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.
WASP-33b is the hottest of the WASP planets, being the only one so far found orbiting a very hot A-type star. A team led by Korey Haynes from NASA’s Goddard Space Flight Center, have used Hubble to show that WASP-33b has a “stratosphere”. The spectrum in the infra-red is best explained by a temperature inversion caused by the presence of Titanium Oxide in the atmosphere.
Titanium Oxide is noted for its ability to absorb light, which is why it is often used in sunscreen lotion. NASA’s graphic shows how an absorbing layer in the atmosphere produces a “temperature inversion” with a hotter layer higher up:
WASP-33b’s stratosphere was detected by measuring the drop in light as the planet passed behind its star (top). Temperatures in the low stratosphere rise because of molecules absorbing radiation from the star (right). Without a stratosphere, temperatures would cool down at higher altitudes (left). [Image: NASA/GSFC]
By comparing models with and without a temperature inversion to the spectrum of WASP-33b, as observed with Hubble’s WFC3 instrument, Haynes et al “make a very convincing case that we have detected a stratosphere on an exoplanet”.
The figure shows the spectrum of WASP-33b (left) and the temperature profile of the atmosphere (right), both for models with a temperature inversion (red) and without an inversion (blue). (From Haynes et al 2015)
WASP-6b was WASP-South’s third planet, announced in 2009 by Gillon et al. It is a good target for studying exoplanet atmospheres since it is a bloated planet, only half a Jupiter mass but 20% larger than Jupiter.
Nikolov et al (2014) have now pointed the Hubble Space Telescope at WASP-6b in transit, using the STIS spectrograph. They find that the transit depth varies with colour; effectively the planet looks slightly larger in blue light, since small particles in the planet’s atmosphere are scattering blue light more than red light.
The strong blue slope in the plot is characteristic of Rayleigh scattering, the same effect that causes Earth’s atmosphere to look blue (in the plot the red line is a Rayleigh-scattering model, though other model fits are possible).
Nikolov etal state that: “With a broad-coverage optical transmission spectrum measured from HST and Spitzer broad-band transit spectrophotometry, WASP-6b joins the small but highly valuable family of hot-Jupiter exoplanets with atmospheric constraints.”
The field of exoplanet atmospheres is growing rapidly in importance, and it is good to see WASP planets being chosen as prime targets for such work.