The discovery and atmosphere of the hot Jupiter Kepler-12 b

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An outline of the paper:  Discovery and Atmospheric Characterization of Giant Planet Kepler-12b: An Inflated Radius Outlier by J. J. Fortney et al.

The Kepler space telescope has found a large number of new transiting exoplanets from its first data release (Borucki et al. 2011).  Fortney et al. recently announced the characterisation of Kepler-12b, an inflated hot Jupiter, including some inferred atmospheric properties.

Data from Kepler and HIRES constrain the planet to 1.70 ± 0.03 RJ and 0.43 ± 0.04 MJ, giving a density less than a tenth that of Jupiter (0.11 ± 0.01 g cm -3)! This makes it one of the most “puffed up” hot Jupiter that we know of.

These measurements also inform us on the bloated hot Jupiter problem: the figure below shows the location of Kepler-12 b on a plot of planet size as a function of incoming flux from the host star. The very large radius of Kepler-12 b is consistent with the distribution of the other inflated hot Jupiters, but makes an extreme case even when taking into account the steep slope of radius inflation as a function of irradiation.

Radius-incident stellar flux diagram of constrained mass-radius giant exoplanets, coloured by mass. The boxed region is where the suggested irradiation-radius relation holds. Adapted from Fortney et al. (2011), with parameters tabulated in www.inscience.ch/transits.

The depth of the secondary eclipse is measured by Kepler and Spitzer as  0.0031+0.0007% , 0.137 ± 0.020% and 0.116 ± 0.034%.  In the figure shown below, the authors attempt to fit several atmospheric models to the occulation depths (and thus the planet’s thermal emission). In red is a model with a built in temperature inversion (due to absorption by TiO and VO vapour). The green and blue models have no inversion, but the blue model has negligible redistribution of heat to the night side, while the green model includes efficient heat redistribution to the night side.

Transmission spectrum of Kepler-12b showing radius as a function of wavelength from Spitzer and Kepler observations. The three models (red, green, blue) are described in the text. Adapted from Fortney et al. (2011).

The authors conclude that the non-inverted and low energy redistribution model is favoured by the data. The small number of measurements, coupled with the relatively small differences between the models make it difficult to truly eliminate any one model.

Feature image: Kepler’s mirror. Credit: NASA and Ball Aerospace

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I am a PhD student in the Exeter astrophysics department. I studied as an undergraduate in Durham university. I specialise in statistical properties of radial velocity datasets and theoretical atmospheric line features. (More)