Second generation planets

TL;DR

This paper explores the possibility of second generation planet formation in evolved binary systems, highlighting unique environments and observational signatures that distinguish them from first generation planets.

Contribution

It introduces the concept of second generation planets forming from accretion disks in evolved binary systems and discusses their potential observational signatures and candidate systems.

Findings

Second generation planets can form in environments inaccessible to first generation planets.

Such planets may be found in white dwarf binaries and globular clusters.

Candidate systems include PSR B1620-26, Gl 86, and HD 27442.

Abstract

Planets are typically thought to form in protoplanetary disks left over from protostellar disk of their newly formed host star. However, an additional planetary formation route may exist in old evolved binary systems. In such systems stellar evolution could lead to the formation of symbiotic stars, where mass transferred from the expanding evolved star to its binary companion could form an accretion disk around it. Such a disk could provide the necessary environment for the formation of a second generation of planets in both circumstellar or circumbinary configurations. Pre-existing first generation planets surviving the post-MS evolution of such systems may serve as seeds for, and/or interact with, the second generation planets, possibly forming atypical planetary systems. 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. In addition planets could form in metal poor environments such as globular clusters and/or in double compact object binaries. Observations of planets in such forbidden or unfavorable regions may serve to uniquely identify their second generation character. Finally, we point out a few observed candidate second generation planetary systems, including PSR B1620-26 (in a globular cluster), 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.