A giant impact as the likely origin of different twins in a compact exoplanet system

Abstract

Understanding the origin of the astonishing diversity in the composition of small planets (R p < 3 Earth radii) is one of the most important issues in exoplanetary science. Discoveries range from low-density sub-Neptunes containing volatile elements 1 to higher density rocky planets with Earth-like 2 or iron-rich 3 (Mercury-like) compositions. The diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet formation process and/or different evolutionary paths that altered the planetary properties after formation 4 . Planet evolution may be especially affected by photo-evaporative mass loss induced by high stellar X-UltraViolet (XUV) flux 5 or giant impacts 6 . While there is some evidence for the former 7,8 , there are no unambiguous findings to date about the occurrence of giant impacts in an exoplanet system. Here we report on the characterization of the compact near-resonant system Kepler-107 9 in which the two innermost planets Kepler-107b and Kepler-107c have orbital periods of 3.2 and 4.9 days, nearly identical radii (R p =1.5-1.6 Earth radii), but different densities (ρ p ~5.3 and 12.6 g cm -3 ). In consequence, Kepler-107c must have a larger iron core fraction than Kepler-107b. This feature cannot be explained by the stellar XUV irradiation but is consistent with a giant impact event on Kepler-107c. Such an impact would have stripped off part of the silicate mantle of Kepler-107c whose radius and mass indeed match the theoretical predictions from collisional mantle stripping 10 . This is the first evidence of planetary bulk density diversity in the same system that has likely been generated by a giant impact event.