Odd planets seem to be sprouting in our data like mushrooms. Take the case of XO-3b. It’s got the mass of thirteen Jupiters but orbits its star in less than four days, making it the largest, most massive planet ever found in such a tight orbit. But XO-3b also seizes the attention because its orbit is significantly elliptical rather than circular. Is this evidence for the gravitational effects of another object in the same system?
We should be able to learn a lot more about this and other questions because XO-3b is also a transiting world, passing between its star and the Earth. This is the third transiting planet identified by the XO Project, which uses two small telescopes at Haleakala (operated by the University of Hawaii) to identify transit candidates before passing the data on to a network of amateur astronomers for further study. After sufficient evidence is gathered, the work goes back to large telescopes at McDonald Observatory (University of Texas) for confirmation.
Announced at the American Astronomical Society meeting in Honolulu, the new planet is close enough to a brown dwarf in size to make its actual status uncertain. Sustainable hydrogen fusion seems to need about eighty Jupiter masses to occur, marking the upper limit between brown dwarf and star. But XO-3B may be right at the lower mass limit. Christopher Johns-Krull (Rice University), says this:
“The controversy lies at the lower end of the scale. Some people believe anything capable of fusing deuterium, which in theory happens around 13 Jupiter masses, is a brown dwarf. Others say it’s not the mass that matters, but whether the body forms on its own or as part of a planetary system.”
All of which may help us with a mystery. The so-called ‘brown dwarf desert’ is the term coined for the unexpected dearth of brown dwarfs in our exoplanet hunts. Radial-velocity techniques ought to be able to spot objects of XO-3b’s size and larger relatively easily given their mass, but the number of brown dwarfs so identified has remained small. Finding objects on the borderline may help us sort out some of the constraints on brown dwarf formation both in and out of planetary systems.
Let’s linger for a moment on the collaboration that’s happening here. Telescope time on the largest and best instruments is scarce, but time is precisely what planet-hunting astronomers need to firm up their findings. “…[T]hat’s where our amateur partners come in,” says XO Project director Peter McCullough (Space Telescope Science Institute), “culling our long lists of candidates down to more manageable size to observe with the big telescopes.” That kind of synergy is likely to produce a continuing harvest of transits, another confirmation of the role amateurs working with commercially available equipment can play in delivering good data.