Astronomers have found that there is a new class of exoplanets that could have the right conditions for life, although they wouldn’t look very homely to us. They are calling them Hycean planets. They are ocean worlds with a dense atmosphere of hydrogen, with masses, temperatures, and conditions very different from Earth.
As reported in The Astrophysical Journal, members of this new class are both bigger and hotter than Earth but they have the right conditions to have large oceans. And those oceans could be supporting life forms, like those found on Earth’s most extreme oceanic environments.
“Hycean planets open a whole new avenue in our search for life elsewhere,” lead researcher Dr Nikku Madhusudhan from Cambridge’s Institute of Astronomy, said in a statement.
The discovery of exoplanet K2-18b and the discovery of water in its atmosphere is what pushed the team to investigate certain massive planets as life-supporting worlds. These are in the broad categories of Super-Earths or Mini-Neptunes (depending on sizes) but with thick hydrogen atmospheres. They can also have a scorching atmosphere. But maybe life finds a way around all that.
The team crunched the numbers and worked out that planets with a radius up to 2.6 times that of Earth and an atmospheric temperature of 200 °C (392 °F), can still have oceans that would be habitable to certain microbes. Even tidally-locked planets, with one side in constant starlight, could still have habitable regions.
But being a Hycean planet is not just about size and temperature. The atmosphere composition is also crucial. A Hycean planet should have traces of other compounds beyond hydrogen in their atmospheres. These include oxygen, ozone, methane, nitrous oxide, methyl chloride, dimethyl sulfide. Molecules that could hint at life.
“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,” added Madhusudhan. “But we think Hycean planets offer a better chance of finding several trace biosignatures.”
Exoplanet K2-18b is going to be observed by Hubble’s successor, JWST, after it launches in October. Atmospheric analysis might actually reveal some interesting molecular signatures. Their larger size and higher temperatures make atmospheric signatures easy to detect.
“It's exciting that habitable conditions could exist on planets so different from Earth,” said co-author Anjali Piette, also from Cambridge.
The fact that a planet or a class of planets can have the conditions for life doesn’t mean that they do. And even if they do, life might not have evolved there. But the possibility is well worth investigating. We don’t know how life comes to be in the Universe, so it is important to look for it everywhere.