The very hot Jupiter WASP-12 b


The “very hot Jupiter” prototype

Even amongst the already extreme “hot Jupiters”, WASP-12 is an extreme case. It orbits a hot F-star in only one day, which results in an equilibrium temperature close to 3000 K! This temperature makes the planet a favourable target for atmospheric study through both eclipse and transit observations.

WASP-12 b parameters:

Mass 1.41 ± 0.09 MJ
Radius 1.79 ± 0.09 RJ
Period 1.09 days
Orbital distance 0.0229 AU
Vm 11.7 mag


Mass 1.35±0.4 MSun
Temperature ~6250 K

Eccentricity, size, tides and mass loss

WASP-12 b is extreme not only in period, but also in size. With a radius of 1.7  MJ, it reinforces the correlation between inflated hot Jupiters and closeness to the parent star. Initially, the planetary orbit was reported to have a small but detectable eccentricity of a few percent (Hebb et al. 2009), seemingly confirmed by a delay in the secondary eclipse from the ground (Lopez-Morales et al. 2010). A non-zero eccentricity would have been very surprising for such a close-in planet, because the tidal forces are strong enough to circularise the orbit rapidly. Both results turned out, however, to be attributable to under-estimated uncertainties, and new data showed the orbit to be circular (Campo et al. 2011 for the timing of the secondary eclise with Spitzer, Husnoo et al. 2011 for the radial velocity orbit),

Li, Lin et al. (2010) modelled WASP-12 b. They show that the planet fills such a large fraction of its Roche lobe that it is expected to be very elongated and loosing a substantial flux of mass to the star. They also find that a significant eccentricity would explain the inflated radius, because tidal effects inject a massive amount of energy in the planet. Unfortunately, the eccentricity detection being spurious, this explanation no longer holds.

The planet WASP-12 b and its Roche Lobe. From Li et al. (2010).

Following up on this, there have been several attempts to detect the mass outflow. Lai, Helling et al. (2010) study how the outflow can lead to detectable changes in the shape of the transit in UV. Vidotto et al. (2010) interpret a possible early UV ingress in terms of a magnetic bow shock, that could be indicative of the magnetic field of the planet. However Fossati et al. (2010), using the spectropolarimeter ESPADON on the CFHT, find no structured magnetic field on the host star.



WASP-12 b has also attracted attention in another respect. The brightness of the planet has been measured in several passbands from the ground and with Spitzer. Madhusudhan et al. (2010) have explored the range of model spectra that could fit the data with various abundances for the main molecules expected, mainly water, CO, CO2, and  CH4 (methane). They find that the spectrum is best accounted for by strong CO and methane lines and a very low water content, and tie this in with the idea of “carbon planets”. In such planets, the C/O ratio would be higher than unity, so that the oxygen would be tied in CO rather than water, and there would be excess carbon to form methane. In solar-system planets, the C/O ratio is around one-half, so that there is enough oxygen to tie all the carbon, and the rest of the oxygen can form water.

At the “Extreme Solar Systems II” meeting in September 2011, though, new Spitzer observations were presented that seemed to invalidate the features in the emission spectrum on which those conclusions were based, so the issue is still open.

Another interesting measurement on WASP-12 b is that of Fossati et al. (2010). In the near-UV part of the transmission spectrum, they measure a marginally significant (2.5-sigma) excess of absorption in the areas were some metallic absorption lines are expected.

WASP-12 b is a stimulating object, which has produced several interesting and provocative hypotheses. To be fair, most of these have yet to be corroborated.


Feature Image: From Li et al. (2010), found here.


About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.

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