In 2023, an exoplanet made international headlines because James Webb Space Telescope (JWST) observations suggested that the planet has a “biosignature” – signs of gas in its atmosphere produced by “life.”
A new study published in the Astrophysical Journal Letters pricks a hole in this hypothesis.
K2-18b is a planet orbiting a star more than 110 light-years from Earth. The planet is in the habitable “Goldilocks” zone – that is, its orbit lies within the distance where it is not too cold and not too hot, and can sustain liquid water. It was discovered in 2015 by the Kepler Space Telescope.
More recent observations by NASA’s JWST helped reveal the planet’s atmospheric make up.
K2-18b’s atmosphere is mostly hydrogen, unlike Earth’s mostly nitrogen atmosphere. It is, therefore, believed to be a “Hycean” world – that is a world with a hydrogen-dominant atmosphere and water oceans.
But it was less abundant gases discovered by a University of Cambridge team last year which got everyone talking. The astronomers reported that JWST data showed traces of methane and carbon dioxide, elements associated with several different processes̶ including life on Earth.
Even more excitingly, though, the Cambridge team also noticed a possible detection of dimethyl sulphide, or DMS. This molecule is present in Earth’s atmosphere, produced by phytoplankton.
“The DMS signal from the Webb telescope was not very strong and only showed up in certain ways when analysing the data,” says University of California, Riverside (UCR) astronomer Shang-Min Tsai, lead author of the new paper. “We wanted to know if we could be sure of what seemed like a hint about DMS.”
Unfortunately, computer modelling by the UCR suggests that last year’s excitement that JWST found signs of life on K2-18b might be misplaced.
“The signal strongly overlaps with methane, and we think that picking out DMS from methane is beyond this instrument’s capability,” Tsai says.
For DMS to be detectable on an exoplanet using current instruments – even those on the powerful JWST – the astronomers calculated that it would have to be 20 times more abundant than it is on Earth.
“The best biosignatures on an exoplanet may differ significantly from those we find most abundant on Earth today,” says senior author and UCR astrobiologist Eddie Schwieterman. “On a planet with a hydrogen-rich atmosphere, we may be more likely to find DMS made by life instead of oxygen made by plants and bacteria as on Earth.”
