Brown dwarfs are intermediate between planets and stars. They are not massive enough to undergo hydrogen fusion in their cores, as required to be a star, but are too massive to be planets, and can fuse deuterium. Those conditions produce a range from about 13 Jupiter masses to about 80. Some people, however, argue that the distinction between a planet and a brown dwarf should not be about their mass, but about whether they formed in a star-like way, by gravitational collapse, or in a planet-like way, by accumulation of planetesimals in a proto-stellar disc.
Comparative sizes. Credit: NASA Goddard Space Flight Center
WASP was designed to look for transiting Jupiter-sized planets, but brown-dwarf stars are much the same size as Jupiter and so produce planet-like transits. That means we only discover which is which by measuring the mass of the transiting body by radial-velocity techniques.
So we should find brown dwarfs as readily as planets. But we’ve found only two, WASP-30b and now WASP-128b, compared to over 150 planets. That means that closely orbiting brown dwarfs must be much rarer than planets. It seems that star-like, gravitational-collapse formation rarely produces objects with a mass as low as 30 to 50 Jupiters (that’s not enough mass to collapse easily), while planet-like accumulation of planetesimals rarely builds up to mass that high (there aren’t enough planetesimals).
Masses and radii of known brown dwarfs. WASP-128b is the object with a mass of 37 Jupiters, while WASP-30b has a mass of 61 Jupiters. The coloured regions denote theoretical models for the mass–radius relation at different ages.
Which means that WASP-128b, newly announced on arXiv today in a paper by Vedad Hodžić etal, is a very rare object, being a brown dwarf with a mass of 37 Jupiters in a 2-day orbit around a G-type star. The nearest comparable object is KOI-205b, at 40 Jupiter masses, though that transits a star that is 2 magnitudes fainter and so is harder to study.
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?