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.
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”.
Most of the best detections of features in the atmospheres of transiting exoplanets have come from the Hubble Space Telescope, but time on hugely expensive satellites is in high demand and limited. Thus a recent paper led by Nikolay Nikolov from Exeter University is a welcome development. Nikolov and his team observed WASP-39b and detected a strong Sodium line from the planet, which indicates a clear atmosphere. The result came from the newly upgraded FORS2 spectrograph on ESO’s Very Large Telescope.
The important feature of the plot is that the VLT data (black) are every bit as good as those from a previous detection of the same line using the Hubble. While Hubble has the advantage of being in space, the VLT has a much larger mirror and can observe whole transits without the gaps seen in Hubble data owing to its low-Earth orbit.
The similar result from a very different facility also gives confidence in the correctness of such detections of features in exoplanet atmospheres, which are, after all, pushing current technology to its limits.
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.
A new paper by Joshua Kammer et al reports observations of 5 transiting hot-Jupiter planets with the Spitzer Space Telescope. The Spitzer infra-red observations looked for the occultation of the planet, when it passes behind its host star. By comparing the observed emission in and out of the occultation one can deduce the temperature of the planet’s atmosphere.
Kammer and colleagues chose to look at 5 relatively cool hot-Jupiter planets (ones around cooler stars, or orbiting further from the star), with expected temperatures in the range 900 to 1200 K. Of the 5, four were WASP planets (WASP-6b, WASP-10b, WASP-39b and WASP-67b).
The point of looking at cooler planets is that the ratio of the light in two Spitzer pass-bands, 3.6 and 4.5 microns, is expected to depend on the metallicity (the abundance of elements heavier than hydrogen and helium) of the planet’s atmosphere.
The authors found a tentative but possible relation between that ratio and the mass of the planet.
The plot shows the brightness ratio in the two pass-bands against planet mass. The named planets are also colour-coded by the planet’s temperature (where the top bar shows the scale in Kelvin). There is a possible trend to a higher ratio at higher masses (WASP-8b is a clear outlier to the trend, and the authors suggest that this might be because it is in a highly eccentric orbit).
Kammer et al say that “If this trend can be confirmed, it would suggest that the shape of these planets’ emission spectra depends primarily on their masses, consistent with the hypothesis that lower-mass planets are more likely to have metal-rich atmospheres.”