper mission in 2024 to study Jupiter’s moon Europa. Attempts to look for conditions suitable for life (that too only life as we know it) and the presence of specific molecules from a distance can only be suggestive of habitable conditions.

Many uncertainties in these studies also arise from astronomers’ instruments and the accuracy of models they use to simulate conditions in outer space. Hitherto unknown chemical processes may be capable of creating the molecules scientists consider to be biomarkers. Or JWST not being able to detect certain compounds could mean they’re present at concentrations below its detection threshold.