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suggest that terrestrial planets may form even in closer systems, near the host star (up to ~0.2 q$_{b}$ , where q$_{b}$ is the stellar binary pericenter distance; Quintana et al. 2007). Indeed the smallest separation observed for planet host- ing MS binaries is of ~20 AU. We can conclude that first generation circumstellar giant planets are not likely to form in the binary system considered here. In our example the pericenter distance of the initial system is q = a$_{b}$ (1 − e ) = 8 . 4 AU, i.e. planets, and especially gas giants, are not likely to form in such system. 

Back to our example, at a later epoch, the more mas- sive stellar component in this system evolves off the main sequence to end its life as a WD of ~0.65 M ⊙ (see e.g. Lagrange et al. 2006). Due to the adiabatic mass loss from the system it may evolve to a larger final separa- tion, given by (e.g. Lagrange et al. 2006, and references therein) 

where m$_{f}$ (= 0 . 8 + 0 . 65 = 1 . 45 M ⊙ ) is the final mass of the system after its evolution, and m$_{i}$ (= 1 . 6 + 0 . 8 = 2 . 4 M ⊙ ) and a$_{i}$ (= a$_{b}$ = 12 AU ) are the initial mass and separation of the system, respectively (where the ec- centricity does not change in this case). We now find a$_{f}$ = 2 . 4 / 1 . 45 a$_{i}$ = 19 . 8 AU [more detailed calculations, using the binary evolution code by Hurley et al. (2002) give a similar scenario]. At such a separation even cir- cumstellar giant planets could now form in the system (Kley & Nelson 2008), i.e. second generation planets could form in in this binary either at a few AU around the star, or closer if they migrated after their forma- tion). Therefore, any circumstellar giant planet observed around the MS star in this system would imply that such a planet must be a second generation planet, since it could not have formed as a first generation planet in the pre-evolved system. Thus, such cases could serve as a smoking gun signature and a unique tracer for the existence and identification of second generation plan- ets. In fact, the example chose here is not arbitrary. The final configuration of the system in this example is very similar to that of the planetary system observed in the WD binary system Gl 86. A giant planet of 4 M$_{Jup}$ have been found at ∼ 0 . 1 AU from its MS host star, which has a WD companion, at a separation of ∼ 18 AU (Mugrauer & Neuhäuser 2005; Lagrange et al. 2006). The configuration of this system possibly indi- cates that this system is indeed a Bone fide second gen- eration planetary system (see also section 7) . 

In Fig. 2, we use more detailed stellar evolution cal- culations (using the binary stellar evolution code BSE; Hurley et al. 2002) to show a range of final vs. ini- tial semi-major axis of circular binary systems with ini- tial masses M 1 = 0 . 8 M ⊙ and M 2 = 1 . 6 M ⊙ in which the higher mass star evolves to become a WD (of mass ∼ 0 . 6 M ⊙ ) . Also shown in this figure is the critical semi- major axis a$_{c}$ in both the initial (pre-evolved MS-MS binary) and the final (evolved; MS-WD binary) configu- ration. As can be seen, the pre and post evolution critical semi-major axis differ significantly, presenting a region of orbital phase space open only to second generation plan- ets but forbidden for first generation ones. 

## 3.1.2. Circumbinary planets 

<img src='content_image/421915.jpg'>

<img src='content_image/421918.jpg'>

In a sense, the requirements from a stable circumbi- nary planetary system present a mirror image of those needed for a circumstellar system in a binary. The orbital separation of circumbinary planets should be typically a few (>2-4) times the binary separation (Holman & Wiegert 1999; Moriwaki & Nakagawa 2004; Pierens & Nelson 2007, 2008; Haghighipour 2009), in or- der not to be perturbed by the binary orbit. Specifically, Holman & Wiegert find 

<img src='content_image/421916.jpg'>

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. Circumbinary planets are therefore not expected to be observed very close to their host stars. As with circumstellar second generation planets, the or- bital phase space available for second generation plan- ets differs from that of pre-existing first generation stars. The post-MS orbit of a relatively pre-evolved close binary (e.g. separation of 1-2 AU) could be shrunk through it’s evolution due to angular momentum loss in a common envelope or circumbinary disk phase [e.g. Ritter (2008) in the context of forming cataclysmic variables]. Second generation planets could therefore form much closer to such evolved binaries than any pre-existing first genera- tion planets. Therefore, similar to the circumstellar case 

