Spectral contamination from starspots on WASP-4

Here’s a topic we’ll be hearing much more about: how the observed spectrum of a transiting exoplanet is affected by transiting across star-spots. In “transmission spectroscopy” the starlight shines through the planet’s atmosphere during transit, and the easiest thing to do is assume that the star itself is a uniform light source.

But as discussed by papers led by Ben Rackham, if the planet passes over a dark region (star spot) or bright region (faculae), this would change the observed spectrum.

A new paper led by Alex Bixel about WASP-4b is the first to attempt to correct for this effect. The authors’ transit observations show a clear crossing of a starspot (the feature is shown in blue, the spot shows as a upward bump since the planet is then removing less light):

And here is the difference it makes. The blue curve is the observed spectrum, presumed to be of the planet’s atmosphere. The orange curve is then the spectrum corrected for the presence of the star spot.

The details of how to do this are complex, and are discussed at length in the above papers. The central message is that “active FGK host stars can produce such features and care is warranted in interpreting transmission spectra from these systems”.

However, there is good news in that: “stellar contamination in transmission spectra of FGK-hosted exoplanets is generally less problematic than for exoplanets orbiting M dwarfs”, and that such signals “are generally minor at wavelengths of planetary atomic and molecular features”. Overall the authors say that their study “bodes well for high-precision observations of these targets”.

Helium in WASP-69b, HAT-P-11b and HD 189733b

Earlier this year helium was found in the outer atmosphere of WASP-107b, the first detection of helium in an exoplanet. Several teams have now used similar techniques to find helium in WASP-69b, HAT-P-11b and HD 189733b, leading to a slew of papers and accompanying press releases from the Instituto de Astrofísica de Andalucía, the University of Exeter and others (see [1], [2], [3] and [4]).

Artist’s impression of an escaping envelope of helium surrounding WASP-69b. (Credit: Gabriel Perez Diaz, IAC)

Lisa Nortmann, lead author of the WASP-69b paper, explains that the helium is escaping from the atmosphere, forming a comet-like tail: “We observed a stronger and longer-lasting dimming of the starlight in a region of the spectrum where helium gas absorbs light. The longer duration of this absorption allows us to infer the presence of a tail.”

The press releases have led to extensive coverage including by CNN, the Daily Mail and Tech Times.

The IAA press release includes a video illustration of WASP-69b, created by Gabriel Perez Diaz of the IAC:

Is WASP-12b’s orbital decay driven by obliquity tides?

Tidal interactions between hot-Jupiter exoplanets and the host star should be causing their orbits to decay, such that the planet gradually spirals inwards. For most systems the change would be too small to detect in the decade or so that we’ve been observing them. However, WASP-12b is an exception, showing a clear change in its orbital period.

In a new paper on arXiv, Gracjan Maciejewski et al present the latest data for WASP-12b:

The graph records the change in transit time (“observed minus calculated” times, or O–C), showing that the transits are now occurring eight minutes early owing to a decreasing orbital period.

Such a rate is far faster than observed in other systems, and too large to be explained by the standard theory of tidal interactions.

However, a new paper led by Sarah Millholland suggests an answer. She suggests that the planet is tilted over, so that the axis around which it spins is tilted with respect to the plane of the planet’s orbit.

This means that the star will give rise to strong “obliquity tides” on the planet, and the dissipation of those tides could explain the decay of the orbit. For this to work something must be keeping the planet tilted over. Millholland suggests that a second planet in an outer orbit might be perturbing WASP-12b, keeping it in the high-obliquity state. This scenario requires some fine tuning, but if WASP-12 is the only system known to show this behaviour then the explanation is plausible.

WASP-18 is observed by TESS

The TESS mission will survey the entire sky for new transiting exoplanets, and as a by-product will produce space-quality lightcurves of all the WASP exoplanet systems. The first such paper has just appeared on arXiv, where Avi Shporer et al report on the TESS lightcurve of WASP-18.

WASP-18b is the most massive planet found by WASP, a 12-Jupiter-mass planet in a very tight orbit lasting only 0.94 days. This means it has the strongest planet–star tidal interaction of any known planetary system, such that the planet’s gravity gives rise to large tidal bulges on the host star. Here are the TESS data folded on the orbital cycle:

The out-of-transit data are clearly not flat (shown on a larger scale in the middle panel), and show the “ellipsoidal modulation” caused by the tidal bulges on the star. The heated face of the planet is also eclipsed by the star at phase 0.5, producing a secondary eclipse.

