Some exoplanets may be covered in weird water that’s between liquid and gas

Small worlds around other stars may come in more than two varieties.

Using exoplanet densities, astronomers have largely sorted planets that are bigger than Earth but smaller than Neptune into two categories: denser, rocky super-Earths and larger, puffy mini-Neptunes (SN: 6/19/17). Mini-Neptunes are generally thought to be padded in thick layers of hydrogen and helium gas, like the giant planets in our own solar system. But astronomers have detected clear evidence of hydrogen on only some mini-Neptunes — and, curiously, seen traces of water on others (SN: 5/11/17).

Now, new simulations indicate that some planets that look like gaseous mini-Neptunes could actually be rocky planets covered in superheated oceans, where the water is in an exotic state between liquid and gas. Such extreme saunalike worlds could bridge the divide between rocky and gaseous planet types, researchers report in the June 15 Astrophysical Journal Letters.

Olivier Mousis, a planetary scientist at Aix-Marseille University in France, and colleagues ran simulations of ocean-covered worlds in close orbits around their stars, where mini-Neptunes are often found. Intense stellar radiation would cause water on the planets to puff up into a diffuse layer of “supercritical” water between liquid and gas, topped by a steamy water vapor atmosphere, the team found. On Earth, supercritical water can be used to break down toxic waste.

The puffiness of each simulated planet depended on factors like its water content. But Mousis’ team simulated water worlds with a range of sizes and densities that match nearly all of the hundreds of mini-Neptunes discovered so far. This hints that water-containing mini-Neptunes may be better explained by supercritical oceans than by hydrogen and helium gas layers. For instance, the authors note that the mini-Neptune K2 18b, where water has been spotted, fits the profile of a supercritical ocean planet of about 37 percent water (SN: 9/11/19).

More detailed observations of exoplanets in the future may prove whether this new planetary model holds water.