Against all odds? Forming the planet of the HD196885 binary

TL;DR

This study examines how a planet in the extreme binary system HD196885 challenges existing planet formation theories, especially during the sensitive accretion phase of planetesimals, and explores potential explanations for its formation.

Contribution

It provides the first detailed numerical analysis of planetesimal impact velocities in a highly perturbed binary environment, highlighting the difficulties of planet formation in such regions.

Findings

Most of the circumprimary disc is hostile to planetesimal accretion.

At 2.6AU, impact velocities exceed 1 km/s, hindering growth.

Multiple hypotheses are proposed to explain the planet's existence despite hostile conditions.

Abstract

HD196885Ab is the most "extreme" planet-in-a-binary discovered to date, whose orbit places it at the limit for orbital stability. The presence of a planet in such a highly perturbed region poses a clear challenge to planet-formation scenarios. We investigate this issue by focusing on the planet-formation stage that is arguably the most sensitive to binary perturbations: the mutual accretion of kilometre-sized planetesimals. To this effect we numerically estimate the impact velocities $dv$ amongst a population of circumprimary planetesimals. We find that most of the circumprimary disc is strongly hostile to planetesimal accretion, especially the region around 2.6AU (the planet's location) where binary perturbations induce planetesimal-shattering $dv$ of more than 1km/s. Possible solutions to the paradox of having a planet in such accretion-hostile regions are 1) that initial planetesimals were very big, at least 250km, 2) that the binary had an initial orbit at least twice the present one, and was later compacted due to early stellar encounters, 3) that planetesimals did not grow by mutual impacts but by sweeping of dust (the "snowball" growth mode identified by Xie et al., 2010b), or 4) that HD196885Ab was formed not by core-accretion but by the concurent disc instability mechanism. All of these 4 scenarios remain however highly conjectural.