Mini Neptunes may have solid surfaces

Humans know of over 6,000 exoplanets, out of which over 2,000 are similar to Neptunes. Worlds that are smaller than Neptune are expected to make up most of the exoplanets in the galaxy, made up of a mixture of rock and metal with thick atmospheres dominated by hydrogen, helium and maybe water. While these planets are expected to be common in orbits around other stars, scientists understand very little about them because of a lack of similar worlds in our own Solar System. It was previously believed that mini-Neptunes were generally covered in planet-wide oceans of molten magma. New research however indicates that most of them may actually have solid surfaces because of the extreme pressure of the thick atmospheres. 

The launch of the James Webb Space Telescope moved exoplanetary science from an era of discovery, where new exoplanets were found, to an era of characterisation, where it became possible to start better understanding these distant worlds. The masses of the exoplanets can be determined by their gravitational influence on their host stars, and the makeup of the atmospheres can be determined by careful analysis of the light from the system during transits and secondary eclipses. Due to the high temperatures and dense atmospheres, it was previously believed that these worlds were covered by magma oceans, that prevent the growth of the planet. 

Observations of an exoplanet called GJ 1214 b by Webb indicates that the planet's atmosphere may contain larger molecules than hydrogen and helium, indicating a heavy atmosphere. Such atmospheres would create high-temperature, high-pressure conditions on the planet, with rock transitioning to solids, much like carbon condenses into diamond deep into the surface of the Earth. By simulating a range of planets with different conditions, the researchers discovered that a wide variety of mini-Neptunes may actually have solid surfaces. A paper describing the findings has been published in The Astrophysical Journal Letters.