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Measures of exoplanet bulk densities indicate that small exoplanets withradius less than 3 Earth radii ($R_\oplus$) range from low-density sub-Neptunescontaining volatile elements to higher density rocky planets with Earth-like oriron-rich (Mercury-like) compositions. Such astonishing diversity in observedsmall exoplanet compositions may be the product of different initial conditionsof the planet-formation process and/or different evolutionary paths thataltered the planetary properties after formation. Planet evolution may beespecially affected by either photoevaporative mass loss induced by highstellar X-ray and extreme ultraviolet (XUV) flux or giant impacts. Althoughthere is some evidence for the former, there are no unambiguous findings so farabout the occurrence of giant impacts in an exoplanet system. Here, wecharacterize the two innermost planets of the compact and near-resonant systemKepler-107. We show that they have nearly identical radii (about$1.5-1.6~R_\oplus$), but the outer planet Kepler-107c is more than twice asdense (about $12.6~\rm g\,cm^{-3}$) as the innermost Kepler-107b (about$5.3~\rm g\,cm^{-3}$). In consequence, Kepler-107c must have a larger iron corefraction than Kepler-107b. This imbalance cannot be explained by the stellarXUV irradiation, which would conversely make the more-irradiated andless-massive planet Kepler-107b denser than Kepler-107c. Instead, thedissimilar densities are consistent with a giant impact event on Kepler-107cthat would have stripped off part of its silicate mantle. This hypothesis issupported by theoretical predictions from collisional mantle stripping, whichmatch the mass and radius of Kepler-107c.

A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system