The atmosphere of the inflated hot Jupiter WASP-6b

Atmospheric characterisation of hot Jupiters continues apace, using both ground-based telescopes such as ESO’s Very Large Telescope and satellites such as Hubble.

Aarynn Carter et al have just produced a new analysis of WASP-6b:

The spectrum shows absorption due to sodium (Na), potassium (K) and water vapour, while the modelling implies that the atmosphere is partially hazy. Carter et al state that: “despite this presence of haze, WASP-6b remains a favourable object for future atmospheric characterisation with upcoming missions such as the James Webb Space Telescope.

The spectrum of the bloated, sub-Saturn-mass planet WASP-127b

Here is the latest analysis of the spectrum of WASP-127b, led by Jessica Spake and newly announced on arXiv.

The different datasets come from the Hubble Space Telescope and the Spitzer Space Telescope. Spake et al see obvious features from sodium, potassium, water and carbon dioxide. They conclude that the planet has a super-solar metallicity and that its skies are relatively cloud-free.

WASP-127b is a highly observable target since, despite being less than Saturn’s mass, it is bloated to larger than Jupiter. The puffy atmosphere projected against the host star gives results in a strong signal observable during transit. Spake et al look forward to observing the planet with the James Webb Space Telescope, and say: “the hint of a large absorption feature around 4.5 microns is strong evidence that future observations of WASP-127b with JWST will be able to measure the abundances of carbon-bearing species in its atmosphere”.

The orbit of WASP-12b is decaying

Here’s the latest update on the changes in the orbital period of WASP-12b, from a new paper by Samuel Yee et al.

The times of transit are getting earlier, which means that the period is decreasing slightly. By also considering the times of occultation (when the planet passes behind the star), and also the radial-velocity measurements of the system, the authors deduce that the changes are not the effect of some other planet, but are a real decay in the orbit of WASP-12b. This is expected to occur as a result of tidal interactions between the planet and its host star.

One notable conclusion is that the rate of period decay in WASP-12b is much faster than that in WASP-19b, which shows no detectable period change yet, despite it being an even shorter-period hot Jupiter, which should increase tidal interactions. Yee et al suggest that the difference could arise if the host star WASP-12 is a sub-giant star, whereas WASP-19 is not.

Gravity darkening of planet-host MASCARA-4

MASCARA is one of WASP’s competitor transit-search projects, so let’s celebrate a neat result from TESS data of transits of MASCARA-4b. The host star, MASCARA-4, is a hot, fast-rotating A-type star. As a result of its fast rotation, the equatorial regions are being flung outwards by centrifugal forces, such that the star has a flattened, oblate shape. As a result, the force of gravity will be less at the equator than at the poles of the star, and that means that the equatorial regions will be slightly cooler and so a bit dimmer (in outline, that’s because gravity inward pull is balanced by gas pressure, and so lower gravity means lower pressure, and the temperature of a gas is related to its temperature through the perfect gas law). This effect is called “gravity darkening”.

The star spins around its axis (thick line) while the planet orbits at an oblique angle.

In a new paper, John Ahlers et al have detected the effect of gravity darkening on a transit lightcurve of the hot Jupiter MASCARA-4b. The planet has a misaligned orbit, first coming onto the stellar face near the equator, and then moving towards a pole. That means it moves from slightly cooler regions to slightly hotter regions, and that changes the amount of light occulted by the planet.

If gravity darkening is not taken into account then the model fit is a bit too deep at the start and a bit too shallow at the end of the transit. One of the benefits of detecting this effect of gravity darkening is that it then tells us the true angle between the star’s spin axis and the planet’s orbit (whereas other methods, such as Doppler tomography, only tell us the projection of that angle onto the sky).

Looking forward to WASP-79b with JWST

The bloated hot-Jupiter WASP-79b has been selected as an Early Release Science target for the James Webb Space Telescope, so is being studied with current facilities such as HST and Spitzer.

Here is a simulation of what the spectrum of WASP-79b might look like when observed with JWST, taken from a new paper by Kristin Sotzen et al.

Sotzen et al have collected together data from HST, Spitzer and the Magellan telescope in order to model the atmosphere of the planet and use that to predict the results of the JWST observations. The different coloured symbols are for different instruments of JWST, namely NIRSpec, NIRCam and NIRISS. The main spectral features are caused by water and carbon dioxide molecules. With a partially cloudy atmosphere and detectable water features, Sotzen et al confirm that WASP-79b is a prime target for JWST.

No period change for WASP-19b

Since close-orbiting hot Jupiters are expected to be gradually spiralling inwards, under the influence of tidal interactions with their stars, and since, in addition, the influence of extra, unseen planets in the system could cause changes in transit times, many groups worldwide are monitoring timings of transits of WASP planets.

The latest report on timings of WASP-19b has just been announced by Petrucci et al. The result is the following diagram, showing deviations of timings from a constant ephemeris, plotted against cycle number.

The upshot is that there is no indication of any period change, which then puts limits on how efficient the tidal bulges, caused by the gravitational interaction of the planet with the star, are at dissipating energy.

It is notable, however, that there is clear scatter about the constant-period line, beyond that expected from the error bars on the timings. This means either that the error bars are under-estimating the uncertainties (as would occur if “red noise” in the lightcurves is unaccounted for), or that there is astrophysically real scatter in the timings, perhaps caused by magnetic activity (star spots) on the surface of the star being transited. We need to better understand such timing scatter if we are to be able to judge whether claims of period changes are actually real.

WASP-South detection of transits of HD 219666b

Transiting a bright star, the “Neptune desert” planet HD 219666b was one of the more important early discoveries from the TESS survey. With a depth of only 0.17 per cent, the transits would be a challenge for any ground-based transit survey.

Nevertheless, we think we’ve found them in WASP-South lightcurves dating back to 2010. Here they are:

The orange lines show times of transit, as found by the WASP transit-detection algorithms. The shallow dips seem to be real, since they align both in period and in phase with the dips seen in the TESS lightcurve. The output from the WASP search algorithm is not itself that convincing:

However, the period that it finds (6.03446 days) matches the TESS period to an accuracy of 0.03 per cent, and the WASP ephemeris then predicts the times of the TESS transits bang on (they occur at 420.99999 cycles on the WASP ephemeris), which together mean that the detection must be real. Here are the WASP data folded on a template of the TESS transit:

With a depth of 0.17 per cent, the transits of HD 219666b are the shallowest that WASP has detected.

The benefit of looking for such pre-detections of TESS planets is that we can then produce a transit ephemeris based on data spanning a baseline of 8 years, rather than the 20 days spanned by the TESS transits. This means we can predict future transits to an accuracy of minutes, instead of hours, which is highly useful for future observations. Hence this WASP-South detection of HD 219666b transits is well worth an AAS Research Note.