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.

Long-lasting starspots on exoplanet-host Qatar-2

Planets transiting their star can cross a starspot, and that — since the spot is dimmer than its surrounding — causes an upward blip in the light-curve of the transit. The same starspot can be occulted in consecutive transits, and so is seen later in phase each time because the star has rotated between the transits.

An illustration of a starspot feature in consecutive transits.  Image by Klaus Felix Huber.

A starspot feature in three consecutive transits. Image by Klaus Felix Huber.

Keele University PhD student Teo Močnik has looked at the Kepler K2 lightcurve of Qatar-2, a star known to host a hot Jupiter in a 1.34-day orbit. The lightcurve records 59 consecutive transits over a 79-day period and Močnik finds that most of the observed transits are affected by starspots (link to paper).

In the plot below each numbered lightcurve is from a transit, which occurs between the vertical dashed lines. The transit profile itself, however, has been subtracted in order to better show the starspot features.

Starspots in transits of exoplanet host Qatar-2

The starspots occur in groups, shown by red ellipses, and each group is the same starspot being seen in consecutive transits. Interestingly, though, the groups of spots themselves recur. Thus the starspots are lasting long enough that they pass behind the limb of the star, and then re-appear to be transited again one stellar rotation cycle later!

One particular starspot first causes the features in transits 20 to 22, then comes round again to produce the features in transits 33 to 36, and then comes round once again to produce the features in transits 46 to 50. Thus the starspot must have lasted for at least 40 days.

We thus have one of the best observations yet of a starspot on a star other than our sun. From this information we can calculate the rotation period of the star, place limits on the size, position and longevity of the spots, and also show that the planet’s orbit is closely aligned with the spin axis of the star.

Using the stellar rotation to trace a planet’s orbit

As a transiting exoplanet tracks across its star it progressively blocks out different regions of the face of the star. Since the star will be rotating, one limb of the star will be moving towards us (and so its light will be blueshifted) while the other limb recedes (producing a redshift). The blocking of light by the planet thus changes the spectral lines from the star. This is called the Rossiter–McLaughlin effect, and it can be used to discern the track of the planet’s orbit.

Brett Addison and Jonti Horner have written a nice introduction to such techniques on the widely read The Conversation website. Since large numbers of WASP planets orbit stars bright enough to enable a detection of the Rossiter–McLaughlin effect, around half of the planets with measured orbits are WASP planets.

Addison and Horner illustrate their piece with an artist’s conception of the polar orbit of WASP-79b:

Hot Jupiter exoplanet WASP-79b polar orbit