Planet formation in Alpha Centauri A revisited: not so accretion-friendly after all

TL;DR

This study uses numerical simulations to show that planet formation around Alpha Centauri A is hindered beyond 0.5AU due to high impact velocities preventing accretion, challenging previous assumptions about planet formation in this binary system.

Contribution

The paper provides a detailed numerical analysis of the planetesimals-to-embryos phase in a binary system, highlighting the inhibitory effects of binary perturbations on planet formation.

Findings

Accretion is inhibited beyond 0.5AU in the nominal gas disc case.

Impact velocities are too high for mutual accretion in the affected region.

Planet formation is unlikely unless starting from very large planetesimals (>30km).

Abstract

We numerically explore planet formation around alpha Cen A by focusing on the crucial planetesimals-to-embryos phase. Our code computes the relative velocity distribution, and thus the accretion vs. fragmentation trend, of planetesimal populations having any given size distribution. This is a critical aspect of planet formation in binaries since the pericenter alignment of planetesimal orbits due to the gravitational perturbations of the companion star and to gas friction strongly depends on size. We find that, for the nominal case of a MMSN gas disc, the region beyond 0.5AU from the primary is hostile to planetesimal accretion. In this area, impact velocities between different-size bodies are increased, by the differential orbital phasing, to values too high to allow mutual accretion. For any realistic size distribution for the planetesimal population, this accretion-inhibiting effect is the dominant collision outcome and the accretion process is halted. Results are robust with respect to the profile and density of the gas disc: except for an unrealistic almost gas-free case, the inner accretion safe area never extends beyond 0.75AU. We conclude that planet formation is very difficult in the terrestrial region around alpha Cen A, unless it started from fast-formed very large (>30km) planetesimals. Notwithstanding these unlikely initial conditions, the only possible explanation for the presence of planets around 1 AU from the star would be the hypothetical outward migration of planets formed closer to the star or a different orbital configuration in the binary's early history. Our conclusions differ from those of several studies focusing on the later embryos-to-planets stage, confirming that the planetesimals-to-embryos phase is more affected by binary perturbations.