Ocean planets, a possible type of exoplanet with deep oceans surrounded by a thick hydrogen envelope, could provide the James Webb Space Telescope ( JWST ) with the best chance to detect biosignatures, according to a new study.
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These potential signs of life are a group of chemicals called methyl halides, which on Earth are produced by certain bacteria and ocean algae.
For now, the existence of ocean planets remains hypothetical. Their name is a combination of the words “hydrogen” and “ocean,” first coined in 2021 by planetary scientist Nikku Madhusudhan of the University of Cambridge.
Ocean planets are expected to orbit red dwarfs (stars), and the best candidate is planet K2-18b. This exoplanet, which is categorized as a "sub-Neptune" world, orbits in the habitable zone of a red dwarf, 124 light-years from Earth, in the constellation Leo.
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The Hubble Space Telescope discovered water vapor in K2-18b's atmosphere in 2019, and JWST detected the presence of carbon dioxide and methane in the planet's atmosphere, along with a lack of carbon and ammonia—just as predicted by the ocean planet hypothesis. There is also evidence that a compound called dimethyl sulfide, which on Earth is only produced by ocean plankton, is also present in K2-18b's atmosphere, but this evidence continues to prove controversial.
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, created by microbial life in Earth's oceans, could produce a biosignature—that is, a chemical signature of biological life—in the atmosphere of an ocean world that is more easily detectable than the signature of oxygen on an Earth-like planet.
On ocean planets, however, things could be different. Scientists suspect that conditions on such worlds, if they exist, would allow methyl halides to accumulate in large quantities in the atmosphere. Moreover, methyl halides would have strong absorption characteristics in infrared light, the same wavelengths that JWST is designed to observe.
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There are two caveats to what scientists are proposing. One is that we don't yet know if ocean planets actually exist. The second issue is that we don't know if such oceans could be habitable. A water world there would be hot, and although the extreme conditions beneath the hydrogen envelope would prevent the ocean from evaporating, it's uncertain whether it would support life as we know it. However, a positive detection of methyl halides in the atmosphere of a candidate ocean world would be a strong indication that life could exist there in a deep ocean.
Source: livescience
