There are three basic factors we can measure, with more or less accuracy, about planets in other solar systems. We can, in the right circumstances, measure how long they take to orbit, their mass, and their radius. This lets us then calculate things like a planet’s average density, which allows us to start to imagine how it may be structured. But an average density isn’t the same thing as an actual density. Take Earth, for instance. We’re a weird mix of rock and water surrounded by air. These factors all have very different densities, and we have every reason to believe that other planets are going to be just as weird, if not way weirder. 

It is with this understanding, that the universe seems to like to make things weird, that scientists had a hard think about a trend they were noticing in heavy Earths and mini-Neptunes. These worlds aren’t that different in mass, but they are quite different in radius, with heavy Earths being about 1.3 Earth radii and mini-Neptunes being about 2.4 Earth radii.  

In a new paper in The Astrophysical Journal Letters with lead author Olivier Mousis, a team of researchers suggests that the mini-Neptunes are worlds where the water has been heated to a supercritical state. A rocky core is surrounded by a dense atmosphere of water that is subject to a strong greenhouse effect that balloons out the water vapor around the world. Essentially, these are water worlds (minus Kevin Costner) where the water has been heated into a world-shrouding haze more hot and humid than a bad summer day in Atlanta.

The heavy Earths, however, may be normal ocean worlds or rocky worlds like Mars that have their oceans locked away in the ground or frozen into ice. If this is the case, both these worlds will have formed the same but have completely different fates based on how strongly (or not) they are irradiated by their stars. This is a nice piece of research that shows that since the same science is at play everywhere, we should see worlds with similar histories that diverge due to differences in the details.

More Information

CNRS press release 

“Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations,” Olivier Mousis et al., 2020 June 15, Astrophysical Journal Letters (Preprint on arxiv.org)

“Revised Mass-Radius Relationships for Water-Rich Rocky Planets More Irradiated Than the Runaway Greenhouse Limit,” Martin Turbet et al., 2020 June 9, Astronomy & Astrophysics (Preprint on arxiv.org)