Planets in evolved binary systems

TL;DR

This paper explores the possibility of second-generation planet formation in evolved binary systems, especially around white dwarfs, due to stellar evolution processes creating new environments for planet formation.

Contribution

It introduces the concept that binary stellar evolution can lead to second-generation planets, expanding the understanding of planetary system formation beyond initial protoplanetary disks.

Findings

Second-generation planets may form in environments inaccessible to first-generation planets.

Observational signatures could distinguish second-generation planets from first-generation ones.

Candidate systems like Gl 86 and HD 27442 may host second-generation planets.

Abstract

Exoplanets are typically thought to form in protoplanetary disks left over from protostellar disk of their newly formed host star. However, additional planetary formation and evolution routes may exist in old evolved binary systems. Here we discuss the implications of binary stellar evolution on planetary systems. In these binary systems stellar evolution could lead to the formation of symbiotic stars, where mass is lost from one star and could be transferred to its binary companion, and may form an accretion disk around it. This raises the possibility that such a disk could provide the necessary environment for the formation of a new, second generation of planets in both circumstellar or circumbinary configurations. Pre-existing first generation planets surviving the post-MS evolution of such systems would be dynamically effected by the mass loss in the systems and may also interact with the newly formed disk. Second generation planetary systems should be typically found in white dwarf binary systems, and may show various observational signatures. Most notably, second generation planets could form in environment which are inaccessible, or less favorable, for first generation planets. The orbital phase space available for the second generation planets could be forbidden (in terms of the system stability) to first generation planets in the pre-evolved progenitor binaries. Observations of exo-planets in such forbidden or unfavorable regions could possibly serve to uniquely identify their second generation character. Finally, we point out a few observed candidate second generation planetary systems, including Gl 86, HD 27442 and all of the currently observed circumbinary planet candidates. A second generation origin for these systems could naturally explain their unique configurations.