Hycean worlds, which are a possible kind of exoplanet with deep oceans surrounded by a thick envelope of hydrogen, could provide the best chance for the James Webb Space Telescope (JWST) to detect biosignatures, according to a new study.
Those potential signs of life are a group of chemicals called methyl halides, which on Earth are produced by some bacteria and ocean algae.
“Unlike an Earth-like planet, where atmospheric noise and telescope limitations make it difficult to detect biosignatures, hycean planets offer a much clearer signal,” said Eddie Schwieterman, who is an astrobiologist at the University of California, Riverside, in a statement.
For now, the existence of hycean planets remains hypothetical. Their name is a portmanteau of “hydrogen” and “ocean,” first coined in 2021 by planetary scientist Nikku Madhusudhan of the University of Cambridge.
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Hycean planets are expected to orbit red dwarf stars, and the best candidate for a hycean world is the planet K2-18b. This exoplanet, which is categorized as a “sub-Neptune” world, orbits in the habitable zone of a red dwarf star 124 light-years from Earth in the constellation of Leo, the Lion.
The Hubble Space Telescope discovered water vapor in K2-18b’s atmosphere in 2019, and JWST has detected the presence of carbon dioxide and methane in the planet’s atmosphere, along with a lack of carbon monoxide and ammonia — exactly as predicted by the hycean planet hypothesis. There’s also tentative evidence that a compound called dimethyl sulfide, which on Earth is only produced by ocean plankton, also exists in K2-18b’s atmosphere, but this evidence continues to prove contentious.
Now a team of researchers at the University of California, Riverside and ETH Zurich in Switzerland have gone a step further. They propose that another family of compounds called methyl halides, generated by microbial ocean life on Earth, could produce a biosignature — that is, a chemical signature of biological life — in the atmosphere of a hycean world that’s more easily detectable than the signature of oxygen is on an Earth-like planet.
“Oxygen is currently difficult or impossible to detect on an Earth-like planet,” said Michaela Leung of the University of California, Riverside, the first author of a new paper describing the research. “However, methyl halides on hycean worlds offer a unique opportunity for detection with existing technology.”
Methyl halides are molecules that incorporate carbon atoms and three hydrogen atoms attached to a halogen atom such as bromine, chlorine or fluorine. (Halogens are group of reactive, non-metallic elements.) On Earth, methyl halides are produced by life, but they are far from abundant in our planet’s atmosphere.
On hycean worlds, however, things could be different. Leung’s team suspect that the conditions on such worlds, should they exist, would allow methyl halides to accumulate in large quantities in the atmosphere. Furthermore, methyl halides would have strong absorption features in infrared light, at the same wavelengths that the JWST is designed to observe.
“One of the great benefits of looking for methyl halides is, you could potentially find them in as few as 13 hours with James Webb. That is similar or lower, by a lot, to how much telescope time you’d need to find gases like oxygen or methane,” said Leung. “Less time with the telescope means it’s less expensive.”
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There are two caveats to what Leung’s team propose. One is that we don’t yet know whether hycean worlds actually exist. They were proposed as a possibility to explain certain properties of some warm sub-Neptune-type planets that have average densities that imply a thick hydrogen atmosphere and a deep ocean of liquid water. However, directly observing an ocean beneath such a world’s hydrogen envelope is not currently feasible.
The second issue is that we don’t know if such oceans could be habitable. A hycean world would be hot, and although the extreme conditions beneath the hydrogen envelope would prevent the ocean from evaporating, it is uncertain whether it would be too hot for life as we know it. However, a positive detection of methyl halides in the atmosphere of a candidate hycean world would be a strong indication that life could exist there in a deep ocean.
If life does exist on such a world, it would have to breathe hydrogen, not oxygen.
“These microbes, if we found them, would be anaerobic,” said Schwieterman. “They’d be adapted to a very different type of environment, and we can’t really conceive of what that looks like, except to say that these gases are a plausible output from their metabolism.”
Anaerobic life — i.e., lifeforms making do without oxygen — exist on Earth, so it wouldn’t be truly alien to life on our planet, even if the environment that it would live in is. Earth-like worlds orbiting red dwarfs could be in short supply, since red dwarfs are fierce little beasts, prone to unleashing bursts of harsh radiation that can strip away the atmosphere of an Earth-like planet. However, hycean worlds protected by their thick hydrogen atmospheres might be less vulnerable to attack from their star.
It could therefore be that hycean worlds are where life resides in red dwarf systems, and since red dwarfs make up about three-quarters of all stars in our Milky Way galaxy, there could be many more habitable hycean worlds in the cosmos than Earth-like worlds.
The research by Leung’s team was published on March 11 in The Astrophysical Journal Letters.