Category Archives: exoplanet atmospheres

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.”

The colours of the planets

Here is an interesting image from Donald Mitchell on Twitter. It shows the average colour of planets and moons in the Solar System.

It would be interesting to do the same for exoplanets. Here is Vivien Parmentier showing possible colours of hot Jupiters, depending on their atmospheric composition and temperature (Credit: NASA/JPL-Caltech/University of Arizona/V. Parmentier):

WASP-62b, in James Webb’s continuous-viewing zone, has a clear atmosphere

James Webb’s “Continuous Viewing Zone” is the patch of sky where the satellite can point continuously at a target and so observe it most efficiently. Exoplanets within the CVZ that are suitable for atmospheric characterisation are thus of high importance, and so far WASP-62b is the only gas giant known within the CVZ.

Munazza Alam et al have now pointed the Hubble and Spitzer space telescopes at WASP-62b to see what its atmosphere looks like. Importantly, they find that WASP-62b has clear skies. This matters since cloudy or haze-filled atmospheres tend to produce flat spectra lacking any spectral features, and so don’t tell us much.

Here, Alam et al plot the spectrum near the sodium (Na) line, showing that it has a broad base, akin to that in the clear-skied planet WASP-96b. The broad base of the line means that it is being widened by “pressure broadening”, and that can only happen deep in the planet’s atmosphere where the pressure is high. And we can only see deep into the atmosphere if it is clear rather than cloudy.

Clear skies mean that spectral features produced by the molecules in the atmosphere should be readily detectable with JWST. Here Alam et al simulate what we expect to see with JWST, showing that Na, H2O, NH3, FeH, SiH, CO, CO2, and CH4 can all be detected.

They conclude by saying that: “As the only transiting giant planet currently known in the JWST Continuous Viewing Zone, WASP-62b could prove a benchmark giant exoplanet for detailed atmospheric characterization in the James Webb era.

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.

Dayside spectrum of the ultrahot-Jupiter WASP-121b

Thomas Mikal-Evans et al have released a new paper analysing the heated, dayside face of WASP-121b. Teams studying the atmospheres of exoplanets either look at the transit, when the planet’s atmosphere is projected against the host star, such that molecules produce absorption features in the spectrum, or they study the eclipse, when the heated face of the planet disappear and then reappears. In the latter, atmospheric molecules produce emission features in the spectrum.

Here is the spectrum of the heated face of WASP-121b, based on recording five eclipses using the WFC3 spectrograph on the Hubble Space Telescope. The orange line and yellow banding show the spectrum expected for a pure black body of the same temperature as the planet. The red lines then show model fits, which reveal emission features caused by H ions and water (H2O) molecules.