To date not a single bona fide planet has been identified orbiting an isolated white dwarf (e.g. Hogan et al. 2009), though the discovery of transiting objects around WD 1145+017 (EPIC201563164, Vanderburg et al. 2015) shows that they should exist.We therefore remain ignorant about the final evolutionary configuration of >95% of planetary systems. Theoretical models (e.g. Nordhaus & Spiegel 2013) predict a gap in the final distribution of orbital periods, due to the opposite effects of stellar mass loss (planets pushed outwards) and tidal interactions (planets pushed inwards) during the red giant branch (RGB) and asymptotic giant branch (AGB) phases. If a planet enters the envelope of the expanding giant star, its survival depends a number of poorly constrained parameters, particularly its mass. Currently, the lowest mass brown dwarf companion known to have survived such “common envelope” evolution to the WD stage has a mass of 25‒30 MJ(Casewell et al. 2012), but theoretical models suggest much lower mass gas giants may survive.
Over its five-year primary mission, Gaia is expected to astrometrically detect tens or hundreds of WD planets (Mplanet>1MJ) in long period orbits (Sozzetti et al. 2014), but the likelihood of planets surviving in close orbits around WDs will likely remain an open question for some time. Recently, more than 15 planets around post-RGB stars were detected, orbiting extreme horizontal branch subdwarf-B (sdB) stars, or cataclysmic variables. Most of them were on long-period orbits, and discovered from eclipse or pulsation timing (e.g. Silvotti et al. 2007, while two sdB planetary systems with very short orbital periods of a few hours were detected by Kepler through illumination effects (Charpinet et al. 2011; Silvotti et al. 2014). The Kepler discoveries suggest that ~10% of sdB stars could have close planets (or planetary remnants) and ~2.25% of sdB stars could show a transit. PLATO will collect large-number statistics for these objects, detecting sdB planets not only from illumination effects, but also from transits, giving the first estimates of their radii.
Even more importantly, PLATO has the capability to detect the first WD planet transits, which requires large statistics (Faedi et al. 2011). PLATO can easily detect gas giants eclipsing WDs, placing limits on the masses of planets that can survive “common envelope” evolution. In addition, since WDs are similar in radius to Earth, PLATO can detect transiting bodies down to sub-lunar sizes. Such objects may exist in close orbits to WDs, possibly through perturbations with other planets in a complex and unstable post-main sequence system. Indeed, at periods of ~10‒30 hours, these rocky bodies would exist in the WDs’ HZs (Agol 2011), and their atmospheres would be detectable with JWST (Loeb & Maoz 2013).
Discovery and characterisation of post-RGB planets is essential to study planetary system evolution and planet-star interaction during the most critical phases of stellar evolution: RGB and AGB expansion; thermal pulses; planetary nebula ejection, etc. We note that sdB/WD asteroseismology allows a very good characterisation of these stars and their planets.