A compact system of small planets around a former red-giant star

Abstract

Planets that orbit their parent star at less than about one astronomical unit (1 AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have revealed the existence of post-red-giant host stars with giant planets orbiting as close as 0.116 AU or with brown dwarf companions in tight orbits, showing that these bodies can survive engulfment. What has remained unclear is whether planets can be dragged deeper into the red-giant envelope without being disrupted and whether the evolution of the parent star itself could be affected. Here we report the presence of two nearly Earth-sized bodies orbiting the post-red-giant, hot B subdwarf star KIC 05807616 at distances of 0.0060 and 0.0076 AU, with orbital periods of 5.7625 and 8.2293 hours, respectively. These bodies probably survived deep immersion in the former red-giant envelope. They may be the dense cores of evaporated giant planets that were transported closer to the star during the engulfment and triggered the mass loss necessary for the formation of the hot B subdwarf, which might also explain how some stars of this type did not form in binary systems.

Hydrodynamic simulations of the interaction between giant stars and planets

“…Examples are the 1.25 MJ planet (where MJ is the mass of Jupiter) orbiting 0.116 au from a horizontal branch star (Setiawan et al 2010), two Earth-sized objects orbiting a subdwarf B star at a separation of 0.0060 and 0.0076 au (Charpinet et al 2011), or three earth-sized planets orbiting a subdwarf B pulsator (Silvotti et al 2014). These planets must have been engulfed in the envelope of the giant star that became the subdwarf star today.…”

“…These planets must have been engulfed in the envelope of the giant star that became the subdwarf star today. Charpinet et al (2011) and Passy et al (2012b) showed how planets may have been much more massive initially and lost much of their mass in the CE phase. However, they could not determine how the core of the planets survived the interaction instead of plunging into the core of the giant.…”

Kepler and the seven dwarfs: detection of low-level day-time-scale periodic photometric variations in white dwarfs

“…Villaver & Livio 2009;Nordhaus & Spiegel 2013). For example, Charpinet et al (2011) have used Kepler to deduce the presence of two Earth-sized planets orbiting a hot B subdwarf star (a red giant stripped of its envelope, and destined to become a WD), with 5.7 hr and 8.2 hr periods. Reflection or thermal re-emission of the WD radiation by the planets modulates the WD light by ∼ 50 ppm.…”

An extreme-AO search for giant planets around a white dwarf

“…However, little is known about the evolution of planetary systems beyond the main sequence lifetime of their host stars. Planets have been discovered around (sub)giant stars (Johnson et al 2011;Gettel et al 2012) and horizontal branch stars (Silvotti et al 2007;Setiawan et al 2010;Charpinet et al 2011). There is only one planetarymass object detected with direct imaging around a white dwarf (WD 0806-661;Luhman et al 2011), although theoretical calculations show that many more planets can survive the red giant stage (Burleigh et al 2002;Jura 2008;Mustill & Villaver 2012;Nordhaus & Spiegel 2013).…”