Planet Formation in Binaries

Despite a wealth of articles on planets in binary star systems, the process of the formation of these objects is still poorly understood. The current theories of planet formation focus only on the formation of planets in a circumstellar disk around a single star, and their extensions to binary environments are limited to either the Sun-Jupiter system, where the focus is on the effect of Jupiter on the formation of inner planets of our solar system (Heppenheimer, 1974, 1978; Drobyshevski, 1978; Diakov & Reznikov, 1980; Whitmire et al., 1998; Kortenkamp, Wetherill & Inaba, 2001), or binaries resembling some of extrasolar planets, in which the secondary star has a mass in the brown dwarf regime (Whitmire et al., 1998). Although attempts have been made to extend such studies to binaries with comparable-mass stellar components (Marzari & Scholl, 2000; Nelson, 2000; Barbieri, Marzari & Scholl, 2002; Quintana et al., 2002; Lissauer et al., 2004), the extent of the applicability of the results of these studies has been only to hypothetical cases since, until recently, there had been no observational evidence on the existence of such binary-planetary systems.

In general, it is believed that planet formation proceeds through the following four stages (Fig. 9.11):

1) coagulation of dust particles and their growth to centimeter-sized objects,

2) collisional growth of centimeter-sized particles to kilometer-sized bodies (plan-etesimals),

3) formation of Moon- to Mars-sized protoplanets (also known as planetary embryos) through the collision and coalescence of planetesimals, and

4) collisional growth of planetary embryos to terrestrial-sized objects.

The latter is a slow process that may take a few hundred million years. During the first few million years of this process, at larger distances from the star, planetesimals and planetary embryos may form planetary cores several times more massive than Earth, and may proceed to form giant planets.

In a binary star system with a moderate to small separation, the secondary star will have significant effects on the efficiency of each of these processes. As shown by Boss (2006), a binary companion can alter the structure of a planet-forming nebula, and create regions where the densities of the gas and dust are locally enhanced (Fig. 9.12). Also, as shown by Artymowicz & Lubow (1994), and Pichardo, Sparke & Aguilar (2005), a stellar component on an eccentric orbit can truncate the circumprimary disk of embryos to smaller radii and remove material that may be used in the formation of terrestrial planets (Fig. 9.13). As a result, it used to be believed that circumstellar disks around the stars of a binary may not be massive enough to form planets. However, observations by Mathieu (1994), Ake-son, Koerner & Jensen (1998), Rodriguez et al. (1998), and Mathieu et al. (2000) have indicated that potentially planet-forming circumstellar disks can indeed exist around the stars of a binary system, implying that planet formation in binaries may be as common as around single stars (Fig. 9.14). Among these circumstaller disks, the two well-separated disks of the system L1551 retain the equivalent of approximately 0.03 to 0.06 solar-masses of their original circumstellar materials in a region

Fig. 9.11. The four stages of planet formation.

Fig. 9.12. Structure of a circumprimary disk in a double star system. The masses of the primary and secondary stars are 1 and 0.09 solar-masses, respectively. The secondary star is at 50 AU at the top of the figure, and has an eccentricity of 0.5. The figure shows an area of 20 AU around the primary. The structures inside the disk have appeared after 239 years from the beginning of the simulations. The orange structure on the right edge of the graph is an artifact of numerical simulations (Boss, 2006).

Fig. 9.12. Structure of a circumprimary disk in a double star system. The masses of the primary and secondary stars are 1 and 0.09 solar-masses, respectively. The secondary star is at 50 AU at the top of the figure, and has an eccentricity of 0.5. The figure shows an area of 20 AU around the primary. The structures inside the disk have appeared after 239 years from the beginning of the simulations. The orange structure on the right edge of the graph is an artifact of numerical simulations (Boss, 2006).

Fig. 9.13. Disk truncation in and around binary systems (Artymowicz & Lubow, 1994). The top graphs show circumstellar disks in a binary with a mass-ratio of 0.3. Note the disk truncation when the eccentricity of the binary is increased from 0 to 0.3. The bottom graphs show a similar effect in a circumbinary disk. The mass-ratio is 0.3 and the binary eccentricity is 0.1. The numbers inside each graph represent the time in units of the binary period. The axes are in units of the binary semimajor axis.

Fig. 9.13. Disk truncation in and around binary systems (Artymowicz & Lubow, 1994). The top graphs show circumstellar disks in a binary with a mass-ratio of 0.3. Note the disk truncation when the eccentricity of the binary is increased from 0 to 0.3. The bottom graphs show a similar effect in a circumbinary disk. The mass-ratio is 0.3 and the binary eccentricity is 0.1. The numbers inside each graph represent the time in units of the binary period. The axes are in units of the binary semimajor axis.

with an outer radius of —10 AU (Fig. 9.14, Rodriguez et al., 1998). The masses of these disks are comparable to the minimum solar-mass model of the primordial nebula of our Solar System (Weidenschilling, 1977; Hayashi, 1981), implying that, planet formation in dual-star systems can begin and continue in the same fashion as around our Sun.

L1551 VLA-A 7-mm

20 AU

40.25 40.24 40.23

RIGHT ASCENSION (B1950)

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