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RE: Second generation planets
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We think of stars as having just one shot at forging planets - a narrow window when the infant stars are surrounded by a disc of dust and gas. Now it seems paired stars may regularly spawn two or even three generations of planets.
The mechanism for this, proposed by Hagai Perets at the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, is simple, if somewhat macabre. A first clutch of planets would form as normal from a disc around one or both of the young stars.
When one of the stars dies, it sheds material that then forms a disc around its surviving partner, providing the building blocks for a second generation of planets. Such discs have already been observed.

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Title: Second generation planets
Authors: Hagai B. Perets

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 favourable, 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 unfavourable 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.

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