Exploring formation scenarios for the exomoon candidate Kepler 1625b I

TL;DR

This study investigates whether the candidate exomoon Kepler 1625b I could have formed in-situ, using N-body simulations and considering tidal evolution, challenging the idea that it was captured.

Contribution

The paper demonstrates that in-situ formation of Kepler 1625b I is plausible, providing a detailed simulation-based analysis of formation conditions and tidal effects.

Findings

In-situ formation is viable for Kepler 1625b I.

Wider star-planet separations require more solids in the circum-planetary disc.

Tidal evolution significantly influences the final satellite system architecture.

Abstract

If confirmed, the Neptune-size exomoon candidate in the Kepler 1625 system will be the first natural satellite outside our Solar System. Its characteristics are nothing alike we know for a satellite. Kepler 1625b I is expected to be as massive as Neptune and to orbit at 40 planetary radii around a ten Jupiter mass planet. Because of its mass and wide orbit, this satellite was firstly thought to be captured instead of formed in-situ. In this work, we investigated the possibility of an in-situ formation of this exomoon candidate. To do so, we performed N-body simulations to reproduce the late phases of satellite formation and use a massive circum-planetary disc to explain the mass of this satellite. Our setups started soon after the gaseous nebula dissipation, when the satellite embryos are already formed. Also for selected exomoon systems we take into account a post-formation tidal evolution. We found that in-situ formation is viable to explain the origin of Kepler 1625b I, even when different values for the star-planet separation are considered. We show that for different star-planet separations the minimum amount of solids needed in the circum-planetary disc to form such a satellite varies, the wider is this separation more material is needed. In our simulations of satellite formation many satellites were formed close to the planet, this scenario changed after the tidal evolution of the systems. We concluded that if the Kepler1625 b satellite system was formed in-situ, tidal evolution was an important mechanism to sculpt its final architecture.