The hot Jupiter WASP-43b

The hot Jupiter WASP-43b is emerging from the anonymity of the exoplanet crowd to join a more select club. Some things are now known about the atmosphere of this planet, thanks to an impressive series of space observations, mostly with the WFC3 infrared camera installed in 2009 during the last upgrade of the Hubble Space Telescope. This camera is already responsible for a good chunk of the recent results on exo-atmospheres.

The best clues on exoplanet atmospheres come from spectroscopy during transits and eclipses on the one hand, and phase curves over the whole orbit on the other. The first method yields emission and transmission spectra, the second global temperature maps.

Observers have pulled an impressive trick for WAPS-43b. The system was observed at many different orbital phases,  providing the transit and eclipse spectra, and the phase curve as well. The evolution of the spectrum around all phases of the orbit was measured, providing a stop-motion movie of how the spectrum of the atmosphere evolves with the rotation of the planet.

Why choose the WASP-43b? The planet is a particularly favourable target, with its very tight orbit and small parent star: the orbital period is only 0.8 days, and the star is 60% of the mass and size of the Sun. Three papers sketch the resulting portrait of the atmosphere of WASP-43b [references at the bottom of the post]:

– a decreasing temperature with increasing altitude

– a relatively low albedo (18%)

– a shift of the hottest point eastward

– a night side much cooler than the day side

The first three features correspond to expectations, and to observations for other hot Jupiters such as HD 189733b. The fourth feature is more puzzling: a large day-to-night temperature contrast implies a low re-distribution of heat by winds from the day side to the night side. The two other hot Jupiter atmosphere that we know well in the same temperature range, HD 189733b and HD 209458b, both show a low day-night temperature contrast.

By combining the eclipse and transit spectra, Kreidberg et al. derive a robust determination of the water vapour abundance in the atmosphere. This result is a key landmark in the field. It is far from the first time a water vapour abundance is measured (see e.g. here), but it is the first time that such a determination is secure and reasonably independent of other assumptions. The water vapour abundance found for WASP-43b is a nice fit to the standard scenario of giant planet formation, as it extend the trends observed is Solar System planets (see figure).

Water vapour abundances in planetary atmospheres. The new result places WASP-43b right in the sequence of decreasing water abundance with planet mass defined by Solar System planets. The standard scenario of planet formation by core accretion would lead us to expect that lighter planets have more heavy elements in their atmospheres, starting from a Solar abundance for the heaviest planets. [Kreidberg et al. Fig. 4]

Kataria et al. describes a global model of the atmosphere of WASP-43b, including both winds and radiative transfer. They find that indeed the low temperature of the night side is difficult to explain with standard ingredients. There is one possible catch: WASP-43b  phase curves was measured at different wavelengths than for the other planets (1.1-1.7 microns instead of Spitzer’s 4-8 microns filters), so the difference in day-night temperature contrast could be partly due to a different flux distribution in the infrared compared to what the models predict.

The obvious way to solve this issue is to measure the phases of with Spitzer at 3.6 and 4.5 microns. It might have been done already, and someone might be pouring over these data right now. Place your bets.

Impression of WASP-43b. I obtained this by projecting the temperature maps from the atmosphere models onto a sphere, then using the phase curves to translate temperature into flux. The star would be on the left [adapted form Kataria et al. Fig. 9 and Stevenson et al. Fig. S2]

References

A precise water abundance measurement for the hot Jupiter WASP-43b, Kreidberg, Bean, Désert et al. (2014), Astrophys. Journal Letters 793, L27

Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy, Stevenson, Désert, Line et al. (2014),  Science, in press

The atmospheric circulation of the hot Jupiter WASP-43b: comparing three-dimensional models to spectrophotometric data, Kataria, Showman, Fortney et al. (2014), submitted to Astrophys. Journal .]