Category Archives: Hot Jupiters

WASP-118 is pulsating

The K2 spacecraft is monitoring a series of fields along the ecliptic and so producing Kepler-quality photometry on some of the exoplanet systems previously discovered by WASP.

WASP-118b is an inflated hot-Jupiter planet (0.5 Jupiter masses but 1.4 Jupiter radii) on a 4-day orbit around a bright F-type star of V = 11. It was observed for 75 days in K2‘s Campaign 8. Teo Močnik et al have now analysed the data and see transits of the planet, as expected:

WASP-118 transits as observed with K2

The upper black curve is the raw data, while the lower red curve has been corrected for artefacts caused by drifts in K2‘s pointing. Nineteen transits are seen, recurring with the 4-day orbital period.

But Teo Močnik noticed that the out-of-transit photometry was not as flat as expected. After further investigation he deduced that the host star is pulsating:

WASP-118 is a pulsating star

The pulsations have a timescale of 1.9 days and a very low amplitude of 2 parts in 10 000, only discernable given a lightcurve with Kepler‘s photometric accuracy. Thus WASP-118 appears to be a γ-Doradus pulsator, possibly the first γ-Dor variable known to host a transiting exoplanet.

WASP-43b has an aligned orbit

WASP-43b is the hot Jupiter that is closest to its parent star, around which it orbits in only 19 hours. At such a close location, tidal interactions between the planet and the star will be intense. That means that we expect the planet to be phase locked (with its rotation period equalling the orbital period, so that the same side always faces the star), and we expect the orbit to be circular (any eccentricity having been damped by tides), and we expect the orbit to be aligned with the rotation axis of the star.

Tidal damping of the alignment of the orbit is the subject of much investigation. It seems to be most efficient if the planet orbits cooler stars, and much less efficient if the planet orbits a hotter star. This might be because cooler stars have large “convective zones” in their outer layers, which can efficiently dissipate tidal energy, whereas hotter stars have only very shallow convective zones with little mass in them.

Since WASP-43b orbits a cool star, a K7 star with a surface temperature of only 4400 Kelvin, that’s another reason for expecting its orbit to be aligned. This has now been confirmed by observations with the Italian Telescopio Nazionale Galileo. The way to measure the orbital alignment of a transiting exoplanet is by the Rossiter–McLaughlin effect. As the planet transits a rotating star, it first obscures one limb and then the other, and since the different limbs will be either blue-shifted or red-shifted, according to how the star is spinning, the effect on the overall light of star will reveal the path of the orbit.

Rossiter-McLaughlin effect

A new paper by Esposito et al reports R–M measurements for three planets including WASP-43b. The data show the classic R–M signature of an aligned planet.

Rossiter-McLaughlin effect for exoplanet WASP-43b

The upper panel shows the change in stellar radial-velocity around the planet’s orbit, caused by the gravitational tug of the planet. The lowest panel highlights the data through transit, showing the expected excursion first to a redder light (when blue-shifted light on the approaching limb is occulted) and then to blue light (when the red-shifted receding limb is occulted).

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.

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.