Planets in binary star systems

Many planets have been discovered around binary stars. Some of these planets are in circumstellar, or S-Type, orbits where the planet orbits one star in the binary system (e.g. γ Cephei Ab (Neuh¨auser et al. 2007) and HD 196885 Ab (Chauvin et al. 2007)) and others are in circumbinary, or P-Type, orbits where the planet orbits both stars (e.g. Kepler-16b (Doyle et al. 2011), Kepler-47b and c (Orosz et al. 2012), PH-1 (Schwamb et al. 2013), ROXs 42Bb (Kraus et al. 2014) and OGLE- 2007-BLG-349L(AB)c (Bennett et al. 2016)). As of January 2019,∼700 exoplanets are known to be in binary or multiple star systems and ∼20 of these planets are in circumbinary orbits2_{. Various methods such as transits, binary eclipse timing}

variation, and microlensing have been used to detect these planets. The detection of planets in S-Type orbits is similar to detecting planets around single stars. However, detecting circumbinary planets can be more complex.

1.2 Binary star formation 37

Most circumbinary planets have been detected via the transit method. The light curve of Kepler-16b, the first circumbinary planet discovered by the Kepler space telescope, is shown in Figure 1.11. In this system, the binary star and the planet are aligned, which is why we are able to see the primary and secondary eclipse of the stars (the blue and yellow data), as well as the transit of the planet across the stars (the green and red data). This method requires the inclination of the respective orbital planes of the binary star and planet to be aligned, which may not be the most common configuration of circumbinary planets. Circumbinary discs with very high inclination to the binary orbit have been observed, such as IRS 43 (Brinch et al. 2016) and HD 98800 (Kennedy et al. 2019), implying that circumbinary planets could form in highly inclined orbits. By studying Kepler planet occurrence rates, Armstrong et al. (2014) calculated the occurrence rate for circumbinary planets of Rp > R⊕ and periods < 300 day to be ∼10% for coplanar orbits to > 47% for an isotropic distribution of inclinations. To characterise the distribution of inclinations that circumbinary planets can exist, we need to observe more misaligned circumbinary planets.

Direct imaging and microlensing are other techniques that are less dependent, than the transit method, on the orbital alignment of the system. A coronagraph was used to directly image the circumbinary planets ROXs 42b and FW Tau b (Kraus et al. 2014). Direct imaging is possible with these planets because are very massive and have large separations from the host binary star. The microlensing technique has discovered the lowest mass circumbinary planet to date, OGLE-2007- BLG-349L(AB)c, which has a mass of 80±13 M⊕(Bennett et al. 2016). Most known circumbinary planets have masses less than 1 MJupwhich is peculiar given that more

massive planets would be easier to detect. However, massive planets are also less common and it is not clear whether the observed lack of known giant circumbinary planets it significant.

Mentioned previously, Artymowicz & Lubow (1994) described a region of insta- bility around binary stars that truncates the inner circumbinary disc radius at∼2a. This same instability can be hostile to the survival of circumbinary planets. Many of the known circumbinary planets (particularly theKepler planets) reside near this region of instability, and this is illustrated in Figure 1.12. It is not clear whether

the proximity of circumbinary planets to this instability region is an observational bias, or due to migration. Do most circumbinary planets migrate to the edge of the instability region? Or do many migrate into the instability region to be destroyed or ejected?

Circumbinary planets have not been discovered around very close (P < 7 day) binaries, despite planet detection being easier around these types of systems. Martin et al. (2015) suggested that the lack of circumbinary planets in close binaries may be due to the presence of a third body which shrunk the inner binary orbit by the process of Kozai-Lidov cycles and tidal friction, hindering the formation or stability of circumbinary planets. Mu˜noz & Lai (2015) explored the tightening of binaries via the Kozai-Lidov mechanism with a tertiary companion and the inner binary hosting a circumbinary planet. They found that for initial tertiary inclinations of _∼< 75◦ it was possible for a circumbinary planet to survive this evolution, and the planet would migrate to a higher inclination. For initial tertiary inclinations of _∼> 80◦ the circumbinary planet developed erratically evolving eccentricity and inclination, and was often ejected or collided with a star. Mu˜noz & Lai (2015) conclude that circumbinary planets may exist around very close binaries, but are on inclined orbits making their detection difficult. However, Fleming et al. (2018) argue that the circularisation via tidal evolution of close binaries can expand the region of instability to the region where planets might reside. During this circularisation, circumbinary planets could be destabilised and destroyed leading to a lack of circumbinary planets observed around very close binary stars.

While it appears the formation of circumbinary planets is very sensitive to the environment and evolution of the binary, it also appears that binaries can hinder the formation or survival of planets in circumstellar orbits. Kraus et al. (2012) compiled a census of discs around binaries in the Taurus-Auriga region and found that the disc fraction (circumstellar and circumbinary) of binaries with separations_∼<40 AU was a third of the disc fraction for wider binaries or single stars. This implied that disc dispersal may be very rapid around binaries with these separations, hindering planet formation. The results of Kraus et al. (2012) is reflected by the results of Kraus et al. (2016) who find the planet occurrence rate in binaries with a separation less than 50 AU is approximately one third that of wider binaries or single stars. Despite

1.2 Binary star formation 39

Figure 1.12: A schematic showing the proximity of known circumbinary planets to the region of instability caused by the host binary star described by Artymowicz & Lubow (1994). Taken from planethunters.org.

binaries of separation_∼<50 AU appearing to be hostile to planet formation, Rafikov & Silsbee (2015) suggest that planet formation in binaries down to separations of ∼20 AU is possible if the circumstellar disc is sufficiently massive (_∼> 0.01 M) and the binary has low eccentricity. Fragione (2019) find that the survivability of planets in S-Type orbits for binary separations less than 50 AU is dependent on the mass ratio of the binary. The results of Sutherland & Fabrycky (2016) imply that planets in S-Type orbits likely formed in situ and did not fall into those orbits via some dynamical interaction. They carried out N-body simulations of a Jupiter mass planet in the instability region around 1 AU binary stars of varying mass ratios to determine the likelihood of falling into an S-Type orbit or ejection. They find that

∼80% of the simulations lead to the ejection of the planet,∼20% lead to the planet striking one of the stars and less than ∼1% lead to the capture of the planet around one of the stars.

The diversity of planets in binaries is only beginning to be comprehended. To understand the formation of these diverse planets we must consider the environments

in which they form. This problem is the topic of this thesis.