Monday, October 21, 2013

Patchy Clouds on an Exotic World

Figure 1: Artist’s impression of Kepler-7b (left), a gas giant planet 1.6 times the radius of Jupiter (right). Kepler-7b is the first exoplanet to have its clouds mapped. The cloud map was produced using data from NASA’s Kepler and Spitzer space telescopes. Credit: NASA/JPL-Caltech/MIT.

Using data from NASA’s Kepler and Spitzer space telescopes, a team of astronomers have created a cloud map of a scorchingly hot gas giant planet known as Kepler-7b. This planet has a mass of 0.44 ± 0.04 Jupiter mass and a size of 1.61 ± 0.02 Jupiter radii. The low mass and large size gives Kepler-7b an exceptionally low density of just 14 percent the density of liquid water. Kepler-7b circles its host star in a tight 4.89-day orbit. By detecting infrared light from Kepler-7b, Spitzer was able to measure the planet’s temperature, estimating it to be between 1,100 K and 1,300 K. This is somewhat too cool for a planet that orbits so close to its host star. However, this can be explained by the high reflectivity observed for Kepler-7b, where the planet reflects a larger faction of light coming from its host star and so does not heat up as much. Such a high reflectivity is believed to be due to the presence of reflective high altitude clouds in the planet’s atmosphere.

In fact, Kepler-7b’s measure temperature places it within an exceptionally rich region of condensation phase space where silicate clouds can potentially form in the upper, observable portion of the planet’s atmosphere. The same would not be true for a warmer planet (temperatures on the dayside would be too hot for silicate clouds to condense) or a cooler planet (silicate clouds would only be present in the deep unobservable layers of the atmosphere). Hence, Kepler-7b is neither too hot nor too cold for silicate clouds to form in its observable atmosphere.

Observations of Kepler-7b’s dayside by Kepler and Spitzer reveal the presence of reflective high altitude clouds located west of the planet’s substellar point. “By observing this planet with Spitzer and Kepler [telescopes] for more than three years, we were able to produce a very low-resolution ‘map’ of this giant, gaseous planet,” study co-author Brice-Olivier Demory of the Massachusetts Institute of Technology in Cambridge said in a statement. “We wouldn't expect to see oceans or continents on this type of world, but we detected a clear, reflective signature that we interpreted as clouds,” he said.

Figure 2: Models of the dayside temperature structure of Kepler-7b. Both a cloud-free model (orange) and cloudy model (blue) are shown. (Demory et al., 2013)

Figure 3: Models of the dayside planet/star flux ratio for Kepler-7b. Compared to the cloudy model (blue), the cloud-free model (orange) is fainter in the optical but brighter in the mid-infrared. The cloudy model is brighter in the optical due to the scattering of light by clouds. Dashed curves represent the thermal emission component (i.e. heat from the planet) and solid curves represent the total flux. The optical detection in the Kepler band (red) is shown, along with the Spitzer 1-σ (cyan) and 3-σ (red) upper limits. (Demory et al., 2013)

The tell-tale sign for clouds first came from a westward shift seen in the Kepler visible light curve of Kepler-7b. This corresponds to a bright region on Kepler-7b that is centred 41 ± 12° west of the substellar point. Previously, this bright region was thought to be a more intensely heated part on the planet. However, observations by Spitzer show a lack of thermal emission from Kepler-7b and this suggests that the bright region is largely caused by reflected light rather than heat. The reflected light is believed to be light from the planet’s host star being scattered back into space by reflective high altitude clouds, with the most likely candidate being silicate clouds. East of the substellar point, the skies appear to be relatively cloud-free. Being so close to its host star, Kepler-7b is tidally-locked where the same side of the planet always faces its host star, resulting in a permanent dayside and a permanent night side. As a result, the cloud patterns on Kepler-7b are not expected to change much over time, unlike those on Earth.

Demory et al. (2013), “Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere”, arXiv:1309.7894 [astro-ph.EP]