Collisional formation of massive exomoons of superterrestrial exoplanets
Exomoons orbiting terrestrial or superterrestrial exoplanets have not yet been discovered; their possible existence and properties are therefore still an unresolved question. Here, we explore the collisional formation of exomoons through giant planetary impacts. We make use of smooth particle hydrodynamical collision simulations and survey a large phase space of terrestrial/superterrestrial planetary collisions. We characterize the properties of such collisions, finding one rare case in which an exomoon forms through a graze and capture scenario, in addition to a few graze and merge or hit and run scenarios. Typically however, our collisions form massive circumplanetary discs, for which we use follow-up N-body simulations in order to derive lower limit mass estimates for the ensuing exomoons. We investigate the mass, long-term tidal-stability, composition and origin of material in both the discs and the exomoons. Our giant impact models often generate relatively iron-rich moons that form beyond the synchronous radius of the planet, and would thus tidally evolve outward with stable orbits, rather than be destroyed. Our results suggest that it is extremely difficult to collisionally form currently-detectable exomoons orbiting superterrestrial planets, through single giant impacts. It might be possible to form massive, detectable exomoons through several mergers of smaller exomoons, formed by multiple impacts, however more studies are required in order to reach a conclusion. Given the current observational initiatives, the search should focus primarily on more massive planet categories. However, about a quarter of the exomoons predicted by our models are approximately Mercury-mass or more, and are much more likely to be detectable given a factor 2 improvement in the detection capability of future instruments, providing further motivation for their development.
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Received: 02 April 2019; Revision received: 21 January 2020; Accepted: 21 January 2020; Published: 24 January 2020. We thank the anonymous referee for helpful comments and suggestions that have improved the quality of our manuscript. UM and HBP acknowledge support from the Minerva Center for life under extreme planetary conditions, the Israeli Science and Technology ministry Ilan Ramon grant and the ISF I-CORE grant 1829/12. CS and CB acknowledge support by the high performance and cloud computing group at the Zentrum für Datenverarbeitung of the University of Tübingen, the state of Baden-Württemberg through bwHP, the German Research Foundation (DFG) through grant no INST 37/935-1 FUGG, the FWF Austrian Science Fund project S11603-N16 and appreciate support by the DFG German Science Foundation project no. 398488521.
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