Circumbinary planets from space-based transiting planet surveys

Planets orbiting stellar multiples are a popular science fiction trope, but while detection of the first exoplanet around a Sun-like star was confirmed in 1995, the first circumbinary planet was only announced in 2011.

Since that announcement other circumbinary planets (or CBPs) have been found through transit detection, eclipse timing, and microlensing, but the number of known CBPs is still very small compared to the total number of known exoplanets. This limits our ability to investigate the formation and evolution of CBP systems, which may be very different from the similar processes for single-star planetary systems. The limited number of known CBPs also restricts our knowledge of the underlying parameter distributions, and the current sample is affected by strong observational biases.

Most of the known transiting CBPs were discovered by NASA's Kepler mission, but since that mission's unfortunate demise the search has been taken up by NASA's TESS satellite, which has produced some recent new discoveries. In the future, ESA's PLATO mission is also likely to detect new CBPs and expand our sample of these interesting systems.

However, detection of a transiting circumbinary requires more fortuitous timing than is the case for single-star systems. Not only are the planet's transits dwarfed by the stellar eclipses, but the gravitational interactions between the host binary stars and the orbiting planet lead to rapid orbital precession. Thus, nearly all of the currently known transiting CBPs will, in fact, cease transiting at some point. The process is periodic, however, and given long enough all will eventually transit once more. This also means that previously non-transiting CBPs may appear around apparently planetless eclipsing binaries.

This project will explore the fascinating world of circumbinary planets, focusing on the use of data from space-based observatories to study those that transit one or both of their host stars. The project will start by searching for circumbinary planet candidates in TESS observations of eclipsing binaries. These could be short-period planets with many detectable transits, or single-transit events indicative of planets on orbits misaligned with their host binary, or even non-transiting CBPs detected through eclipse timing variations. Following their identification, candidate CBPs will be modelled to produce preliminary estimates of their characteristics and predict their future evolution, and further observations will be sought to confirm that the candidates are real and improve their characterisation. Further avenues of work could include extension of the search to data from Kepler and K2, or simulation of CBP systems to evaluate their detectability and constrain the likelihood of planets being present in known eclipsing binary systems. Methods for optimising detection and analysis of CBPs, and the potential yield of CBPs from PLATO, may also be explored using simulated data.