Planet formation and long-term stability in a very eccentric stellar binary

TL;DR

This study investigates the long-term stability and formation scenarios of a planet in a highly eccentric binary system, revealing that retrograde orbits can survive gravitational perturbations, challenging conventional planet formation models.

Contribution

It demonstrates that planets in eccentric binaries can remain stable if on retrograde orbits, and explores their possible formation histories, including capture or in situ formation.

Findings

Retrograde orbit stabilizes the planet against binary perturbations.

Direct N-body simulations show the planet's tendency to collide or be ejected without retrograde orbit.

The planet likely formed closer to the star or was captured after formation.

Abstract

Planets orbiting one of the two stars in a binary are vulnerable to gravitational perturbations from the other star. Particularly, highly eccentric companion stars risk disrupting planetary orbits, such as in the extreme system TOI 4633 where close encounters between the companion and a gas giant planet in the habitable zone make it one of the most fragile systems discovered so far. Here, we report that TOI 4633's planet likely survived these encounters throughout the system's age by orbiting retrograde relative to the binary, stabilised by the Coriolis force. Using direct $N$-body simulations, we show it otherwise tends to collide with the binary stars or becomes free-floating after getting ejected. A retrograde planetary orbit has profound implications for TOI 4633's formation and evolution, suggesting an extraordinary history where its eccentric companion was likely randomly captured after planet formation in a single-star system. Alternatively, if stars and planet are born in situ from the same gas clump, we show the planet must have formed at sub-snow-line distances, contrary to the conventional core-accretion model. Our study highlights the importance of considering the long-term stability ($\gtrsim\rm Gyr$) of planets in eccentric binaries and demonstrates that the mere existence in such dynamically hostile environments places strong constraints on their orbital configuration and formation.