SECOND GENERATION PLANETS Hagai B. Perets Harvard-Smithsonian Center for Astrophysics, 60 Garden St.; Cambridge, MA, USA 02138 Draft version October 30, 2018 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. Currently, over 400 extra-Solar planetary systems have been found; most of hem around main sequence stars, with a few tens found in wide binary systems. Typi- cally, such planets are thought to form in a protoplan- etary disk left over following the central star formation in a protostellar disk (e.g. Armitage 2007, for a re- cent review). Several studies explored the later effects of stellar evolution on the survival and dynamics of plan- ets around an evolving star (Debes & Sigurdsson 2002; Villaver & Livio 2007) or the possible formation of plan- ets around neutron stars (NSs; see Phillips & Thorsett (1994); Podsiadlowski (1995) for reviews). Others stud- ied the formation and stability of planets in binary sys- tems (see Haghighipour 2009, for a review). In this study we focus on the implications of stellar evolution in bina- ries on the formation and growth of planets. One of the most likely outcomes of stellar evolution in binaries is mass transfer from an evolved donor star [which later becomes a compact object; a white dwarf (WD), NS or a black hole (BH); here we mostly focus on low mass stars which evolve to become WDs] to its binary companion. If the binary separation is not too large, this process could result in the formation of an ac- cretion disk containing a non-negligible amount of mass around the companion. Here we suggest that such a disk could resemble in many ways a protoplanetary disk, and could therefore produce a second generation of planets and debris disks around old stars. In addition, the re- newed supply of material to a pre-existing (’first gener- ation’) planetary system (if such exists after surviving In the following we discuss the role of binary stellar evolution in the formation of a second and third gener- ation of planets in evolving binary systems (a schematic overview of this scenario is given in Fig. 1). We begin by discussing the conditions for the formation of a second generation protoplanetary disk and its properties (§2). We then explore the role of such disks in the formation of new planets (§3), their effects on pre-exiting planetary systems (§4), and on the formation of planets around compact objects (§5) . We then review the observational The later evolution of evolved binaries could, in some cases, even lead to a third generation of planet formation in the system. When the previously accreting star (the lower mass star in the pre-evolved system) goes through its stellar evolution phase, it too can expand to become a mass donor to its now compact object companion. A new disk of material then forms around the compact object and planet formation may occur again, this time around the compact object (see also Beer et al. 2004 which tries to explain the formation the pulsar planet system PSR B1620-26). One could even suggest further next genera- tions of planets in multiple systems (triples, quadruples etc.), in which the post-MS evolution in each of the stel- lar components could provide new material for a another generation of planet formation. Such further generation of planets, however, is less likely, requiring much more fine tuned conditions than those existing more robustly in binary systems. the post-MS evolution of the host star), is likely to have major effects on this system, possibly leading to the re- growth/rejuvenation of the planets and planetesimals in the system as well as possibly introducing a second epoch of planetary migration. 1. INTRODUCTION arXiv:1001.0581v2 [astro-ph.EP] 27 Jan 2010 Draft version October 30, 2018 Preprint typeset using L A T E X style emulateapj v. 11/10/09 hperets@cfa.harvard.edu