A distant planet once touted as a potential location to look for alien life is actually most likely to be inhospitable, according to astronomers who have found it probably lacks a solid surface.
In 2015, astronomers discovered a planet 110 light years away called K2-18b, which later analysis estimated to be a super-Earth or mini-Neptune about eight times the mass of our world. Astronomers in 2019 then found evidence for water vapour on the planet. Since K2-18b is thought to be in the habitable zone of its star – the region in which water can exist as a liquid on the surface of a planet – that led to suggestions it might be an ocean world. Prospects were further buoyed by 2023 research finding evidence of dimethyl sulphide, a molecule that, on Earth, is only produced by life, particularly marine phytoplankton.
But now, at the NASA Ames Research Center in California and his colleagues have re-analysed the planet using data from the James Webb Space Telescope (JWST) and they suggest that the likelihood of the planet being a habitable ocean world is low. They say that an found by JWST points to the planet being a gas-rich mini-Neptune with no surface, rather than a habitable super-Earth with an ocean and sea floor, because such gases would be broken down by a chemical process called photolysis if it were an ocean or “hycean” world.
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The researchers write in their paper that there are two options, with the planet being either a “Hycean world inhabited by methanogenic life” – that which produces methane – “or a mini-Neptune with no defined surface. The latter case is less complex and requires fewer assumptions.”
“Overall, we favour the mini-Neptune explanation,” writes the team. In both cases, the evidence for the dimethyl sulphide may simply be a false signal.
at the University of Wisconsin-Madison says this follow-up work highlights the rigorous tests that any claims of habitability or biosignatures on a planet will have to go through. “It’s going to be very difficult to get a really positive affirmation that life is present on a planet,” he says. “We’re going to have to eliminate all non-life possibilities so that the most likely outcome is life.”
Other recent research on K2-18b suggested another possibility, that it might be a hycean world with oceans made of magma instead of water – making it equally unlikely to support life.
Wogan and his team note that if dimethyl sulphide were detected with more confidence in future, it “might be difficult to account for its presence” without the planet being a habitable ocean world. Further observations by JWST could help determine for sure which scenario is correct.
arXiv
![Nature Astronomy, doi:10.1038/s41550-022-01634-x Fig. 4 | SCExAO/CHARIS images of AB Aur at different wavelengths and observing modes. Left: polarized intensity wavelength-collapsed image obtained one day later. A pure scattered-light disk feature would have been detected at the position of AB Aur b (green circle). Instead this region shows no concentrated emission, indicating that AB Aur b is not detected. Right: emission at the approximate position of AB Aur b from VAMPIRES H? data using RDI/KLIP for PSF subtraction. From left to right, the intensity scaling is [0, 0.0925] mJy, [0, 0.055] mJy and [?0.007, 0.007] mJy, normalized to the source?s apparent FWHM. The x and y axes are in units of arcseconds east (along the x axis) and north (along the y axis).](/wp-content/uploads/2022/04/04144318/SEI_97110504.jpg?w=300&h=225&crop=1)
