Planetary Formation And Dynamics In Binary Star Systems

As of today, over 500 exoplanets have been detected since the first exoplanet dis-covered around a solar-like star in 1995. Planets in binaries could be common as stars are usually born in binary or multi-stars systems. Although current observations show that the multiplicity-rate of the detected exoplanet host stars is around 17%, this frac-tion should be considered as a lower limit because of noticeable selection effects against binaries in planet searches. Most of the currently known planet-bearing binary systems are wide S-types, meaning the companion star acts as a distant satellite, typically orbit-ing the inner star-planet system over 100 AU away. Nevertheless, there are currently four systems with smaller separation of～20 AU, including the y Cephei, GJ 86, HD 41004 and HD 196885. In addition to the planets in circumprimary (S-type) orbits discussed above, planets in circumbinary (P-type) orbits have been found in only two systems. In this thesis, we mainly focus on the S-type to study how planet formation proceeds in binary star systems.In Chapter 1, we first summarize current observational facts of exoplanets both in single and binary star systems, then review the theoretic models of planet formation, with special attention to the application to binary star systems. Perturbative effects from stellar companions render the planet formation process in binary systems even more complex than that in single-star systems. The perturbations from a binary com-panion can excite planetesimal orbits and increase their mutual impact velocity,δV, to values that might exceed their escape velocity or even the critical velocity for the onset of eroding collisions. The intermediate stage of the formation process—from planetes-imals to planetary embryos—is thus the most problematic. In the following chapters, we investigate whether and how planet formation goes through such a problematic stage.In chapter 2, we study the effects of gas dissipation on planetesimals'mutual accretion. We show that in a dissipating gas disk, all the planetesimals eventually con- verge toward the same forced orbits regardless of their size, leading to much lower impact velocities. This process of decreasingδV progressively increases net mass ac-cretion and can even trigger runaway growth for large planetesimals.In chapter 3, for the first time, we adopt a 3-dimensional approach to investigate planetesimals' mutual accretion in binary systems. We find that inclusion of a small inclination (iB) between the binary orbital plane and the circumstellar disk plane leads to differential orbital phasing is realized in the 3-dimensional space. In such a case, impacts mainly occur between similar-sized bodies with impact velocities (δV) being significantly reduced, and thus planetesimal accretion is more favored.In chapter 4, we investigate the planet formation in a specific system, the habitable zone of a Centauri B. For the first time, we develop a scaling method to estimate the planetesimal collisional timescale in binary systems. We find that accretion-favorable conditions reached at 1-2 AU from a Centauri B after the first～105 yr. However, planetesimal accretion is significantly less efficient as compared to the single star case. Our results suggest that formation of Earth-like planets through accretion of km-sized planetesimals is possible in a Centauri B, while formation of gaseous giant planets is not favorable.In chapter 5, we outline a new concept, which we call the "snowball" growth mode. In this snowball phase, isolated planetesimals move in Keplerian orbits and grow solely via the direct accretion of subcentimeter-sized dust entrained with the gas in the protoplanetary disk. Using a simplified model in which planetesimals are progressively produced from the dust, we find snowball growth phase can be the dominant mode to transfer mass from the dust to planetesimals. Snowball growth could provide an alternative explanation for the turnover point in the size distribution of the present-day asteroid belt. For the specific case of close binaries such as a Centauri, the snowball growth provides a safe way for bodies to grow through the problematic～1-50 km size range.In chapter 6, we investigate the intermediate stages-from planetesimals to plan-etary embryos/cores-of planet formation in highly inclined cases (30°