Stripped planets around a former red giant star

An artist's impression of planets orbiting close to a hot subdwarf star
Illustration courtesy S. Charpinet

An international research team, including Dr. Roy Østensen of the University of Leuven found a system of compact planets around a former red giant star. The orbits of the planets are so close to the star that they must have been engulfed in the outer layers of the star's atmosphere when it was a red giant. The red giant star's atmosphere ripped off the atmosphere and surface of the planets, leaving the planets stripped and compact. The team reports about the discovery in the journal Nature, in the issue of 21 december 2011.

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.

The team reports 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.

BBC: Newly found planets are 'roasted remains'

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

S. Charpinet, G. Fontaine, P. Brassard, E. M. Green, V. Van Grootel, S. K. Randall, R. Silvotti, A. S. Baran, R. H. Østensen, S. D. Kawaler & J. H. Telting