Apparent signs of alien life on the exoplanet K2-18b may just be statistical noise, according to a new analysis of data from the James Webb Space Telescope.
On 17 April, at the University of Cambridge and his colleagues made the stunning claim that K2-18b, a super-Earth 124 light years away, showed strong evidence of an atmosphere containing dimethyl sulphide, a gas that on Earth is only produced by living things.
But at the University of Oxford has re-analysed the data that Madhusudhan and his team released 鈥� which was produced from an unpublished raw dataset 鈥� using a basic statistical test, and he found that it doesn鈥檛 appear to show the presence of any molecules at all.
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To detect molecules on alien worlds, astronomers observe the light that has passed through the planet鈥檚 atmosphere, producing what is called a transmission spectrum. This comes in the form of a collection of data points for different frequencies of light, and astronomers then trace a line through these points to match them to known molecules.
Many of these lines will look a bit like bell-shaped curves, known as Gaussian distributions. Taylor used a statistical test to see whether the data better matched a Gaussian distribution or a flat line, which would indicate no molecules have been detected.
In six tests comparing Madhusudhan鈥檚 data with different bell-shaped curves and flat lines, five of them showed a better match with a flat line. 鈥淭his indicates that the data does not have the precision, yet, to detect a spectral feature [indicating a particular molecule],鈥� says Taylor.
Madhusudhan disagrees that tests like these say anything useful, because the simplicity of Taylor鈥檚 modelled curves doesn鈥檛 reflect the complexity of real data. 鈥淭he model itself is unphysical, as I have never seen a physically plausible model that can give rise to the Gaussian peaks and troughs in a transmission spectrum over this wavelength range,鈥� says Madhusudhan.
But Taylor argues that simple models, like Gaussian distributions, are routinely used to identify chemicals such as sodium in exoplanet atmospheres, and that most planetary models use simplified models to some extent.
Another question over the habitability of K2-18b concerns whether liquid water can exist on its surface. at the University of Cambridge and his colleagues argue that the reflectivity of K2-18b鈥檚 atmosphere, based on its transmission spectrum, is too high to retain a liquid ocean and implies a scorchingly hot interior.
鈥淭hose spectra tell us that the atmosphere is absorbing light from the star, and that鈥檚 bad news from the perspective of the planet鈥檚 climate, because it wants to be reflecting to stay as cool as possible,鈥� says Shorttle.
Madhusudhan counters that Shorttle and his team can only measure the reflectivity for the part of the planet鈥檚 atmosphere that light passes through on its way to us, rather than the day side of the planet, which permanently faces the star. This makes its 鈥渋mpractical鈥� to infer information about the whole planet, he says.
It鈥檚 possible that the day side of the planet is cooler than the point they measured, says Shorttle, but that would imply that heat can鈥檛 efficiently move around the planet, which would be another problem for the likelihood of finding life, he says. 鈥淚n solving that problem, you just create a different one, which is you then trap all the heat there,鈥� he says.
The raw data that Madhusudhan and his team used from JWST will be released on 25 April, which will allow other teams to scrutinise their findings further.
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