Second generation planets 7 however, to second generation exo-planets, and could be possible in first generation planetary systems hosted by binaries (e.g. Marzari et al. 2005). Infall of planets into their host stars may spin them up; stars in WD binary systems may therefore show higher rotational velocities. This possible signature, however, could also be induced by the material accreted from the second generation disk, rather than a planet infall. Given the possible misalignment between the second generation disk and the pre-existing planetary system, unique disk-planets configurations could be produced. Planets misaligned with the gaseous disk may both af- fect the disk through warping, and could be affected by it (Marzari & Nelson 2009). Small relative inclinations are likely to be damped through the planets interactions with the disk, re-aligning the planets (Cresswell et al. 2007). Observations of two co-existing misaligned plan- etary systems or even counter rotating planets in the same system could, in principle, serve as a spectacular example of second generation planetary formation with pre-existing first generation planets. However, dynami- cal interactions in multiplanetary systems could possibly produce somewhat similar effects, although likely not the counter rotating configurations (Chatterjee et al. 2008; Jurić & Tremaine 2008). Mass loss from the donor star in the binary can result in the dynamical evolution of relatively wide orbits into even wider orbits, due to the reduced gravitational poten- tial of the system (see section 3.1). Alternatively, close binaries could become even closer due to gas drag and common envelope evolution. The change in the dynami- cal configuration of the binary could therefore have major impact on the evolution of pre-existing planets, not dis- cussed above (where it was assumed that the binary sys- tem have already evolved to a stable configuration, and only planetary formation and evolution processes take place). Pre-existing stable planetary systems could be destabilized at this stage due to the migration of the host- ing stars, involving direct scattering by the stars and/or going through resonant interactions, similar, but with a greater amplitude, to asteroids and Kuiper belt objects in the Solar system, thought to scatter resonantly due to planet migration. These complex issues are beyond the scope of this paper, and will be explored elsewhere. Finally, we note in passing that in binaries of small separations (a few AU) even very low mass companions such as brown dwarfs or even massive planets (rather than a stellar binary companion) could develop accre- tion disks, under favorable conditions, such as low wind velocities. Such disks would have very low masses; nev- ertheless, they might suffice for the formation of a new generation of moons or rings around these planets. Few planetary systems and debris disks have been found to exist around compact objects, such as the pulsar planets(Wolszczan & Frail 1992), debris disks around the neutron star (Wang et al. 2006), planets around evolved stars (Silvotti et al. 2007; Lee et al. 2009) and the pos- sible planets observed around WDs (Qian et al. 2009, 2010a,b). These systems are generally thought to ei- ther form in a fall back disk of material following the post-MS evolution of the progenitor star (e.g. fall back From the above discussion we can try to summarize and formulate basic expectations regarding second gen- erations planetary systems and their host stars. These could serve both to identify the second generation origin of observed planetary systems and to provide guidelines and directions for targeted searches for second generation planets. This alternative scenario for the formation of plane- tary systems around compact objects suggests a differ- ent interpretation for planets in compact object system; observations of such planetary systems might not reflect the survival of planets through post-MS evolution of their host star, but rather their formation at even a later stage from the ashes of a companion star. Such systems (e.g. Schmidt et al. 2005; Maxted et al. 2006; Silvotti et al. 2007; Geier et al. 2009) should therefore be targets for searches of compact object companions. Note, however, that formation of sdB stars may require the existence of a close companion (see Heber 2009, and references therein), such as these observed planets. This, in turn, would suggest that a planet have already been in place prior to the sdB star formation in these systems. Mass transfer in binaries poses an alternative and ro- bust way of providing disk material to compact objects. Second (or possibly third) generation planets could there- fore naturally form around many compact objects, and should typically exist around compact objects in doubly compact binary systems. Such systems may show differ- ences relative to planets formed around MS stars due to the quite different radiation from the hosting (compact) star, as well as the difference of their magnetic fields. Some studies explored planet formation around neutron stars (see Sigurdsson et al. 2008, for a recent review) and WDs (Livio et al. 1992), but these issues and the is- sue of binarity in these systems are yet to be studied in more depth. In this context Tavani & Brookshaw (1992); Banit et al. (1993) made some pioneering efforts relating to planet formation in NS binaries, although focusing on pulsar planets and the evaporation of stellar companion to the NS as a source for protoplanetary disk material. Much more directly related are the studies done later by Livio et al. (1992) and Beer et al. (2004) in order to explain the pulsar planet PSR B1620−26 (see section 7). material from a supernova; Lin et al. 1991, bus see also Livio et al. 1992); or form around the MS star and sur- vive the post-MS evolution stage of the star. 1. Second generation planets should exist in evolved binary systems, most frequently in WD-MS or WD- WD binaries. Such systems should therefore be the prime targets for second generation planetary sys- tems searches. Since WD binaries are relatively frequent(Holberg 2009), second generation planets could be a frequent phenomena (with the caution that our current understanding of planet forma- tion, especially in such systems, is still very lim- ited). Evolved binary systems may show a differ- ent frequency of hosting planetary systems (around the MS star in a binary MS-WD systems) than MS binaries with similar orbital properties. Planet hosting compact objects are more likely to be part of double compact object binaries rather than be 6. OBSERVATIONAL EXPECTATIONS 5. SECOND/THIRD GENERATION PLANETS FORMATION AROUND COMPACT OBJECTS AND EVOLVED STARS