Category Archives: exoplanets

The spectrum of hot Jupiter WASP-79b

Bloated hot-Jupiter WASP-79b, one of the largest known exoplanets with a radius near twice that of Jupiter, is among the planets scheduled to be observed with the keenly-awaited JWST. In a new paper, Alexander Rathcke et al report on observations made with Hubble. Here’s the spectrum:

The clearest spectral feature, in the range observed with the WFC3 G141 grating, is attributed to water vapour. The authors also interpret the spectrum as showing opacity due to H ions and the effects of faculae on the host star, that are 500 K hotter than most of the star’s surface. They say that this “underscores the importance” of observing a wide wavelength range in order to “disentangle the influence of unocculted stellar heterogeneities from planetary transmission spectra”.

No clouds on the dayside of WASP-43b

As you’ll likely know from flying in an aircraft above the weather, clouds are bright, they reflect a lot of sunlight. This means that if a hot-Jupiter exoplanet has a cloudy atmosphere, then it should also be relatively bright, and so we should be able to detect a discernible drop in light when it it eclipsed behind its host star.

A new paper by Jonathan Fraine et al analyses data obtained with the Hubble Space Telescope WFCS/UVIS instrument to look for the eclipse of WASP-43b. Here is the result (with the data compared to model eclipse profiles):

The authors find no significant eclipse, deriving only an upper limit to any drop in light of 67 parts-per-million, which means that the dayside face of the planet is reflecting less than 6% of the illuminating starlight. And that means “that we can rule out a high altitude, bright, uniform cloud layer”.

Fraine et al remark that “Because of its observational and atmospheric viability for spectroscopic detections, WASP-43b has become a benchmark planet for current and future hot Jupiter observations. Upcoming … JWST observations [will] map the thermal structure and chemical composition of this exoplanet with exquisite detail … We expect that no other exoplanet has or will be observed with this much precision and wavelength coverage for many years to come.”

The importance of cloud-free skies is that one can then see atomic and molecular spectral features much more readily, and so learn much more about the atmosphere’s composition.

Weather on ultra-hot-Jupiter WASP-121b?

Here’s a plot of the spectrum of the ultra-hot-Jupiter WASP-121b. It’s from a new paper led by Jamie Wilson of Queen’s University Belfast.

The plot compares results from different instruments at different times. In particular the green points are from the ground-based Gemini/GMOS instrument, and are fitted by the model in red. The light-blue points (and fitted purple model) are from the space-based HST/STIS instrument.

Clearly the two datasets are not consistent. One possible explanation would involve instrumental systematics that are not properly accounted for in the analysis. Such analyses are right at the edge of what can be done, pushing the instruments beyond their designed capabilities, and reducing the datasets to a properly calibrated spectrum is a demanding task.

The other possible explanation is that WASP-121b really was different on the two occasions, and that “weather” on the planet is affecting its atmosphere. Just as Earth’s atmosphere can change from clear to cloudy, we expect that the same could be occurring on exoplanets.

The authors say that: “WASP-121b is expected to have wind speeds of 7 km/s and a pressure–temperature profile which lies near the condensation curves of a number of species”, and thus: “It is therefore perhaps not all that surprising that small temperature fluctuations could result in significant spatial and temporal variations in atmospheric constituents and could lead to measurable variations in transit measurements.”

TESS observes the WASP-148 system

The hot Jupiter WASP-148b is rather unusual, since it has a sibling planet, WASP-148c in a 35-day orbit (Hébrard et al. 2020). The system was recently observed by TESS leading to a new paper by Gracjan Maciejewski et al. (Nicolaus Copernicus University and the Instituto de Astrofísica de Andalucía).

The gravitational tug of the outer planet WASP-148c perturbs the orbit of the hot Jupiter WASP-148b. Here are deviations in the timings of the hot-Jupiter’s transit (the green points are new timings from TESS, the blue points are from observations from the Sierra Nevada Observatory, the red line is a model based on the masses and orbits of the planets):

The great boon of transit-timing information is that it leads to measurements of the masses of the planets, which can be combined with radial-velocity measurements to give a better overall characterisation of the system.

Maciejewski et al. also searched the TESS data for transits of the outer planet. The yellow areas are the predicted time of transit, should the planet’s orbital inclination be sufficiently high (the red line is a model showing the predicted depth of the transit; the black triangle marks a transit of the hot Jupiter WASP-148b). There is no indication that WASP-148c transits.

