The first spatial map of clouds in an exoplanet atmosphere

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An outline of the paper: Inference of inhomogeneous clouds in an exoplanet atmosphere by Demory et al.

This letter by Brice-Olivier Demory (MIT) and collaborators presents a really neat result: the first spatial map of clouds in an exoplanet’s atmosphere. The planet in question is Kepler-7 b, one of the “starting five” discovered by the NASA Kepler mission back in 2009. With a radius of 1.6 Jupiter radii and a mass of only 0.4 Jupiter masses, Kepler-7 b is a low density gas giant, orbiting a sub-giant G star every 4.9 days.

Quite early on, it was realised that Kepler-7 b has an albedo of ~0.4 at visible wavelengths, with a number of authors reporting the detection of its secondary eclipse in the Kepler data (e.g. Kipping & Bakos 2011, Demory et al 2011). This hinted at the presence of reflective condensates in the atmosphere, because cloud-free hot Jupiter models predict lower albedos across the Kepler bandpass. In this latest study, Demory et al have put the cloud hypothesis to the test, analysing the entire ~1200 days of available Kepler data. This plot shows the phase-folded light curve that they come up with:

kepler7b_phasec
Figure 1. Kepler phase curve for Kepler-7 b, binned into 5 minute intervals. Solid lines show best-fit models: green is assuming a uniform dayside hemisphere that scatters light isotropically, while blue and red allow for the brightness of the dayside hemisphere to vary with longitude – see the paper for the details. Note that the gap at phase 0 corresponds to the primary transit and the drop in flux at phase 0.5 is the secondary eclipse.

The green line shows the best-fit model assuming a uniform dayside hemisphere that scatters incident starlight isotropically. The red and blue lines show best-fit models  in which the brightness of dayside hemisphere is allowed to vary in longitude. It turns out that these latter models do a much better job of explaining the data than the simple uniform-brightness model. In particular, you can see by eye that the peak in the phase curve data occurs well after the secondary eclipse, by approximately 13 hours.

Here’s the map of longitudinal brightness variation corresponding to the red line in the above figure:

kepler7b_map
Figure 2. The brightness map that best explains the Kepler phase curve for Kepler-7 b. Colours indicate the relative brightness of three fixed longitudinal bands that were allowed to vary in the fit – see the paper for more details on the model.

The asymmetry in the dayside brightness, which decreases from west to east, is difficult to explain with anything other than clouds. For instance, if the high albedo was caused by Rayleigh scattering from hydrogen molecules, we’d expect the brightness to be uniform across the dayside hemisphere. Instead, Demory et al suggest that clouds of condensed silicates or iron are causing the reflection, as these materials are expected to condense high in Kepler-7 b’s atmosphere.

It’s interesting that the clouds seem to be most significant in the western hemisphere of the planet. Hot Jupiter circulation models universally predict that strong equatorial jets develop in the atmospheres, moving around the planet in an easterly direction. If this is the case for Kepler-7b, then the map above seems to be showing that the thickest clouds develop from the gas that is flowing from the nightside onto the dayside. This is a useful empirical constraint as we continue trying to understand the structure and dynamics of clouds in hot Jupiter atmospheres.

More generally, this study by Demory et al is a good example of the detailed information that can be recovered from exoplanet reflection signals. Without actually resolving the planet from the star, we can start building up a picture of its weather system. And this measurement was only performed in a single bandpass – imagine the level of detail that could in principle be extracted if a similar measurement was made using a spectrograph and a polarimeter. Although currently there aren’t any instruments that could do this, it’s the kind of thing we can look forward to in the future.

Feature Image: An artist’s impression of the broad cloud structure in Kepler-7b’s atmosphere, as inferred from the Kepler photometry. Jupiter is shown for size comparison.

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About Author

I'm a PhD student at the University of Oxford. My work focuses on transiting exoplanets and, in particular, what we can learn about the atmospheres of these systems. A large part of this involves getting a better handle on the various instrumental systematics that contaminate the small signals we're trying to measure, and devising methods to remove them from the data. I'm also investigating ways of correcting for the effect of star spots on planetary transmission and emission spectroscopy measurements. My supervisor is Suzanne Aigrain.