With most of them orbiting a mere 5-10 stellar radii from their host stars, hot Jupiters get bombarded by so much stellar radiation that it dominates their atmospheric energy budgets. This external energy source is the most fundamental feature that distinguishes them from the gas giants in our own Solar System, and understanding how the incident energy is absorbed and redistributed is a basic question that’s still far from answered.
In 2011, Nicolas Cowan and Eric Agol published a paper trying to make some sense of the available observational data. For each of the 24 hot Jupiters in their sample, they attempted to put constraints on two basic quantities that give a first order characterisation of the global atmospheric energy budget: the Bond albedo and the circulation efficiency. The Bond albedo is the fraction of incident stellar radiation that gets scattered back to space by the planetary atmosphere, integrated across all wavelengths. The circulation efficiency then quantifies the fraction of absorbed energy that gets redistributed to the nightside of the planet by winds.
For the majority of cases, the authors only had secondary eclipse depths at infrared wavelengths to work with, mostly from Spitzer/IRAC datasets. These measurements probe the planetary dayside emission, but the constraints they provide are degenerate between the Bond albedo and circulation efficiency. This degeneracy is quite intuitive, but see Figure 1 of the paper for further details if you’re interested. It can be broken if the planet’s nightside emission is also known, but this was only the case for 3 of the planets in the sample because it’s more expensive to measure, requiring full-phase observations.
The resulting likelihood map for the distribution of Bond albedo versus circulation efficiency is shown in the figure above. Despite the degeneracy between the two parameters, there’s clearly a strong preference for Bond albedos less than about 0.5. This is perhaps reassuring, because it’s broadly consistent with what’s been predicted from models (see for instance Sudarsky et al 2000). On the other hand, high Bond albedos of ~0.5 have been predicted for the hottest (>1700K) hot Jupiters, which may form reflective layers of silicate haze in their upper atmospheres. Given that over half the planets in the sample have estimated dayside temperatures in the >1700K range, the fact that they’re consistent with Bond albedos below 0.5 could be a useful constraint for such models.
In terms of reducing the degeneracy seen in the plot above, one line of attack will be to measure more secondary eclipses at wavelengths closer to the peak of the host star spectra. This is because the continuum emission from a planet will have two peaks: one corresponding to reflected starlight and another corresponding to intrinsic thermal emission. Provided the wavelength separation between these two peaks is wide enough, the contribution from the reflected starlight can be isolated, giving a measure of the atmosphere’s reflective properties independent of the circulation efficiency. Unfortunately, this isn’t always straightforward for hot Jupiters, simply because they’re so hot, and their thermal emission profiles are shifted to shorter wavelengths. As a result, the planetary thermal emission can seep into the passband you’re trying to measure the reflected starlight in, and disentangling the relative contributions can be difficult. This is a particular problem for secondary eclipse signals that have been detected in Kepler lightcurves, as Kepler has a broad passband that extends to 850nm (e.g. Christiansen et al 2010, Demory et al 2011, Kipping & Spiegel 2011, Désert et al 2011).
Despite challenges such as these, Frédéric is currently leading an HST program to try measure the reflected light signal of HD189733 b with the STIS camera. Those data were obtained on 21 December 2012 for a single eclipse and the analysis is ongoing. Combined with the infrared full-phase observations already published by Agol et al (2010) and Knutson et al (2012), we’ll hopefully be able to get a handle on the Bond albedo of this well-studied hot Jupiter. This will be an important piece of the puzzle when it comes to understanding the reflective properties of the haze material that must be responsible for the scattering signature seen so prominently in the optical transmission spectrum.