A second planet for WASP-107

WASP-107b is a hugely bloated planet, with a mass of only two Neptunes, but a radius near that of Jupiter, making it one of the puffiest planets known. As such it has been heavily studied, and indeed was the exoplanet showing the first detection of helium.

Long-term monitoring of the WASP-107 system with the Keck telescope has now revealed a companion planet, WASP-107c, as announced by Caroline Piaulet et al.

The new planet is in a much wider orbit, with a period of 1088 days and a high eccentricity of e = 0.26. It likely does not transit, and has a mass of perhaps a third that of Jupiter.

The discovery of a second planet is important for understanding the nature of WASP-107b itself. The tight, 5.7-d orbit, and the fact that the orbit is mis-aligned with the star’s rotation, might be explained by gravitational interactions with the second planet. The bloated size could then result from tidal interactions with the host star, as the planet circularised in its orbit, after the interactions with WASP-107c.

The authors conclude that, “Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion”, starting with a planned observation with the soon-to-be-launched JWST.

ESPRESSO looks at ultra-hot-Jupiter WASP-121b

ESPRESSO is ESO’s state-of-the-art spectrograph for the Very Large Telescope, specifically designed to get the best data possible on planetary systems.

Francesco Borsa et al have pointed ESPRESSO at transits of the ultra-hot-Jupiter WASP-121b, and the ESPRESSO team have put out a series of Tweets explaining the paper:

Nightside clouds explain hot-Jupiter phase curves

Vivien Parmentier (Oxford University, @V_Parmentier) has produced an explanatory Twitter thread on his latest paper with Jonathan Fortney (@jjfplanet). Since this concerns WASP exoplanets, let’s reproduce it here:














[The symbols include data points for WASP-43b, WASP-14b, WASP-19b, WASP-18b and WASP-103b, along with other hot Jupiters.]


Which exoplanets do we have atmospheric spectra for?

Here’s an interesting plot created by Zafar Rustamkulov (@exoZafar), a PhD student at Johns Hopkins University. He has added up all the exoplanets for which we have either transmission spectra (blue), emission spectra (red) or both (pink), and plotted the planet’s size and orbital period.

Most atmospheric characterisation has been done on the hot Jupiters (top left of the plot), since these are the easiest to study. Their large size and often bloated, fluffy outer layers produce the largest spectral signals. Smaller planets are harder to study, unless their host stars are very bright or very small (such that the fraction blocked by the planet during transit is relatively large).

For the planets for which we have over 50 spectra Zafar has added the planet’s name (though the lettering is rather small!). This shows that roughly half of the most-studied exoplanets come from the WASP survey. WASP-12b, WASP-33b and WASP-39b are in the Northern Hemisphere and came from the SuperWASP-North survey. WASP-17b, WASP-19b, WASP-31b, WASP-43b, WASP-80b, WASP-107b, WASP-121b and WASP-127b are in the South and so are from the WASP-South survey.

First results from ESA’s Cheops: WASP-189b

ESA’s Cheops satellite (the Characterising Exoplanet Satellite) started observing this year, and ESA has just put out a press release announcing its first science results. Cheops looked at transits and occultations of WASP-189b, an ultra-hot Jupiter in a polar orbit transiting a bright star.

“Only a handful of planets are known to exist around stars this hot, and this system is by far the brightest,” says Monika Lendl of the University of Geneva, Switzerland, lead author of the new study. “WASP-189b is also the brightest hot Jupiter that we can observe as it passes in front of or behind its star, making the whole system really intriguing.”

At a visual magnitude of V = 6.6, WASP-189 is the brightest host star of all the WASP planets. The discovery of the transiting hot Jupiter was announced in 2018 in a paper led by David Anderson. The exceptional nature of WASP-189 thus made it a prime target for Cheops.

The Cheops study shows that: “the star itself is interesting – it’s not perfectly round, but larger and cooler at its equator than at the poles, making the poles of the star appear brighter,” says Dr Lendl. “It’s spinning around so fast that it’s being pulled outwards at its equator!”

“This first result from Cheops is hugely exciting: it is early definitive evidence that the mission is living up to its promise in terms of precision and performance,” says Kate Isaak, Cheops project scientist at ESA.

Press coverage has included articles in CNN, CTV, the International Business Times, The Sun, The Mirror, The Daily Mail, The Express and over 30 other news sites.