Evolution of Red Giants

REFERENCE: Charpinet et al. “A compact system of small planets around a former red-giant star.” Nature (2011) 480, pgs 496 – 499.    

Red giants are low to intermediate mass stars late in their lifetimes that have swelled to massive sizes.  It has been predicted that the Sun will become a red giant is 7.5 billion years at which time its radius will become 200 times larger than it is now.  Typically, planets that are orbiting a star at a radius less than 1 astronomical unit (AU) will be engulfed by the swelling red giant.  However, some post red giant stars still have giant planets orbiting them at radii much closer than 1 AU.

                Recent work by Telting and colleagues suggests both how planets can survive immersion in the red giant envelope and influence the evolution of the star.  KIC 05807616 (also known as KPD 1943+4058) is a B subdwarf star, which is much hotter and brighter than a typical subdwarf star and represents a rarer way a red giant star can evolve.  This type of subdwarf evolves when a red giant loses its outer envelope prematurely.  Charpinet et al. show that KIC 05807616 has two planets, which they name KOI 55.01 and KOI 55.02, slightly smaller than Earth still orbiting it.

                They authors further offer a reasonable scenario as to how the current system came to be.  They feel that both bodies were originally large, gaseous planets that were swallowed by the red-giant envelope.  The immersion triggered the red giant’s outer shell premature loss and evolution into a B subdwarf, however this process stripped the planets of their gaseous layers and left only the inner cores, which the authors see orbiting the star today.

                An alternative scenario involving the merging of two white dwarfs that resulted in planet formation followed by a secondary planet is possible, but highly unlikely.