The article compares WASP-67b and HAT-P-38b, noting how similar they are in size and temperature, both orbiting similar stars at a similar orbital distance. But then Hubble’s Wide Field Camera 3 found that WASP-67b has a very cloudy atmosphere whereas HAT-P-38b has much clearer skies.
From the press release: Perhaps one planet formed differently than the other, under a different set of circumstances. “You can say it’s nature versus nurture,” explains co-investigator Kevin Stevenson. “Right now, they appear to have the same physical properties. So, if their measured composition is defined by their current state, then it should be the same for both planets. But that’s not the case. Instead, it looks like their formation histories could be playing an important role.”
“Astronomers measured how light from each parent star is filtered through each planet’s atmosphere. HAT-P-38 b did have a water signature indicated by the absorption-feature peak in the spectrum. This is interpreted as indicating the upper atmosphere is free of clouds or hazes. WASP-67 b, has a flat spectrum that lacks any water-absorption feature, suggesting most of the planet’s atmosphere is masked by high-altitude clouds.”
The NASA press release has been picked up and reported on several dozen science-related websites.
Credits: Artwork: NASA, ESA, and Z. Levy (STScI); Science: NASA, ESA, and G. Bruno (STScI)
Orbiting a hot F-star in only 1.27 days, WASP-121b is a highly irradiated hot Jupiter found by Laëtitia Delrez et al using the WASP-South survey. A team led by Tom Evans at Exeter has now pointed the Hubble Space Telescope at WASP-121b and found that its atmosphere shows a “stratosphere”. That is, the higher layers of the atmosphere appear to be hotter than the lower layers.
This is possible if molecules high in the atmosphere absorb radiation very efficiently. The “stratosphere” interpretation comes from finding spectral features caused by water, but seeing them in emission (as expected if the atmospheric temperature increases with height) rather than in absorption (expected if the temperature declines with height).
The data show the Hubble spectrum observed during transit using the WFC3 instrument. The red line is a model including a stratosphere. The blue lines are, for comparison, colder “brown dwarfs” which don’t have a stratosphere. The WFC3 data (circles with error bars) clearly favour the stratosphere interpretation.
NASA have put out a press release about the discovery, while the press team at Exeter have produced an illustration of the highly irradiated planet:
Many forefront facilities such as the Hubble Space Telescope and ESO’s Very Large Telescope are being pointed at exoplanets to try to find out what their atmospheres are made of. Yet such work is right at the limit of what can currently be done (though we hope that the James Webb Space Telescope will soon change that). So to what extent can we trust the results?
Here is an interesting puzzle. A new paper by Neale Gibson et al reports a spectrum of the atmosphere of WASP-31b, obtained with the FORS2 instrument on the VLT.
The spectrum is mostly flat, implying that the planet has a fairly cloudy atmosphere, but towards the right-hand side the orange line (a computed model) shows a strong emission line owing to potassium. The problem is that while one data point from previous HST data (small grey circle) indicates the presence of a strong potassium line, the new data from the VLT (the green-square data point) is incompatible with the HST data and would mean that there is no strong potassium line.
Gibson and co-authors put a lot of effort into trying to resolve the discrepancy, and consider whether Earth’s atmosphere might be contaminating the ground-based data, or whether unknown systematic uncertainties might be affecting the Hubble data. Overall they can only “highlight the need for caution” in interpreting such features. This illustrates that science at the cutting edge is never easy, and that much of an astronomer’s time is spent investigating whether one can trust the data one is working with.
NASA’s Jet Propulsion Laboratory have put out a press release suggesting that clouds in exoplanet atmospheres might be preventing the detection of water that lies beneath the clouds, thus explaining why some hot Jupiters show signs of water while others don’t.
The release is based on work by Aishwarya Iyer et al, published in the Astrophysical Journal in June. Iyer et al made a comprehensive study of Hubble/WFC3 data for 19 transiting hot Jupiters, including many WASP planets.
Clouds in Hot-Jupiter atmospheres might be preventing space telescopes from detecting atmospheric water. Image credit: NASA/JPL-Caltech
The press release has been extensively reported, being carried on over 40 news websites. In the UK the Daily Mail covered the story, and included a note about the recent Keele University-led discovery of five new hot Jupiters, WASP-119b, WASP-124b, WASP-126b, WASP-129b and WASP-133b.
The hot Jupiter WASP-121b, discovered recently by Laetitia Delrez et al, is a very good opportunity for learning what the atmosphere of an exoplanet is made of. Being in a close, 1.27-day orbit around a hot star makes the atmosphere hot, while being a bloated planet of 1.9 Jupiter radii makes the atmosphere puffy. That means one can observe the planet in transit, projected against its star, and readily observe spectral features caused by the atmosphere absorbing star light.
Thomas Evans et al have pointed the Hubble Space Telescope at WASP-121b. To model the resulting spectrum they find they need an atmosphere containing titanium oxide, vanadium oxide, and iron hydride. In the plot below, models with these molecules are plotted red and yellow, and fit the observations, while models without, plotted in green and purple, do not.
The model also shows that WASP-121b has clear skies, rich in water vapour. It looks as though WASP-121b will become one of the most important exoplanets for such atmospheric characterisation work.
Transmission spectroscopy of exoplanet atmospheres — looking at the atmosphere of a planet in transit, backlit by the light of its star — is one of the major growth areas in studying WASP planets.
The latest such study is by Patrick Fischer and colleagues, who pointed the Hubble Space Telescope with its STIS spectrograph at WASP-39b in transit.
The plot shows the resulting data compared with three models of WASP-39b’s atmosphere (depending on how clear or hazy it is, and on the metal abundance compared to the Sun).
Unlike some hot Jupiters, which have very hazy atmospheres with few spectral features, WASP-39b shows a clear detection of potassium and sodium, as expected in largely clear skies.
Comparing to the hazier planets HD 189733b and WASP-6b, Fischer et al remark: “These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres”.
NASA have put out a press release regarding the largest-ever study of hot-Jupiter atmospheres by the Hubble Space Telescope and the Spitzer Space Telescope. Of the ten planets studied, six are WASP discoveries.
The results, published in Nature, report that hot Jupiters are a diverse group that have atmospheres ranging from clear to cloudy. Strong water absorption lines are seen when the planets have a clear atmosphere, but less so when the atmospheres are dominated by clouds and hazes.
Planets such as WASP-17b and WASP-19b have clear atmospheres and show the strongest water features, whereas planets such as WASP-12b and WASP-31b are more cloudy.
The NASA press release has so far resulted in articles on over 110 news websites worldwide. The paper was lead-authored by David Sing of the University of Exeter.