Tag Archives: planet formation

Long-period brown dwarfs for WASP-53 and WASP-81

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.

Radial velocity monitoring of WASP-53 and WASP-81

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?

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The birthplace of hot-Jupiter exoplanets

The WASP-discovered Jupiter-sized exoplanets all orbit very close to their star, having orbital periods of typically a few days. Yet they are not thought to form there because it is generally too hot. It is likely that they form further out, where it is colder, where ices can form and start sticking together to form the planetesimals that then clump together to form planets.

The ALMA array is designed to observe the disks of cold material around young stars, disks that planets form out of. Here’s an artist’s impression, released by the National Astronomical Observatory of Japan, of a Jupiter-sized exoplanet forming out of a protoplanetary disk. Note that the planet sweeps up all the disk material near its orbit, creating a hole in the disk.

Artist's illustration of planet formation in TW Hya

The image below, from work led by Takashi Tsukagoshi and described in a NAOJ press release, then shows the real proto-planetary disk surrounding the young star TW Hydrae, as imaged by ALMA.

ALMA image of TW Hya

The image shows gaps in the disk, just as expected if planets are forming there. The planets themselves are too small to be seen directly, but this is among the best images yet of the likely birthplace of giant exoplanets.