A new class of Super-Earths formed from high-temperature condensates: HD219134 b, 55 Cnc e, WASP-47 e

Abstract

We hypothesize that differences in the temperatures at which the rocky material condensed out of the nebula gas can lead to differences in the composition of key rocky species (e.g. Fe, Mg, Si, Ca, Al, Na) and thus planet bulk density. Such differences in the observed bulk density of planets may occur as a function of radial location and time of planet formation. In this work, we show that the predicted differences are on the cusp of being detectable with current instrumentation. In fact, for HD 219134, the 10 per cent lower bulk density of planet b compared to planet c could be explained by enhancements in Ca-, Al-rich minerals. However, we also show that the 11 per cent uncertainties on the individual bulk densities are not sufficiently accurate to exclude the absence of a density difference as well as differences in volatile layers. Besides HD 219134 b, we demonstrate that 55 Cnc e and WASP-47 e are similar candidates of a new Super-Earth class that have no core and are rich in Ca and Al minerals that are among the first solids that condense from a cooling proto-planetary disc. Planets of this class have densities 10-20 per cent lower than Earth-like compositions and may have very different interior dynamics, outgassing histories, and magnetic fields compared to the majority of Super-Earths.