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?

Magnetic activity on planet-host stars

One interesting question is whether close-in hot-Jupiter planets have an effect on the magnetic activity of the host star. There have been suggestions that star–planet interactions might increase magnetic activity on the star, or that tidal interactions might decrease it. Further, if mass lost from planets forms an absorbing cloud around the star, then it might reduce observable signs of magnetic activity, even if it doesn’t affect the magnetic activity itself.

A new paper by Daniel Staab et al, led by the Open University, investigates the issue by looking for markers of magnetic activity in spectra of host stars WASP-43, WASP-51, WASP-72 and WASP-103. In the following plot, RHK is a marker of magnetic activity, plotted against the temperature (B–V) of the star. The green dots are a large sample of field stars, while the four WASP host stars are labelled in red.

Chromospheric activity on planet-host stars.

The result is that at least one planet-host, WASP-43, has an abnormally high degree of magnetic activity, while another one, WASP-72, has abnormally low magnetic activity. Staab et al conclude that there is no single factor explaining the differences, and that more than one effect might be in play. As often, a much larger sample of data is needed to investigate the issue.

The atmospheres of WASP planets with JWST

The James Webb Space Telescope is expected to revolutionise the study of exoplanet atmospheres following its launch in 2018, and WASP planets will be among the prime targets. Paul Mollière et al have been simulating the data expected, and have produced this illustration of the atmospheric emission spectrum of WASP-18b.

Spectrum of exoplanet WASP-18b as observed with JWST

The different coloured curves result from different assumptions about WASP-18b’s atmosphere. The lines along the bottom illustrate the spectral coverage of the different JWST instruments. In contrast to existing data (Spitzer results are shown as black squares), the JWST data will have both the spectral resolution and signal-to-noise to differentiate clearly between different models.

Mollière et al have also simulated spectra for cooler planets, such as WASP-10b and WASP-32b.

WASP-10b and WASP-32b simulated atmospheres observed with James Webb Space Telescope.

The different models are for different abundances of carbon relative to oxygen (C/O), showing that JWST should be able to settle the issue of which exoplanets have enhanced abundances of carbon relative to the Solar System.

Such simulations show that the results from JWST should be spectacular, opening up whole new areas of enquiry.

WASP-20 is a binary star

The host star of the hot Jupiter WASP-20b has been found to be a binary star. A new paper by Daniel Evans et al finds WASP-20 to be a binary separated by 0.26 arcsecs, sufficiently close that the second star had not previously been noticed. Evans et al used the SPHERE instrument on ESO’s Very Large Telescope to find the slightly dimmer companion:

WASP-20 is a binary star

The two stars, WASP-20 A and WASP-20 B seem to be gravitationally bound, and the planet appears to orbit the brighter star. The companion star is 61 astronomical units from the planet-hosting star, close enough that it might have had a gravitational effect on the orbit of the planet.

This is relevant since hot Jupiters are thought to have been created much further from their star than their current close-in orbits, and gravitational perturbations from a third body is one suggested mechanism for causing them to migrate inwards.

WASP’s “Super Saturn” feature for kids

The possible discovery of an exoplanet ring system, a “Super Saturn”, has featured in a Frontiers for Young Minds article aimed at scientists aged 8 to 15 years.

The suggestion of an exoplanetary ring system was an interpretation of the multiple dips in the lightcurve of a star, catalogued as 1SWASP J140747.93–394542.6, as observed in WASP-South data in 2007. The article gives a good introduction to the WASP project at an accessible level, complete with this image of the “Super Saturn”:

Illustration of the "Super Saturn" found in WASP data.

The clear atmosphere of WASP-39b, seen from the ground

Most of the best detections of features in the atmospheres of transiting exoplanets have come from the Hubble Space Telescope, but time on hugely expensive satellites is in high demand and limited. Thus a recent paper led by Nikolay Nikolov from Exeter University is a welcome development. Nikolov and his team observed WASP-39b and detected a strong Sodium line from the planet, which indicates a clear atmosphere. The result came from the newly upgraded FORS2 spectrograph on ESO’s Very Large Telescope.

Sodium in the atmosphere of exoplanet WASP-39b

The important feature of the plot is that the VLT data (black) are every bit as good as those from a previous detection of the same line using the Hubble. While Hubble has the advantage of being in space, the VLT has a much larger mirror and can observe whole transits without the gaps seen in Hubble data owing to its low-Earth orbit.

The similar result from a very different facility also gives confidence in the correctness of such detections of features in exoplanet atmospheres, which are, after all, pushing current technology to its limits.

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.