Introduction

The concept of a "world with two suns" has been of interest to astronomers for many years. Many scientists tried to understand whether planets could form in binary star systems, and whether the notion of habitability, as we know it, could be extended to such environments. Although as a result of their respective works, many dynamical features of binary-planetary systems1 have been discovered, until recently, the subjects of their studies were, in large part, hypothetical. There was no detection of a planet in and/or around a binary system, and planet detection techniques had not advanced enough to successfully detect planets in dual-star environments.

1A binary-planetary system is a dual-star system that also hosts planetary bodies.

Fig. 9.1. Velocity residuals to 7 Cephei after subtracting a second order fit to its original radial velocity data (Campbell, Walker & Yang, 1988). The residuals show periodicity implying the possible existence of a planetary companion.

The discovery of extrasolar planets during the past decade has, however, changed this trend. Although the candidate planet-hosting stars have been routinely chosen to be single, or within wide (>100 AU) binaries 2, the precision radial velocity technique has been successful in detecting planets around the primaries of three moderately close (<40 AU) dual-star systems. As a result, during the past few years, the topic of planets in binaries, once again, found its way to the mainstream research and has now become a real scientific issue that demands theoretical explanations.

The first detection of a planet in a binary system was reported by Campbell, Walker & Yang in 1988. In an attempt to identify planetary objects outside our solar system, these authors measured the variations in the radial velocities of a number of stars, and reported the possibility of the presence of a Jovian-type body around the star 7 Cephei (Fig. 9.1, Campbell, Walker & Yang, 1988). This star, that is a K1 IV sub-giant with a mass of 1.59 solar-masses (Fuhrmann, 2004), is the primary of a binary system with a semimajor axis of 18.5 AU and an eccentricity of 0.36 (Griffin, Carquillat, & Ginestet, 2002; Hatzes et al., 2003). The secondary of this system is an M dwarf with a mass of 0.44 solar-masses (Neuhauser et al., 2007; Torres, 2007). Initial radial velocity measurements of 7 Cephei implied that this star may be host to a giant planet with a probable mass of 1.7 Jupiter-masses, in an orbit with a semimajor axis of 1.94 AU (Campbell, Walker & Yang, 1988).

Unfortunately, the discovery of the first binary-planetary system, which could have also marked the detection of the first planet outside our solar system, did not withstand skepticism. In an article in 1992, Walker and his colleagues attributed

2As shown by Norwood & Haghighipour (2002), the perturbative effect of the stellar companion on the dynamics of a planetary system around a star becomes important when the binary separation is smaller than 100 AU. At the present, approximately 25% of extrasolar planetary systems detected by radial velocity technique are in binary systems with separations ranging from 250 to 6000 AU.

their measured variations of the radial velocity of 7 Cephei to the chromospheric activities of this star, and announced that the possibility of the existence of a giant planet around 7 Cephei may be none (Walker et al., 1992). It took observers an additional 12 years to monitor 7 Cephei and measure its radial velocity to arrive at the conclusion that the previously observed variations were not due to stellar activities and were in fact representative of a planetary companion (Fig. 9.2). It was the discovery of a giant planet in the binary system of Gl 86 (Queloz et al., 2000), and the (re-)announcement of the detection of a giant planet in 7 Cephei system (Hatzes et al., 2003) that opened a new chapter in the theoretical and observational studies of extrasolar planetary systems.

The fact that giant planets exist in moderately close (< 40 AU) binary systems has confronted dynamicists with many new challenges. Questions such as, how are these planets formed, can binary-planetary systems host terrestrial and/or habitable planets, how are habitable planets formed in such dynamically complex environments, and how do such planets acquire the ingredients necessary for life, are among major topics of research in this area. This chapter is devoted to review these issues and present the current status of research on the formation of planets in dual-star systems and habitability of terrestrial bodies in and around binary stars.

The chapter begins with a review of the dynamics of a planet in a binary star system. In general, prior to constructing a theory for the formation of planets, it proves useful to study whether the orbit of a planet around its host star would be stable. In a binary system, such studies are quite important since in these systems the perturbation of the stellar companion may dictate the possibility of the formation of planetary bodies by affecting the stability and dynamics of smaller objects.

The formation of planets in binary star systems is reviewed in the third section. Although the study of the dynamics of planets in and around binary stars dates back to approximately forty year ago, the formation of planets in these systems is an issue that is still unresolved. In spite of the observational evidence that indicates a majority of main and pre-main sequence stars are formed in binaries or clusters (Abt, 1979; Duquennoy & Mayor, 1991; Mathieu, 1994; Mathieu et al., 2000; White & Ghez, 2001), and the detection of potentially planet-forming environments in and around binary stars (Fig. 9.3, also see Mathieu, 1994; Akeson, Koerner & Jensen, 1998; Rodriguez et al., 1998; White et al., 1999; Silbert et al., 2000; Mathieu et al., 2000), planet formation theories are still unclear in explaining how planets may form in multi-star environments. The focus of Sect. 9.3 is on discussing the formation of giant and terrestrial planets in moderately close binary-planetary systems, and reviewing the current status of planet formation theories in this area.

The habitability of a binary system is presented in Sect. 9.4. The notion of habitability is defined based on the habitability of Earth and life as we know it. Such a definition requires a habitable planet to have the capability of retaining liquid water in its atmosphere and on its surface. The latter is determined by the luminosity of the central star, the size of the planet, and also the distribution of water in the protoplanetary disk from which terrestrial-class objects are formed. In Sect. 9.4, a review of the current status of the models of habitable planet formation

Fig. 9.2. Radial velocity measurements of the primary of 7 Cephei binary system. The graph on the top shows the orbital solution for the planet (solid line) and the residual velocity measurements after subtracting the contribution due to the binary companion (data points). The graph on the bottom depicts the phased residual radial velocity measurements (data points) compared to the planet orbital solution (solid line) (Hatzes et al., 2003).

Fig. 9.2. Radial velocity measurements of the primary of 7 Cephei binary system. The graph on the top shows the orbital solution for the planet (solid line) and the residual velocity measurements after subtracting the contribution due to the binary companion (data points). The graph on the bottom depicts the phased residual radial velocity measurements (data points) compared to the planet orbital solution (solid line) (Hatzes et al., 2003).

Fig. 9.3. Image of the circumbinary disk of GG Tau (Krist et al., 2005) taken by Advanced Camera for Surveys (ACS) on board of Hubble Space Telescope (HST). The binary separation is 35 AU. The locations of the binary components are marked with crosses.

in and around binary systems are presented, and their connections to models of terrestrial planet formation and water-delivery around single stars are discussed. Finally the chapter ends by discussing the future prospects of research in the field of planets in binaries.

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