HD 202206 is a yellow dwarf star approximately 151 light-years away in the constellation Capricornus. The star is orbited by two companions with substellar masses in a near-resonant configuration.
Measurements by the Hipparcos astrometric satellite give a parallax of 21.58 milliarcseconds, which corresponds to a distance of 46.3 parsecs. It is similar in mass, radius, and luminosity to the Sun, though estimated to be older at around 5.6 Gyr old. It is also more metal-rich than our Sun based on the amount of iron relative to hydrogen.
In 2000, analysis of radial velocity measurements of the star revealed the existence of a substellar companion (designated HD 202206 b) with at least 17 times the mass of Jupiter around the star in an eccentric orbit with a period of around 256 days. This mass exceeds the 13 Jupiter mass limit above which an object can undergo deuterium fusion in its core, which some take to be the dividing line between planets and brown dwarfs, a definition which is supported by the IAU. However, some authors have suggested that the core accretion process (the traditional model for planet formation) can form objects above this limit, up to around 25-30 Jupiter masses. The classification of HD 202206 b as a brown dwarf or "superplanet" is currently unclear.
Even after HD 202206 b was accounted for, the star still showed a drift in the radial velocity measurements, indicating another companion in a longer-period orbit. In 2004 after further observations, the parameters of a companion was announced. This companion is about Jupiter's size, has a planetary mass, at least 2.44 times that of Jupiter, and has an eccentric orbit taking around 1384 days to complete. The planet is located in a 5:1 orbital resonance with the inner companion.
The formation history of this planetary system is interesting: depending on the method of formation of the inner companion, the system can either be regarded as consisting of a superplanet and a planet, or alternatively a planet revolving in a circumbinary orbit.
* PSR B1620-26
1. ^ "HIP 104903". The Hipparcos and Tycho Catalogues. ESA. 1997. http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIP%20104903. Retrieved 26 June 2006.
* HD 202206 on SIMBAD