Category Archives: exoplanet atmospheres

WASP-18b has a smothering stratosphere without water

NASA Goddard Space Flight Center and NASA Jet Propulsion Laboratory have put out press releases about observations of WASP-18b with the Hubble Space Telescope and the Spitzer Space Telescope.

The main finding is that WASP-18b, a highly irradiated hot Jupiter in a tight orbit around a hot F-type star, is “wrapped in a smothering stratosphere loaded with carbon monoxide and devoid of water”.

The team determined this by detecting two types of carbon monoxide signatures, an absorption signature at a wavelength of about 1.6 micrometers and an emission signature at about 4.5 micrometers.”

The findings have been reported in many media outlets including: Newsweek, The Independent, The Sun, the Daily Mail, the International Business Times, phys.org, and more than 20 other websites including Forbes magazine, who have produced the following infographic:

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Wide-coverage spectrum of exoplanet WASP-39b

WASP-39b is turning out to be one of the more important WASP discoveries, being observed with the Hubble Space Telescope, the Spitzer Space Telescope and large ground-based telescopes such as the VLT. This is because, as a Saturn-mass planet with a bloated radius, it has a low surface gravity and so is ideal for atmospheric characterisation. Further, it has relatively clear skies showing spectral features.

Now a team led by Hannah Wakeford from Exeter University have put the different data-sets together to produce the widest-coverage spectrum of the planet so far:

The dominant spectral features are due to water vapour, while there are narrower lines due to sodium (Na) and potassium (K) and a Rayleigh-scattering slope at the blue end.

The main finding from fitting the water features is that the atmospheric metallicity must be at least 100 times that of the sun. This high value shows the diversity of exoplanets. The authors conclude that “WASP-39b is an ideal target for follow-up studies with the James Webb Space Telescope”.

WASP planets selected for James Webb Space Telescope ERS and GTO

Studying the atmospheres of exoplanets is one of the main goals of the James Webb Space Telescope, now scheduled for launch in mid 2019. The mission recently asked for proposals for “Early Release Science”, observations to test out the instruments, show what JWST can so, and supply the community with data to start analysing.

Of 13 ERS proposals accepted, the “The Transiting Exoplanet Community ERS Program”, led by Kepler lead-scientist Natalie Batalha, got all the time it asked for.

WASP planets feature heavily in the ERS program, since many transit relatively bright stars. Large, puffy gaseous planets will also give the strongest and clearest signals of atmospheric features, and so are optimum early targets. While JWST will want to look also at atmospheres of smaller, rocky planets, “Astronomers initially will train their gaze onto gaseous Jupiter-sized worlds like WASP-39b and WASP-43b because they are easier targets on which to [look for the chemical fingerprints of the atmosphere’s gases]”.

The target list for the ERS proposal is currently being finalised in the light of the recent delay in JWST launch from 2018 to 2019, though an earlier draft of the proposal featured 7 WASP planets out of 12 targets.

Further, the four GTO teams have also selected WASP planets for early JWST observations. GTO time (“Guaranteed Time Observations”) is time allocated to the teams who built the JWST instruments as a reward and incentive. All four instrument teams have picked WASP planets, including WASP-17b, WASP-52b, WASP-43b, WASP-69b, WASP-77Ab, WASP-80b, WASP-107b and WASP-121b.

Meanwhile, Kevin Heng, of the University of Bern, has written a popular-level account for American Scientist of how JWST is expected to revolutionise the study of exoplanet atmospheres.

Hot Jupiter irradiation and the efficiency of heat recirculation

Here’s an interesting plot from a new paper by Michael Zhang et al.

The x-axis is the irradiation temperature for a sample of hot-Jupiter exoplanets; that is, how blasted the day-side of their atmosphere is by irradiation from the host star. This depends on the temperature of the star, its size, and the closeness of the orbit.

The heat of the day side of the planet is then transported to the night side by winds (the planets are phase-locked, so the same side always faces the star). The efficiency of this re-circulation of heat then determines whether the hottest regions of the planet are directly facing the star, or whether they are offset by some angle. This angle can by measured by looking at the “phase curve” radiation in the infra-red.

The y-axis then shows the observed offset angle as a function of the irradiation. The plot shows that the offset angle appears to be highest for cooler planets, and then decreases as irradiation increases, but then perhaps increases again for the very hottest planets such as WASP-33b.

There is, however, also a lot of scatter in the plot. The authors speculate that this might result from differing metallicities of the planets, which affects how well they form clouds, which can then determine the albedo of the planet, and thus how much irradiation is simply reflected.

First results on the atmosphere of WASP-107b

Being a Neptune-mass planet (0.12 MJ) bloated to a near-Jupiter radius (0.94 RJ) makes WASP-107b’s atmosphere very fluffy, and that, coupled with it transiting a moderately bright K star (V = 11.6) makes it a superb target for atmospheric characterisation.

Laura Kreidberg et al have pointed the Hubble Space Telescope at WASP-107b to make the first atmospheric study. Here’s the WFC3 spectrum:

Hubble Space Telescope spectrum of WASP-107b

The broad features at 1.15 and 1.4 microns are due to water absorption in WASP-107b’s atmosphere. Kreidberg et al model the features, finding that they are compatible with expectations given solar abundances. They are not deep enough, though, to be produced by fully clear skies, and a layer of high-altitude cloud is also required.

WASP-107b is one of the prime exoplanets already chosen for early observations with the imminent James Webb Space Telescope, so it is exciting to know that its atmosphere does show prominent molecular features.

Titanium oxide in the atmosphere of WASP-19b

The European Southern Observatory have put out press release about observations of WASP-19b with the Very Large Telescope. A team led by ESO Fellow Elyar Sedaghati have found titanium oxide in the atmosphere of an exoplanet for the first time.

ESO’s graphic (credit: ESO/M. Kornmesser) illustrates how observations during transit allow us to analyse an exoplanet’s atmosphere. The star light shines through the atmosphere, where light at particular wavelengths is absorbed by molecules, causing the light that we see to carry a distinctive signature of the atmosphere’s composition.

The team observed three different transits of WASP-19b, each in a different colour, to produce one of the best transmission spectra of an exoplanet so far. The titanium oxide (TiO) features are marked, along with those from water (H2O), sodium (Na) and scattering due to haze.

ESO’s press release has led to coverage on several dozen news- and science-related websites. ESO have also produced an artist’s impression of WASP-19b:

WASP-12b a “Blistering Pitch-Black Planet”.

NASA has put out a press release about Hubble Space Telescope observations of WASP-12b. Taylor Bell et al find that WASP-12b “traps at least 94 percent of the visible starlight falling into its atmosphere”, making it “as black as fresh asphalt”.

WASP-12b “as black as asphalt” (Credit: NASA, ESA, and G. Bacon, STScI)

The article explains that WASP-12b, in a very close, 1.2-day orbit, is so irradiated by its host star that “clouds probably cannot form to reflect light back into space. Instead, incoming light penetrates deep into the planet’s atmosphere where it is absorbed by hydrogen atoms and converted to heat energy”. NASA’s press release has led to coverage on several dozen websites.

WASP-12b is one of the more important of the WASP discoveries, with over 30 refereed papers so far focused on understanding it. Most notably, the fierce stellar irradiation means that material is boiling off the planet and forming a cloud surrounding it.