4 Perets, H. B. over, double compact object binaries (WD-WD, WD-NS etc.) may show a larger frequency of planets than single compact objects $^{1}$. Correlation between planetary companions and their host star metallicity could show a difference between WD binary systems and MS binary systems. This may be a weak signature, given that the host star itself may accrete metal rich material from the companion. Nevertheless, the latter case could be an advantage for targeting second planet searches, through looking for them around chemically peculiar stars in WD systems (e.g. Jeffries & Smalley 1996; Jorissen 1999; Bond et al. 2003). The high metallicity of second generation disks originate from the evolved star that produced them and need not be related to the metallicity of their larger scale environments. For this reason, second generation planets can form even in metal poor environments such as glob- ular clusters, or more generally around metal poor stars. This possibility could be tested by searching for WD companions to planet hosting, but metal poor, stars (e.g. Santos et al. 2007) . Reversing the argument, one can di- rect planet searches around metal poor stars (which typ- ically do not find planets; Sozzetti et al. 2009) to look for them in binary systems composed of a metal poor star and a WD. Similarly, planet hosting stars in the metal poor environments of globular clusters are likely to be members of binary WD systems. Note, however, that given their formation in binaries, second generation plan- ets are not expected to exist in the globular cluster cores, where dynamical interactions may destroy even relatively close binaries. Such second generation planetary systems should still be able to exist at the outskirts of globular clusters. Interestingly, the only planet found in a globu- lar cluster (PSR B1620−26; Backer et al. 1993) is a cir- cumbinary planet around a WD-NS binary, found in the outskirts of a globular cluster, as might be expected from our discussion (see section 7 for further discussion of this system). The age of second generation planets, if could be mea- sured, should be inconsistent with and much younger than their stellar host age (such implied inconsistencies could be revealed in some cases, e.g. WASP-18 plane- tary system; Hellier et al. 2009). Their typical composi- tion is likely to show irregularities and be more peculiar and metal rich, relative to that of typical first generation planets. The second generation protoplanetary disk in which second generation planets form does not have to be aligned with the original protostellar disk of the star and its rotation direction, but is more likely to be aligned with the binary orbit (but not necessarily; the accretion disk from the wind of a wide binary may form in a more arbitrary inclination). Studies on the formation and stability of planets in circumbinary orbits (see Haghighipour 2009 for a re- 1 Planets around an evolving donor star could be engulfed by the star and be destroyed. We are not interested in comparing the planet frequency around these donor stars, but rather the planet frequency around the accretor. Specifically, the frequency of plan- ets around the MS star in MS-WD systems should be compared with the planet frequency around either MS stars in MS-MS sys- tems; similarly, the planet frequency around the WDs in WD-WD systems should be compared with planet frequency around the WD in WD-MS systems. where a$_{c}$ is critical semi major axis at which the orbit is still stable, m = M 2 / ( M 1 + M 2) , a$_{b}$ and e$_{b}$ are the semi major axis and eccentricity of the binary, and M1 and M2 are the masses of the primary and secondary stars, respectively. Giant planets are not likely to form in such a system, since such planets are thought to form far from their host star (although they may migrate later on to much smaller separations, e.g. forming hot Jupiters), where icy material is available for the initial growth of their planetary embryos (Pollack et al. 1996). In fact, it is not clear if any type of planet could form under such hostile conditions in which strong disk heating is induced by perturbations from the stellar companion. The small- est binary separations in which planets could form are thought to be ~20 AU for giant planets (Haghighipour 2009, and references therein), although some simulations Let us illustrate this by a realistic example. Con- sider a MS binary with stellar components of 1.6 and 0.8 M ⊙ and a separation of a$_{b}$ = 12 AU on a an or- bit of 0.3 eccentricity. The secondary protoplanetary disk in this binary would be truncated at about 2- 2.5 AU in such a system (Artymowicz & Lubow 1994), and a planetary orbit would become unstable at simi- lar separations (Holman & Wiegert 1999). Specifically, Holman & Wiegert find Stable circumstellar planetary systems in binaries could be limited to a close separation to their host star, since planets may be prohibited from forming at wider orbits or become unstable at such orbits which are more susceptible to the perturbations from the stellar binary companion (Haghighipour 2009, and references therein). During the post-MS evolution of a wide binary, its orbit typically widens (due to mass loss), therefore allowing for planets to form and survive at wider circumstellar orbits. This larger orbital phase space, however, is open only to second generation planets formed after the post-MS evo- lution of the binary (see also Thébault et al. 2009, for a somewhat related discussion on a dynamical evolution of such forbidden zone due to perturbations in a stellar cluster). view) suggest that they can form and survive in such sys- tems. Recently, several circumbinary planets candidates have been found (Qian et al. 2009, 2010a,b; Lee et al. 2009), possibly confirming such theoretical expectations. Circumbinary second generation disks such as discussed above could therefore serve, in principle, as nurseries for the formation and evolution of second generation planets. The evolution of second generation planets in circum- stellar and circumbinary disk could be very different. In the following we highlight some important differences be- tween the types of orbits possible for second generation planets, and the way in which these could serve as a smoking gun signature for the effects and evolution of second generation planets and/or disks. 3.1. Orbital phase space of second generation planets 3.1.1. Circumstellar planets a$_{c}$/a$_{b}$ = (0 . 464 ± 0 . 006) + ( − 0 . 38 ± 0 . 01) µ + ( − 0 . 631 ± 0 . 034) e$_{b}$ + (0 . 586 ± 0 . 061) µe$_{b}$ + (0 . 15 ± 0 . 041) e 2 b + ( − 0 . 198 ± 0 . 047) µe 2 $_{b}$, (2)