Abstract
Exoplanets Exoplanets with masses between those of Earth and Neptune come in two types that display a bimodal distribution of densities: “super Earths,” with rocky bodies and small (or no) atmospheres, and “mini Neptunes,” with extended atmospheres of hydrogen and helium surrounding rocky cores. That difference might reflect the removal of light gasses from some of their atmospheres by stellar activity. Gandolfi et al. have discovered two exoplanets orbiting the star HD15337. Planet c is a mini Neptune with a 50% larger radius than planet b, a super Earth. As both planets have identical masses (within the uncertainties) and orbit the same star, studying them will help explain the atmospheric loss processes that lead to these planet types. Astrophys. J. Lett. 876 , L24 (2019).
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