New class of exoplanet could put search for ET into hyper-drive

Astronomers have identified a new class of exoplanet they believe could be a candidate for the search for extra-terrestrial life, according to a new study in The Astrophysical Journal.

Traditionally, the search for extra-terrestrial life has focused on the terrestrial: looking for other “Earths” with similar dimensions and conditions. The planets in question, however, dubbed “Hycean” exoplanets by the authors of the paper, are hot, ocean-covered planets with hydrogen-rich atmospheres, and which are both more numerous and larger than Earth-like planets.

The researchers, from the University of Cambridge, UK, believe that despite being hotter than Earth, these planets could host large oceans that could in theory host microbial life similar to that found in Earth’s more extreme aquatic environments.

By modelling the atmospheres of various types of Hycean worlds, the team established that the planets allow for a far wider habitable zone (the “Goldilocks Zone”, the orbit around the planet’s star that’s the right temperature for life) than Earth-like planets.

“Hycean planets open a whole new avenue in our search for life elsewhere,” says lead author Nikku Madhusudhan, from Cambridge’s Institute of Astronomy.

“It’s a really important reminder that we need to keep exploring what could be rather than just focusing on the things that we definitely know,” says Jonti Horner, an astronomer at the University of Southern Queensland who was not involved in the study.

“One of the problems we have in astrobiology is that no matter how diverse life on Earth is, it’s only one kind of life – it’s Earth life,” Horner says. “And so we’re really strongly biased to think that the place you must look to find life must be a place like Earth.”

Mini-Neptunes and Super-Earths

There are thousands of exoplanets (planets outside our solar system) that are already known to science. Most of these are mid-size between Earth and Neptune and are thus called, perhaps uncreatively, either mini-Neptunes or super-Earths.

Most of these mini-Neptunes are around 1.6 times the size of Earth, and were thought to be far too hot and pressurised for life, thanks to their hydrogen-rich atmospheres.

But when Madhusudhan and his team conducted a recent study on mini-Neptune K2-18b, they found, perhaps surprisingly, that under certain conditions the planet could theoretically support life similar to the life found on Earth. Buoyed by this finding, the team began to investigate the various conditions under which mini-Neptunes could feasibly host life, leading them to develop the Hycean planet theory.

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These Hycean planets could be up to 2.6 times larger than Earth, and could have atmospheric temperatures as high as 200C. But their oceans may have conditions closer to those on Earth, and could be a place to start looking for life.

Hycean exoplanets may be a dime a dozen

The astronomers believe Hycean worlds are likely very common: more so than the Earth-like planets ET-enthusiasts have tended to search for. This means Hycean worlds open up the possibility of analysing far more planets, and potentially hastening the discovery of extra-terrestrial life, if it is indeed out there.

Identifying Hycean exoplanets requires more than just establishing their size, however: astronomers need to determine the habitable zone of the parent star, and search for molecular signatures that betray the planet’s atmosphere and structure.

Then, they will look for signs of life, bio-signatures including oxygen, ozone, methane and nitrous oxide, which are all present on Earth and could be the by-products of living organisms. Other bio-signatures will be considered, including methyl chloride and dimethyl sulphide, both of which are less abundant on Earth but could be promising signals on hydrogen-rich planets.

“Essentially, when we’ve been looking for these various molecular signatures, we have been focusing on planets similar to Earth, which is a reasonable place to start,” says Madhusudhan. “But we think Hycean planets offer a better chance of finding several trace biosignatures.”

The team have already identified a sample of potential Hycean worlds which could be prime candidates for future study. These planets all orbit red dwarf stars between 35-150 light-years away which, by astronomical standards, is practically in our backyard. The James Webb Space Telescope, due to launch this year, will observe the most promising candidate – K2-18b – once it becomes operational.

“A biosignature detection would transform our understanding of life in the universe,” says Madhusudhan. “We need to be open about where we expect to find life and what form that life could take, as nature continues to surprise us in often unimaginable ways.”