Friday, 10 February 2012

KOI-13b, a big, hot planet not a Brown Dwarf.

The KOI-13 (Kepler Object of Interest-13, otherwise BD+46 2629) system is composed of a pair of A-type White Dwarf stars roughly 1630 light years from the Earth. The larger of these stars, KOI-13 A, has a mass 2.05 times that of our sun, is 30.5 times as luminous, and has 2.55 times the sun's radius. The smaller star, KOI-13 B, has a mass 1.95 times that of the sun, is 23 times as luminous and has a radius 2.38 times that of the sun.

Image of the KOI-13 system from the 1m RCC telescope at Konkoly Observatory in Hungary. Image from Szabó et al. (2011). See below.

In 2011 the Kepler Space Telescope detected an object orbiting KOI-12 A. This was described in a paper published on the arXiv database at Cornell University Library by a team lead by Gyula Szabó of the Konkoly Observatory of the Hungarian Academy of Sciences and the Department of Experimental Physics at the University of Szeged, as being a Brown Dwarf, or possibly a very small Red Dwarf Star. Brown Dwarfs are objects to large to be considered planets, but to small to be considered stars; they are thought to be able to fuse deuterium in their cores, but not hydrogen. Red Dwarfs are the smallest type of stars, they burn dimly, but can be very long lived. KOI-13b (or KOI-13.01) was envisaged as an object borderline between these two types of objects, with a mass 20% of that of the sun and a radius 2.2 times that of Jupiter. Such an object orbiting so close to a very hot A-type star would probably receive more heat than it emitted.

The path of KOI-13b across the face of KOI-13 A, as detected by Kepler. From Szabó et al. (2011)

This month, in a paper published on the arXiv database and in the Monthly Notes of the Royal Astronomical Society, Dimitris Mislis and Simon Hodgkin of the Institute of Astronomy at the University of Cambridge, describe a new study of the KOI-13 system, which reveals KOI-13b as a large planet rather than a Brown Dwarf or small star.

Mislis and Hodgkin conclude KOI-13b is a super-Jovian planet with a mass of 8.3 times that of Jupiter and 1.4 times Jupiter's radius. This is big and dense for a planet, but well short of the mass needed to fuse deuterium, so it is not a Brown Dwarf. KOI-13b orbits KOI-13 A every 25.4 hours at a distance of 0.0367 AU (3.67% of the distance between the Earth and the sun. Since KOI-13 A is very hot, with a surface temperature of 8511K (compared to 5778K for our sun), and KOI-13b is very close to it, KOI-13b is thought likely to have a very high surface temperature itself, modeled by Mislis and Hodgkin as 2864K, warmer than the coolest Red Dwarf stars.


  1. This was a very useful summary and clarification of the papers, which saved me much time. Mr. Bauwens, of course, in the first sentence means (A-type White) Dwarf stars and not A-type (White Dwarf) stars. This parsing problem is often avoided by putting the "white" first and/or not using the "dwarf" sobriquet for main sequence stars, referring to such objects as, for instance, white A-type main sequence stars.

    1. Another nit, I'm afraid. While I'm sure the Szabo diagram is accurately reproduced, one would get a more accurate notion of the system if one reversed the colors, with the large oblate star blue-white and the crossing planet red-orange.