Planetary Systems in Wide Binaries

A significant fraction of the stars in the galaxy have companions with many of them hard to detect. A large fraction of these binary/triple/quadruple stars could host planetary systems. Planetary systems surrounding two stars in a close binary pair (circumbinary planets) have been detected as well as planets that orbit one star in a wide binary. The gravitational perturbation from a companion star in a wide binary can have a significant effect on the orbital evolution of any planets or planetesimals in the system which can lead to different evolution from that of our solar system.

One of the secular effects that a wide orbit companion star can exert is the Kozai-Lidov (K-L) mechanism. This causes large amplitude oscillations in the eccentricity and inclinations of any particles in the system. If the perturbing star is on an eccentric orbit, then the octupole component of the gravitational potential experienced by a particle is potentially important. This can cause the particle's eccentricity to reach values very close to one. This in turn causes very low pericentre distances (relative to its host star).

One application the project is looking at is Tabby' star. It exhibits aperiodic dips in its light curve and one explanation is that this is caused by transiting exocomets. Any exocomets would start as planetesimals left over from planet formation and would have to get very close to the host star to generate the required transit depths. One potential way this could work is via the K-L mechanism as Tabby's star is observed to have a companion nearby on the sky which is hypothesised to be bound. The project will first examine whether this is likely by examining the behaviour of test particles under the influence of the K-L mechanism using N-body integrations and numerical integrations of the differential equations for orbital elements derived from theory. Then a parameter space exploration will take place to find where the necessary transit depths arise and thus how likely it is that the K-L mechanism is causing these transits given the available data.

Another application the project can look at is whether the K-L mechanism could deliver the rocky material to white dwarfs which is needed to explain how they become polluted with metals. The observations have shown that some of these have an infrared excess interpreted to be a disc of metal rich planetary material which must have been delivered from further away.

Other areas of interest to examine within wide binary planetary systems are where the zones of instability are for particles due to the perturbation of a companion, how hot Jupiters evolve and migrate close to the host star and how cometary material behaves when under the influence of both a companion and scattering planets.