A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system
Creators: name: Bonomo, Aldo S.
name: Zeng, Li
name: Damasso, Mario
name: Leinhardt, Zoë M.
name: Justesen, Anders B.
name: Lopez, Eric
name: Lund, Mikkel N.
name: Malavolta, Luca
name: Aguirre, Victor Silva
name: Buchhave, Lars A.
name: Corsaro, Enrico
name: Denman, Thomas
name: Lopez-Morales, Mercedes
name: Mills, Sean M.
name: Mortier, Annelies
name: Rice, Ken
name: Sozzetti, Alessandro
name: Vanderburg, Andrew
name: Affer, Laura
name: Arentoft, Torben
name: Benbakoura, Mansour
name: Bouchy, François
name: Christensen-Dalsgaard, Jørgen
name: Cameron, Andrew Collier
name: Cosentino, Rosario
name: Dressing, Courtney D.
name: Dumusque, Xavier
name: Figueira, Pedro
name: Fiorenzano, Aldo F. M.
name: García, Rafael A.
name: Handberg, Rasmus
name: Harutyunyan, Avet
name: Johnson, John A.
name: Kjeldsen, Hans
name: Latham, David W.
name: Lovis, Christophe
name: Lundkvist, Mia S.
name: Mathur, Savita
name: Mayor, Michel
name: Micela, Giusi
name: Molinari, Emilio
name: Motalebi, Fatemeh
name: Nascimbeni, Valerio
name: Nava, Chantanelle
name: Pepe, Francesco
name: Phillips, David F.
name: Piotto, Giampaolo
name: Poretti, Ennio
name: Sasselov, Dimitar
name: Ségransan, Damien
name: Udry, Stéphane
name: Watson, Chris
Publisher: name: arXiv
Year: 2019
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii ($R_\oplus$) range from low-density sub-Neptunes containing volatile elements to higher density rocky planets with Earth-like or iron-rich (Mercury-like) compositions. Such astonishing 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. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux or giant impacts. Although there is some evidence for the former, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets of the compact and near-resonant system Kepler-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 as dense (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 core fraction than Kepler-107b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping, which match the mass and radius of Kepler-107c.
description: Published in Nature Astronomy on 4 February 2019, 35 pages including Supplementary Information material