It was two years ago that astronomers at the Geneva Observatory in Sauverny, Switzerland, released their findings on the star HD 202206. The conclusion: An object 17 times as massive as Jupiter orbits the star at 0.82 AU. A second planet has now been found at 2.55 AU, this one over twice as massive as Jupiter.
So is the heavier object a planet or a brown dwarf? The issue is called into question by the second discovery. Science News is running a story in its November 27 issue pointing out that by the standards of the International Astronomical Union, brown dwarfs must be heavy enough to burn deuterium at their core, while remaining light enough so as not to burn any other nuclear fuel. The heavy object around HD 202206 fits this category well, since brown dwarfs normally range from 13 times the mass of Jupiter to 75.
But there’s a twist. The two objects are gravitationally locked in a peculiar synchrony that has not previously been observed, with the inner body orbiting the star exactly five times for every single orbit of the more distant one. And here’s where it gets intriguing: the orbital mechanics imply that both objects were born simultaneously, in which case, the supposed brown dwarf would actually be the heaviest planet ever discovered.
Here is a precis of this work, which was led by Alexandre Correia of the University of Aveiro in Portugal:
Long-term precise Doppler measurements with the CORALIE spectrograph reveal the presence of a second planet orbiting the solar-type star HD 202206. The radial-velocity combined fit yields companion masses of 17.4 M_Jup and 2.44 M_Jup, semi-major axis of a = 0.83 AU and 2.55 AU, and eccentricities of e = 0.43 and 0.27, respectively. A dynamical analysis of the system further shows a 5/1 mean motion resonance between the two planets. This system is of particular interest since the inner planet is within the brown-dwarf limits while the outer one is much less massive. Therefore, either the inner planet formed simultaneously in the protoplanetary disk as a superplanet, or the outer Jupiter-like formed in a circumbinary disk, the former explanation being favored by the observed mean-motion resonance. We believe this singular planetary system will provide important constraints upon planetary formation and migration scenarios.
The abstract for “A pair of planets around HD 202206 or a circumbinary planet?,” can be found here, with a link to the full paper (PDF warning).
We are left with a question of definition. Can some massive objects that otherwise fit our understanding of brown dwarfs actually be planets, while others match our theories of brown dwarf formation? The jury on this one is still out, at least until we understand more about synchrony and its causes in extrasolar planetary systems.
