The WASP project has just released the discovery paper for the systems WASP-53 and WASP-81, led by Amaury Triaud. We’ve known about close-in hot-Jupiter planets around these two stars for several years, but the paper had been delayed owing to an interesting development: the radial-velocity monitoring showed that the planets both had longer-period brown-dwarf companions. Several years of data have been needed to prove the reality of these brown dwarfs, now dubbed WASP-53c and WASP-81c.
The plot shows the “radial velocities” — how much the star is tugged about by the gravity of orbiting bodies — as a function of time (in BJD, a count of days). WASP-53 is on the left and WASP-81 on the right. The red line is a fit to the data. The close-in hot Jupiters (WASP-53b and WASP-81b, with orbits of 3.3 and 2.7 days respectively) cause short-period variations, so fast that they appear as a solid red swathe.
In addition, though, WASP-53 shows a variation owing to a more-massive brown dwarf with an orbital period of about ten years and a mass of at least 16 Jupiters. Similarly, WASP-81 shows a variation caused by a 57-MJup brown dwarf in a 3.5-yr orbit. Both outer orbits are highly eccentric.
The presence of the brown dwarfs has interesting consequences for ideas about how planets form. It is generally accepted that hot Jupiters form further out, where it is colder, where ices can stick together and form a planetesimal. But the presence of eccentric brown dwarfs, disrupting the proto-planetary disc in that region, would have made that hard. So maybe the planets formed further in? Or maybe the brown dwarfs were originally elsewhere, and moved to their current orbits later on?