Wednesday 21 December 2011

Kepler discovers two Cthonian Planets orbiting a subdwarf star.

KIC 05787616 (Kepler Input Catalogue 05787616) is a Hot B-type subdwarf star, 3850 light years from the Earth. A Hot B-type subdwarf is a Red Giant star that has lost it's outer layers prematurely, before it begins to fuse helium; thus when they do start to fuse helium they do it unshielded by the cooler, darker outer layers of the Red Giant, making them some of the hottest, brightest objects in the sky. The precise cause of this loss of outer layers is unclear, and it is doubtful that all Hot B-type subdwarfs have formed in the same way.

As a former Red Giant KIC 05787616 was an unlikely candidate for a planetary system, particularly one close enough in to the star to be detected easily, but it was in the Kepler field of view (Kepler stares constantly at the same bit of space), so Kepler was gathering data on it, it is an unusual and interesting object, and telescope time is a precious resource to astronomers, so the data gathered was bound to be used.

This week a team lead by Stephane Charpinet of the Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse publish a paper in the journal Nature detailing the (highly unexpected) discovery of a pair of small planets orbiting KIC 05787616. These two roughly Earth-sized objects orbit the star at 0.60% and 0.76% of the distance between the Earth and the Sun, and were, unusually, detected not by the dimming of the star as they passed in front of it, but by the heat and light they were emitting. Many planets have been found orbiting close to their stars in the past (such planets are the easiest to detect), but typically around small, cool, Red Dwarf type stars, not anything as hot as KIC 05787616 (a B-type subdwarf has a surface temperature of between 20 000 and 40 000 K, compared to 5778 K for our sun). Consequently these planets are far hotter than anything previously discovered, with estimated surface temperatures of between 8000 and 9000K; i.e. the planets themselves are considerably hotter than the surface of our sun. This causes the planets to emit a considerable amount of light. The two planets orbit the star every 5.76 and 8.23 hours respectively.

These planets must, somehow, have survived the star passing through a Red Giant phase, something that scientists were not certain was possible prior to this discovery (and which they shall probably argue about for some time to come). The most likely explanation is that they were formerly large, gas giant type planets further out in the system, and that they were swallowed by the star in its expansive phase, had their outer layers stripped away and fell towards the star as friction with the gas giant's corona slowed them down (orbiting planets are in a dynamic relationship with their star, held in place by a combination of centrifugal force and gravity; if they are for some reason slowed down they fall towards the star, if they speed up they move away). Somehow this process caused the star to shed it's outer layers, leaving the B-type subdwarf at the centre, being orbited by the two reduced planets. Gas giants that have lost their outer layers are called Cthonian Planets; these also were only a theory until the discovery of the KIC 05787616 system (two previously discovered planets, COROT-7b and Kepler-10b are considered possible Cthonian Planets, the new planets differ in that we have no other explanation for them).

A model of the evolution of the KIC 05787616 system.