Is there life beyond Earth? It seems extremely likely, although we do not yet have evidence of it. But astronomers said on August 26, 2021 that they had identified a new class of exoplanets that did a big step forward in search of life. They call them Hicean worlds, in words hydrogen i ocean. Oceans around the planet and hydrogen-rich atmospheres could cover these worlds. And yet, the researchers said, they could be habitable.
Researchers at Cambridge University led this intriguing research. The peer review Astrophysics journal published it on August 26th.
Hycean planets: like Earth but different
According to researchers, the Hycean worlds could greatly accelerate the search for life elsewhere. They somehow resemble the Earth, largely or even completely covered by the oceans. However, they are also rare: up to 2.6 times the diameter of the Earth, with temperatures of up to 200 degrees C (with an intensity of 400 degrees F) and thick atmospheres of hydrogen. This places them somewhere between Earth and giant planets like Neptune or Uranus.
In fact, many of these worlds of this size range are known to exist, super-Earths, and mini-Neptunes. Scientists say planets in this size range are the most common in our galaxy.
There can also be different types of icis, including “dark” and “cold”. As stated in the document:
Our research includes “Dark Hycean” worlds closed in order that allow living conditions only in their permanent nocturnal and “Cold Hycean” worlds that see insignificant irradiation [receive little radiation from their stars].
Superterrests, mini-Neptunes and Hiceans
Super-Earths are rocky but larger than Earth. Even now little is known about what types of atmospheres they have, although some have been found in the habitable areas of their stars where temperatures could allow liquid water. Although it has long been believed that Mini-Neptunes is inhospitable to life as we know it. Most mini-Neptunes do not have a solid surface and the temperatures and pressures of their atmospheres would hinder the evolution of life.
What the new study suggests, however, is that some of these worlds may be able to sustain life after all. These are the hiccups. As lead author Nikku Madhusudhan said at Cambridge University:
The Hycean planets open a new path in our search for life elsewhere.
Are the Hycean planets habitable?
So how can Hicean planets be habitable? Having plenty of liquid water is, of course, a good start. These planets, unlike most mini-Neptunes, can have solid surfaces, such as Earth. Many of the known Hycean candidates are larger and hotter than Earth, but they could still host large oceans, according to researchers. The conditions could be similar to some of the most extreme aquatic environments on our planet, but theoretically they could at least sustain microbial life.
By their nature, these planets would also mean that the habitable zones around their stars could be much larger than those of systems with Earth-like planets. This is another advantage for the possibility of life. The habitable zone is the region around a star where temperatures would be adequate for liquid water on the surface of a rocky planet.
Another previous study, on the mini-Neptune K2-18b, supports the possibility of habitable Hycean worlds. Based on this study, scientists identified the new class of exoplanets, the hyceans. Although some scientists refer to K2-18b as a super-Earth, most now classify it as a mini-Neptune.
Looking for biosignatures
So how would astronomers look for evidence of life in any of these worlds? They will look for biosignatures, chemical fingerprints of biological processes in the planets’ atmospheres. Some common ones are oxygen, ozone, methane, and nitrous oxide, as well as methyl chloride and dimethyl sulfide. The latter two are not common on Earth, but may be on hydrogen-rich planets. According to Madhusudhan:
Essentially, when we’ve been looking for these various molecular signatures, we’ve focused on Earth-like planets, which is a reasonable place to start. But we believe that the Hycean planets offer better chances of finding various traces of biosignatures.
Madhusudhan and his team claim that many of these biosignatures should be easily detectable on the 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 detectable than on Earth-like planets. From the paper:
We found that a number of terrestrial biomarkers that could be expected to be present in hycea atmospheres would be easily detectable by a modest observation time with the James Webb Space Telescope (JWST). We identify a considerable sample of possible nearby hypic planets that may be ideal targets for these observations in search of exoplanetary biosignatures.
Upcoming observations
Interestingly, it may not take long to get good spectroscopic observations of some Hycean planets. Astronomers already have a good sample of candidate-type hiccup worlds to study, and upcoming space telescopes like the James Webb Space Telescope (the Webb) will be able to analyze their atmospheres. All the candidates are quite close, 35 to 150 light-years away, orbiting between red dwarf stars.
Currently, K2-18b is the main Hicean candidate and observations are already planned with the Webb, which will be launched after October 31 this year.
If any of these candidate hiccups really support life, future studies with the Webb and other telescopes have a good chance of detecting signs of them. As Madhusudhan pointed out:
A biosignature detection would transform our understanding of life in the universe. We need to be open to where we hope to find life and what form that life might take, as nature continues to amaze us in often unimaginable ways.
Astronomers also announced last July that another mini-Neptune, the TOI-1231 b, has a deep atmosphere ideal for further study by Webb and the Hubble Space Telescope. It will be exciting to see what future observations of worlds like this, and especially of the Hyceans, reveal.
Conclusion: Cambridge University astronomers have identified a new class of potentially habitable exoplanets called hiceans. These are huge, hot worlds that could be covered by oceans and have thick hydrogen atmospheres.
Source: Habitability and biosignatures of the Hycean worlds
Source (prepress): Habitability and biosignatures of the Hycean worlds
Via University of Cambridge