Making super Earths

Now that almost 4000 exoplanets have been found, it’s more and more clear that our solar system is atypical. The two largest groups of known exoplanets are super-Earths and sub-Neptunes that orbit their stellar hosts more tightly than Mercury orbits the Sun. The two populations could be related. One plausible way for a super-Earth to form is to start with a sub-Neptune and wait for the star’s UV emission to burn away the planet’s gaseous shell of hydrogen and helium and leave a large rocky core. Howard Chen of Northwestern University and John Forbes and Abraham Loeb of Harvard University realized that our galaxy has another source of atmosphere-stripping UV: the 4-million-solar-mass black hole at the center of the galaxy. The black hole is currently quiescent, but if it had been active in the past, it could have transformed planets, at least in the galactic center. To evaluate that possibility, Chen, Forbes, and Loeb modeled the evolution of sub-Neptunes that have silica-iron-magnesium cores surrounded by an envelope of H and He. For typical levels of black hole activity, the researchers found that most of the sub-Neptunes within 70 light-years of the center were converted to super-Earths. Although that region is small compared with the galaxy’s 100 000 light-year diameter, it has a high density of stars. Could all those super-Earths be teeming with life? The UV emission from an active black hole likely leaves a super-Earth devoid of atmosphere. But once the black hole has quieted, a second atmosphere, supplied by colliding planetesimals or the planet’s volcanism, could develop. A super-Earth is massive enough to retain it. (H. Chen, J. C. Forbes, A. Loeb, Astrophys. J. Lett. 855, L1, 2018 .)