We’ve talked a few times on Daily Space about sub-Neptunes and super-Earths, and Astronomy Cast did an entire episode on them late last year. We don’t have either type of planet in our solar system, obviously, but we keep finding them everywhere else. However, there’s this odd gap in sizing where super-Earths disappear and before sub-Neptunes appear. Astronomers at the University of Hawaiʻi at Mānoa’s Institute for Astronomy decided not to “mind the gap” and went looking for an explanation instead.
It’s hard to study planetary formation and evolution due to the massive time scales needed for observations, so as we did with galaxies last week, researchers here looked at lots of different stars and their planets to get as many snapshots of the process as possible. The study, which was published in the Astronomical Journal, used data from NASA’s Kepler and ESA’s Gaia missions to put together a population of planets. Researchers found that sub-Neptunes gradually lose their atmosphere over billions of years, eventually leading them into super-Earth size. This atmospheric loss is due to intense light from host stars.
Lead author Travis Berger explains: The fact that we see planet sizes change on billion-year timescales suggests that there is an evolutionary pathway, where highly-illuminated sub-Neptune-sized planets transition to becoming super-Earth-sized planets.
“The Gaia–Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age,” Travis A. Berger et al., 2020 August 12, The Astronomical Journal