Planets that orbit around both components of a stellar binary were suggested as favourable targets for transit surveys (Borucki 1984) due to the expected alignment between the planetary and the stellar orbital planes, which strongly increases detection probabilities on eclipsing binaries with near edge-on orbits. Some early surveys (e.g. Deeg et al. 1998) subsequently focused on such systems, but it was not until the Kepler mission that the first transiting circumbinary planets (CBPs) were found (Doyle et al. 2011). The discovery of 11 transiting CBPs in 9 systems has been announced as for Sep 2016. Their orbital periods are of the order of several months and planet-masses are relatively low, the most massive being Kepler-16b with Mplanet~ 0.33 MJ. All CBP orbits have an inner limit to their stability (e.g. Dvorak et al. 1989; Chambers et al. 2002), and most of the transiting CBPs orbit close to that limit (Welsh et al. 2012). It is also notable that all planet-hosting binaries have orbital periods on the order of 10 days or longer and have an occurrence frequency probably similar to that in single stars (Armstrong et al. 2014).
An additional photometric method to detect CBPs, based on the detection of the binaries’ eclipses in the planet’s reflected light, was suggested by Deeg & Doyle (2011). In Kepler data this method can potentially detect CBPs that are close to the inner stability limit around short-periodic binaries, with a large range of orbital inclinations. No discoveries have been reported yet, however.
Formation and evolution models predict the formation of circumbinary protoplanets to occur in relatively distant disks, with subsequent migration to the planets’ observed positions being accompanied by the further accretion of matter. In particular, the accumulation of CBPs near the inner stability limit has been foreseen by Pierens & Nelson (2007), who predicted that the inward drift of a protoplanet can be stopped near the edge of the disk cavity formed by the binary stars. In more general terms however, any generic theory on planet system formation and evolution needs to be compatible with planets found around binary stars, making this population of planets an interesting test-bed for many theoretical advances.
The number of CBP detections that was found to date in Kepler data is likely limited by the number of light curves sampled and not by photometric precision, i.e. all known CBP transits can be identified “by eye” in the light curves. One difficulty comes from the precession of the orbit which results in appearance and disappearance of transits.
With the sample size and observing duration of PLATO, we can expect that the sample of transiting circumbinary planets (CBPs) of the types that are currently known will multiply several-fold. We can also expect a subsequent detailed analysis of the planet(s) thanks to their host star brightness, and investigating whether the planets that form in such systems are different from those around single stars.
For PLATO, this presents the following objectives:
- What are the properties of the circumbinary planetary systems? What are their masses, orbital periods, and the types and ages of host stars? Can their special features be explained by existing planet formation theories, and/or do they need modifications?
- Do other classes of CBPs besides currently known ones exist? In particular, no CBPs on short-period binaries have been found to date, although these binaries are by far the most common and there are no special obstacles to the detection of their planets.