Only a tiny fraction of them may be habitable, though.
Arecent paper accepted by The Astrophysical Journal has gotten a lot of attention, and for good reason. Nikku Madhusudhan and his colleagues from the University of Cambridge took a close look at a novel type of habitable exoplanet, which they call Hycean worlds. These planets would fall somewhere between “Super-Earths” and “mini-Neptunes” in terms of mass. They’re thought to have enough gravity to keep a thick hydrogen atmosphere over a huge liquid water ocean.
Planets like these, floating alone in interstellar space—in other words, not orbiting a star—have previously been recognized as being potentially habitable. Madhusudhan provided a more detailed investigation, including modeling of different scenarios, and concluded that habitable temperatures and pressures would be very common on those type of planets. In fact, they argue, many of them could harbor life. The authors suggest that certain molecules indicative of life could be detected by the soon-to-launch James Webb Telescope, even though they would only exist at very low concentrations in their hydrogen-dominated atmospheres.
The authors surmised that Hycean planets up to 10 times as massive as Earth could be habitable even if tidally locked (Dark Hycean Worlds) or lacking a host star (Cold Hycean Worlds). The latter would, in principle, be like other “rogue” planets.
Given the number and variety of exoplanets already cataloged, I agree that there should be many planets that fall in the mass range between Super-Earths and mini-Neptunes. But I think only a very small fraction of these can be expected to be truly habitable, and even fewer would be capable of hosting life.
We do not have a Hycean world in our Solar System, which doesn’t mean they couldn’t be common elsewhere, of course. We do have Neptune, which is thought to have a liquid water ocean in its atmosphere. Yet that’s not a place where we look for life. Habitability requires so much more than benign temperatures and pressures to keep water liquid. It also requires an energy source, suitable organic building blocks, and a mechanism to recycle nutrients, such as plate tectonics. Even if a planet has all this, it doesn’t mean it hosts life, because there are likely many habitable, but uninhabited, planets out there.
The reason is that the constraints for life arising in the first place are much more stringent than for the persistence of life, because once it takes hold it can adapt by natural selection to a broad variety of environments, even extreme ones. And even if life does arise on Hycean worlds, don’t expect anything more than microbes. Given the lack of oxygen or some other compound to provide lots of metabolic energy, animal life is not a realistic option.
So, while I agree with the authors that we should search these proposed Hycean worlds for molecules indicative of life (so-called biosignatures), such as methyl chloride, dimethyl sulfide, and carbonyl sulfide, we shouldn’t get our hopes too high. Keep in mind that the Venusian atmosphere has lots of carbonyl sulfide, but may have no current life, and the detection of these molecules would not be conclusive.
Having said that, however, I think the authors need to be praised for suggesting possible life on worlds very different from ours. One common mistake is that we’re often too Earth-centric in our search for life in the universe.