2 Perets, H. B. Fig. 1.— Second (and third) generation planet formation. The various stages of second generation planet formation are shown schematically. (a) The initial configuration: a binary MS system, possibly having a first generation (I) circumstellar planet around the lower mass on an allowed (stable) orbit. (b) The higher mass stars evolves to the AGB phase, and sheds material which is ac- creted on the secondary, and forms a protoplanetary disk. The bi- nary orbit expands, and the allowed stability region expands with it. The existing first generation planet may or may not survive this stage (see text). (c) Second generation debris disk and planets are formed, in regions previously forbidden for planet formation (in the pre-evolved system; panel a). (d) The secondary evolves off the MS, and sheds material to its now WD companion. A protoplane- tary disk forms. The binary orbit and the planetary allowed region (around the WD) further expand. Second generation planets may or may not survive this stage (see text). (e) Third generation debris disk and planets are formed around the WD, in regions previously forbidden for planet formation (in the pre-evolved system; panel a). expectations for such second generation planetary sys- tems as suggested from our discussion (§6). Finally we suggest several planetary systems as being candidate sec- ond generation planetary systems (§7) and then conclude (§8). The first stage in planet formation would be setting the initial conditions of the protoplanetary disk in which the planets could form. We therefore first try to understand whether appropriate disks could be produced following a mass transfer epoch during the stellar evolution of a binary. Studies of planet formation in binary stars sug- gest that the binary separation should be large, in order for planets to be formed and produce a planetary sys- tem around one of the binary stellar components (see Haghighipour 2009, for a review). This is also consistent with the observational picture in which the smallest sep- 2. SECOND GENERATION PROTOPLANETARY DISKS AGB stars can lose a large fraction of their mass during to their companion. Given a simple estimate, a fraction of ( R$_{a}$/ 2 a ) 2 of the mass, where R$_{a}$ is the radius of the Bondi-Hoyle accretion cylinder (i.e. gas having impact parameter b < R$_{a}$ = 2 GM 2 /v 2 $_{r}$), is transferred to com- panion accretion disk. The total mass transferred from the AGB could therefore range between ∼ 0 . 1 − 20 per- cents of the total mass lost from the AGB star (for wind velocities of 5 − 15 kms − 1 and separation between 10 − 40 AU; with the smallest separation and largest mass corre- where M 1 and M 2 are the masses of the donor and accret- ing stars, respectively. R 2 is the radius of the accreting star, and a is the semi-major axis of the binary. v$_{r}$ is the relative velocity of the wind and the accretor, η is the pa- rameter indicating the reduction in the specific angular momentum of the accreted gas caused by the increase in the cross-section for accretion from the low-density side (where η ∼ 0 . 1 and η ∼ 0 . 3 for isothermal and adiabatic flows, respectively; Livio et al. 1986), and where they ap- proximate v$_{r}$ to be a constant equal to 15 km s − $^{1}$. From this condition, it turns out that a disk around a main sequence star with R 2 ∼ R ⊙ , could be formed up to or- bital separations of a ∼ 37 AU; for a WD accretor an orbital separation of even a ∼ 60 AU is still possible. Note, however, the strong dependence on the wind ve- locity, which could produce a few times wider or shorter accreting systems for the range of 5 − 20 km s − 1 pos- sible in these winds (e.g. Kenyon 1986; Habing 1996). Since the peak of the typical binary separation distri- bution is at this separation range (Duquennoy & Mayor 1991), we conclude that a non negligible fraction of all bi- naries should evolve to form accretion disks during their evolution. 1 < j$_{a}$ j 2 ≃ 7 . 2 ( η 0 . 2 ) ( M 1 + M 2 2 . 5 M ⊙ )( M 2 M ⊙ ) 3 / 2 × ( R 2 R ⊙ ) − 1 / 2 ( a 10 AU ) − 3 / $^{2}$( v$_{r}$ 15 kms − 1 ) − 4 (1) In order to understand the conditions for the formation of circumstellar disks from wind accreted material, we follow Soker & Rappaport (2000). For a disk to form around a star (or compact object), one requires that that j$_{a}$ > j 2 , where j$_{a}$ is the specific angular momentum of the accreted material, and j 2 = ( GM 2 R 2 ) 1 / 2 is the specific angular momentum of a particle in a Keplerian orbit at the equator of the accreting star of radius R 2 . For typical values for a MS accretor and a mass-losing terminal AGB star they find the following condition for the formation of a disk arations for a planets hosting binaries are of the order of ~20 AU (Eggenberger et al. 2004). We therefore mostly focus on relatively wide wind accreting binaries in §2.1. Nevertheless, mass transfer in close binaries could pro- duce circumbinary disks of material, possibly serving as an environment for formation of circumbinary planets, we briefly discuss this latter more complicated possibil- ity in §2.2. We then shortly also discuss the composition of second generation disks. 2.1. Circumstellar disks in wide binaries GLYPH<1> GLYPH<2> GLYPH<3> GLYPH<4> I II III GLYPH<5>GLYPH<6> GLYPH<5>GLYPH<6> GLYPH<7>GLYPH<8>GLYPH<9> GLYPH<7>GLYPH<8>GLYPH<9> GLYPH<10>GLYPH<11> GLYPH<10>GLYPH<11> GLYPH<10>GLYPH<11> GLYPH<10>GLYPH<11> GLYPH<5>GLYPH<6> GLYPH<5>GLYPH<6> GLYPH<12>GLYPH<13>GLYPH<3>GLYPH<14>GLYPH<15>GLYPH<16> GLYPH<1>GLYPH<17>GLYPH<18>GLYPH<2>GLYPH<19>GLYPH<4>GLYPH<20> GLYPH<11>GLYPH<15>GLYPH<2>GLYPH<3>GLYPH<14>GLYPH<21>GLYPH<14>GLYPH<15>GLYPH<22>GLYPH<16> GLYPH<13>GLYPH<1>GLYPH<23>GLYPH<14>GLYPH<24>GLYPH<19> GLYPH<25>GLYPH<21>GLYPH<2>GLYPH<19>GLYPH<1>GLYPH<15>GLYPH<16>GLYPH<23>GLYPH<1>GLYPH<19>GLYPH<1>GLYPH<13>GLYPH<2>GLYPH<15>GLYPH<14>GLYPH<24>GLYPH<19>GLYPH<20> II II III GLYPH<12>GLYPH<13>GLYPH<3>GLYPH<14>GLYPH<15>GLYPH<16> GLYPH<1>GLYPH<17>GLYPH<18>GLYPH<2>GLYPH<19>GLYPH<4>GLYPH<20> GLYPH<26>