# Summary of K2 Program GO16051 Title: A survey to detect first sdB Planetary Transit PI: Silvotti, Roberto (INAF) CoIs: Ostensen, Roy H. To date almost nothing is known about the final (WD) configuration of planetary systems. Although not a single bona-fide planet orbiting an isolated WD has yet been identified, it is now widely accepted that the dusty and/or gaseous disks that we see within one solar radius around many WDs are produced by the tidal disruption of asteroids (Farihi+2010, MNRAS 404, 2123). Recently, a disintegrating minor planet transiting a white dwarf has been detected (Vanderburg+2015, Nature 526, 546; Croll+2015, http://arxiv.org/pdf/1510.06434v1.pdf). An increasing fraction of WDs (>=27%) show a metal enrichment due to the fall into the WD atmosphere of ancient planetary material (Koester+2014, A&A 566, 34), whose chemical composition is similar to that seen in different Solar-system meteorites (Gansicke+2012, MNRAS 424, 333). Theory predicts 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 RGB and the AGB phases (Nordhaus & Spiegel 2013, MNRAS 432, 500). While Gaia will discover WD planets at several AUs from their host stars, those external to the period gap (Silvotti+2015, ASPC 493, 455), this proposal aims to study the inner boundary of the period gap. Substellar companions to sdB stars offer a unique opportunity to disentangle the effects of the RG expansion alone (while WD planets are affected also by AGB expansion, thermal pulses and PN ejection, Mustill & Villaver 2012, ApJ 761, 121), and they are the natural 1st step to study the post-RGB evolution of planetary systems and their final architecture. Star/planet interactions during the RGB may be responsible for the stellar envelope ejection leading to the formation of a single sdB star (Soker 1998, AJ 116, 1308; Han+2012, ASPC 452, 3). Among the first planet/BD candidates around sdB stars (Silvotti+2014, ASPC 481, 13), 2 planetary systems were detected by Kepler measuring the photometric modulation due to reflection/re-emission of the star light (Charpinet+2011, Nature 480, 496; Silvotti +2014, A&A 570, 130). Given that only 7 single sdBs were observed by Kepler for enough time to detect tiny photometric variations of 20-50 ppm, we can assume that 2/7 of sdBs have Earth-size bodies in tight orbits at ~0.005 AU from the star. With a geometric transit probability of 0.2 (Rp<