Planet formation in Binaries

TL;DR

This review explores how binary star systems influence various stages of planet formation, highlighting challenges in planetesimal accretion and habitability, and discusses potential solutions to formation paradoxes in tight binaries.

Contribution

It provides a comprehensive overview of the effects of binary environments on planet formation stages, emphasizing the sensitivity of planetesimal accretion and discussing habitability complexities.

Findings

Binary perturbations affect different planet formation stages variably.

Planetary formation in tight binaries faces challenges explaining observed exoplanets.

Binary environments complicate assessments of planetary habitability.

Abstract

Spurred by the discovery of numerous exoplanets in multiple systems, binaries have become in recent years one of the main topics in planet formation research. Numerous studies have investigated to what extent the presence of a stellar companion can affect the planet formation process. Such studies have implications that can reach beyond the sole context of binaries, as they allow to test certain aspects of the planet formation scenario by submitting them to extreme environments. We review here the current understanding on this complex problem. We show in particular how each of the different stages of the planet-formation process is affected differently by binary perturbations. We focus especially on the intermediate stage of kilometre-sized planetesimal accretion, which has proven to be the most sensitive to binarity and for which the presence of some exoplanets observed in tight binaries is difficult to explain by in-situ formation following the "standard" planet-formation scenario. Some tentative solutions to this apparent paradox are presented. The last part of our review presents a thorough description of the problem of planet habitability, for which the binary environment creates a complex situation because of the presence of two irradation sources of varying distance.