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Is there life beyond Earth? It seems extremely likely, although we don’t have hard evidence yet. But astronomers said on August 26, 2021 that they had identified a new class of exoplanets that are a big step forward in the search for life. They call them Hycean worlds, after the words hydrogen and ocean. Planetary oceans and hydrogen-rich atmospheres could cover these worlds. And yet, according to the researchers, they could be habitable.
Researchers at the University of Cambridge conducted this intriguing research. Peer review Astrophysics Journal posted it on August 26.
Hycean planets: like Earth but different
According to the researchers, the Hycean worlds could significantly speed up the search for life elsewhere. In some ways, they are reminiscent of the Earth, largely or even completely covered by the oceans. Yet they are also particularly extraterrestrial: up to 2.6 times the diameter of Earth, with temperatures up to 200 degrees C (about 400 degrees F) and thick hydrogen atmospheres. This places them somewhere between Earth and giant planets like Neptune or Uranus.
Indeed, many worlds of this size are already known to exist, super-Earths and mini-Neptunes. Scientists say that planets of this size are the most common in our galaxy.
There may also be different types of high school students, including “dark” and “cold”. As stated in the document:
Our investigations include tidal locked “Dark Hycean” worlds which allow habitable conditions only on their permanent night sides and “Cold Hycean” worlds which see negligible irradiation. [receive little radiation from their stars].
Super-Earths, mini-Neptunes and Hyceans
Super-Earths are rocky but larger than Earth. Even now, little is known about the types of atmospheres they have, although some have been found in the habitable areas of their stars where temperatures could allow liquid water. It was long thought that mini-Neptunes were inhospitable to life as we know it. Most mini-Neptunes do not have a solid surface and the temperatures and pressures in their atmospheres would make it very difficult for life to evolve.
What the new study suggests, however, is that some of these worlds may be able to support life after all. These are the Hyceans. As lead author Nikku Madhusudhan said at the University of Cambridge:
The Hycean planets open up a whole new path in our quest for life elsewhere.
Are Hycean planets habitable?
How, then, could the Hycean planets be habitable? Having plenty of liquid water is of course a good start. These planets, unlike most mini-Neptunes, can have solid surfaces, like the Earth. Many known Hycean candidates are larger and hotter than Earth, but would still be able to accommodate large oceans, the researchers say. Conditions could be similar to some of the most extreme aquatic environments on our planet, but could theoretically still at least support microbial life.
By their nature, these planets would also mean that the habitable areas around their stars could be much larger than those in systems with Earth-like planets. This is another plus for the possibility of living. The habitable zone is the region around a star where temperatures would be suitable for liquid water on the surface of a rocky planet.
Another previous study, of mini-Neptune K2-18b, supports the possibility of habitable Hycean worlds. Based on this study, scientists identified the new class of exoplanets, the Hyceans. While some scientists call K2-18b a super-Earth, most now classify it as a mini-Neptune.
Search for biosignatures
So how would astronomers search for evidence of life on any of these worlds? They will look for biosignatures, chemical fingerprints of biological processes in the atmosphere of planets. Some streams are oxygen, ozone, methane and nitrous oxide, as well as methyl chloride and dimethyl sulfide. The latter two are not common on Earth, but could be on planets rich in hydrogen. According to Madhusudhan:
Essentially, when we researched these various molecular signatures, we focused on Earth-like planets, which is a good place to start. But we believe that the Hycean planets offer a better chance of finding multiple traces of biosignatures.
Madhusudhan and his team say many of these biosignatures should be easily detectable on Hycean planets. In fact, the nature of the planets themselves – larger sizes, higher temperatures, and hydrogen-rich atmospheres – means that biosignatures would be even more easily detected than on Earth-like planets. Paper :
We find that a number of traces of terrestrial biomarkers that can be expected to be present in Hycean atmospheres would be easily detectable using modest observation time with the James Webb Space Telescope (JWST). We identify a large sample of nearby potential Hycean planets that may be ideal targets for such observations in search of exoplanetary biosignatures.
Observations to come
Interestingly, it might not be too long before we get good spectroscopic observations of some Hycean planets. Astronomers already have a good sample of Hycean-type candidate worlds to study, and future space telescopes like the James Webb Space Telescope (the Webb) will be able to analyze their atmospheres. All of the candidates are fairly close, 35 to 150 light years away, and orbit around red dwarf stars.
K2-18b is currently Hycean’s lead contender, and sightings with the Webb, which will launch after October 31 of this year, are already planned.
If any of these candidate hyceans are indeed sustaining life, future studies with Webb and other telescopes have a good chance of detecting signs of it. As Madhusudhan noted:
A biosignature detection would transform our understanding of life in the universe. We need to be open about where we expect to find life and what form that life might take, as nature continues to surprise us in ways that are often unimaginable.
Astronomers also announced last July that another mini-Neptune, TOI-1231b, had a deep atmosphere ideal for further study by the Webb and the Hubble Space Telescope. It will be exciting to see what future observations of worlds like this, and of Hyaceans in particular, reveal.
Conclusion: Astronomers at the University of Cambridge have identified a new class of potentially habitable exoplanets called Hyceans. These are huge, hot worlds that could be covered with oceans and have thick hydrogen atmospheres.
Source: Habitability and biosignatures of Hycean Worlds
Source (preprint): Habitability and biosignatures of Hycean worlds
Via Cambridge University
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