Tuesday 28 February 2012

The origin of the cthonian planets orbiting KIC 05807616.

In December 2011 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 discovery of a pair of cthonian planets orbiting the B-type subdwarf star KIC 05807616. A B-type subdwarf star is a very hot small star fusing helium; these are thought to be the result of a Red Giant star (a star that has used up all its hydrogen then begun to fuse helium, consequently causing its outer layers to expand massively due to the additional heat) having somehow lost its outer layers, usually as a result of interaction with a smaller binary companion. Cthonian planets are planets that have been absorbed by the expansion of a star at the end of its hydrogen-fusing life, then re-emerged when the outer layers were shed for some reason (until 2011 an entirely theoretical class of planet.

Model of the origin of the KIC 0580761 system. Charpinet et al. (2011).

However this model leaves several unanswered questions about the origin of the KIC 0580761 system. Firstly KIC 0580761 lacks a binary companion which could have stripped the outer layers from the primary star. Charpinet et al. attributed this to the interaction of the planets with the star, but it is unclear how this could happen. Secondly the two planets are locked in a 3:2 tidal resonance (i.e. one orbits the star twice in the time the other does three times, planets in such orbits 'lock' into a resonance due to tidal effects). It is hard to see how this resonance could have survived while the planets were within the star, which would have exerted a severe braking effect on the planets.

In a paper published on the online arXiv database at Cornell University Library on 6 February 2012, Ealeal Bear and Noam Soker of the Department of Physics at the Israel Institute of Technology suggest an alternate model for the origin of the KIC 0580761 system, in which the star lost its outer layers when its expansion brought it into contact with a massive Gas Giant type planet, that was ripped apart in the process. Under this scenario the two planets originated in this massive interaction, being fragments of the core of the massive planet (there may have been others). The violent reaction between the planet and the star caused the loss of both the outer layers of the Red Giant, and the dense atmosphere of the planet.

Bear and Soker also note that the inner of the two planets appears to be evaporating due to the proximity of the ultra-hot star.