By analysing the lightcurve the authors conclude that very little heat is being redistributed from the heated face of the planet. Strong winds could carry heat to the un-irradiated cooler hemisphere, but there is little sign of this in the data.

So far the results of the analysis are in line with theoretical expectations, though the work points to the potential for similar analyses of other previously-known exoplanet systems.

WASP-166b, a Neptune-desert planet

WASP-166b, newly announced on arXiv, is a planet we’ve been following for a while. As we routinely do, we measure the mass of the planet from how much its gravity tugs the host star around, and we measure that from the Doppler shift of its spectrum. The “radial velocity” data for WASP-166, however, didn’t neatly fit the expected orbital motion (the fitted line in the figure), showing, in addition, deviations from the model.

It took us a while to understand this. One suggestion was that it was caused by a second planet also tugging the host star around. So we obtained more data, hoping to trace out the orbit of the second planet, only to find that that didn’t properly explain the data. Eventually we attributed the deviations to magnetic activity on the host star. If we plot the deviations as a function of time we obtain:

Faculae on our Sun

The green sinusoidal lines are at the 12-day period at which we think the star rotates, as judged from the width of the spectral lines (which tells us how fast the star rotates). This suggests that the radial velocity varies with the rotational period of the star, and thus that the deviations are caused by “faculae”, magnetically active patches on the surface of the star.

WASP-166b is a low-mass planet, only a tenth that of Jupiter and twice that of Neptune. It has a large radius, however, at 63% of that of Jupiter. Thus it is a bloated planet with a low surface gravity.

Such planets are rare, especially in short-period orbits around fairly hot stars, as is WASP-166b. Indeed the lack of such planets is called the “Neptune desert”.

The explanation is thought to involve irradiation, heat from the host star evaporating off the atmosphere of the planet. Jupiter-mass planets can resist this because they have a lot of mass and thus gravity to keep hold of the atmosphere. Similarly, smaller, compact planets can survive because they are rocky and denser. But, in the middle, gaseous Neptune-mass planets find it hard to survive when subjected to high irradiation.

Another notable fact about WASP-166b is that it transits a host star with a bright visual magnitude of V = 9.4. The combination of a fluffy atmosphere and bright host star make it one of the best targets for studying the atmosphere by “transmission spectroscopy”, as can be obtained when the planet is projected against the stellar disc during transit. The plot compares the expected atmospheric signal of WASP-166b to the other best targets already known.

Aluminium Oxide in the atmosphere of WASP-33b?

The Instituto de Astrofisica de Canarias have put out a press release on a new paper by von Essen et al, reporting a study of WASP-33b using the 10-meter Gran Telescopio Canarias.

WASP-33 is a hard system to analyse since the host star is a delta-Scuti star, which means that it pulsates. That produces transit lightcurves like these, where the usual transit profile has pulsations superimposed on it. The figure shows the transit in different wavebands across the optical, from blue to red, as obtained with the OSIRIS spectrograph. That meant that the authors first had to model and subtract the effect of the pulsations.

After doing that they analysed how the transit depth depended on wavelength, which reveals how the planet’s atmosphere absorbs light. “We find that the feature observed between 450 and 550 nm can best be explained by aluminium oxide in its atmosphere” says lead author, Carolina von Essen.

“The current models of exoplanetary atmospheres predict that the Ultra Hot Jupiters should be free of clouds, and present a range of oxides in the visible spectrum, such as vanadium oxide, titanium oxide, and aluminium oxide”. This work on WASP-33b is the first observational indication of the presence of aluminium oxide.

Sulfanyl in the atmosphere of WASP-121b?

The latest Hubble Space Telescope spectrum of a WASP exoplanet has just been published by Thomas Evans et al. The spectrum of WASP-121b extends from near-UV wavelengths through the optical to the infra-red, combining data from three different gratings (shown in different colours in the figure):

Of particular interest is the rapid rise in the data in the near-UV (the extreme left of the plot), which is clearly out of line with the fitted model (purple lines). The rise is too rapid to be attributed to Rayleigh scattering in a clear atmosphere.

Instead, the authors suggest that it is due to sulfanyl, a molecule consisting of one sulfur and one hydrogen. Evans et al conclude that the near-UV absorber “likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere”.

The work points to the ongoing importance of the Hubble Space Telescope, even after the James Webb Space Telescope is launched, since the JWST is designed for infrared astronomy, and can’t see the near-UV wavelengths that can be observed with Hubble.

Update: One of the authors, Jo Barstow, has tweeted the following thread on the @astrotweeps account: