A new paper by Joshua Kammer et al reports observations of 5 transiting hot-Jupiter planets with the Spitzer Space Telescope. The Spitzer infra-red observations looked for the occultation of the planet, when it passes behind its host star. By comparing the observed emission in and out of the occultation one can deduce the temperature of the planet’s atmosphere.
Kammer and colleagues chose to look at 5 relatively cool hot-Jupiter planets (ones around cooler stars, or orbiting further from the star), with expected temperatures in the range 900 to 1200 K. Of the 5, four were WASP planets (WASP-6b, WASP-10b, WASP-39b and WASP-67b).
The point of looking at cooler planets is that the ratio of the light in two Spitzer pass-bands, 3.6 and 4.5 microns, is expected to depend on the metallicity (the abundance of elements heavier than hydrogen and helium) of the planet’s atmosphere.
The authors found a tentative but possible relation between that ratio and the mass of the planet.
The plot shows the brightness ratio in the two pass-bands against planet mass. The named planets are also colour-coded by the planet’s temperature (where the top bar shows the scale in Kelvin). There is a possible trend to a higher ratio at higher masses (WASP-8b is a clear outlier to the trend, and the authors suggest that this might be because it is in a highly eccentric orbit).
Kammer et al say that “If this trend can be confirmed, it would suggest that the shape of these planets’ emission spectra depends primarily on their masses, consistent with the hypothesis that lower-mass planets are more likely to have metal-rich atmospheres.”