NASA have put out a press release entitled: “Hubble’s Tale of Two Exoplanets: Nature vs. Nurture”.
The article compares WASP-67b and HAT-P-38b, noting how similar they are in size and temperature, both orbiting similar stars at a similar orbital distance. But then Hubble’s Wide Field Camera 3 found that WASP-67b has a very cloudy atmosphere whereas HAT-P-38b has much clearer skies.
From the press release: Perhaps one planet formed differently than the other, under a different set of circumstances. “You can say it’s nature versus nurture,” explains co-investigator Kevin Stevenson. “Right now, they appear to have the same physical properties. So, if their measured composition is defined by their current state, then it should be the same for both planets. But that’s not the case. Instead, it looks like their formation histories could be playing an important role.”
“Astronomers measured how light from each parent star is filtered through each planet’s atmosphere. HAT-P-38 b did have a water signature indicated by the absorption-feature peak in the spectrum. This is interpreted as indicating the upper atmosphere is free of clouds or hazes. WASP-67 b, has a flat spectrum that lacks any water-absorption feature, suggesting most of the planet’s atmosphere is masked by high-altitude clouds.”
The NASA press release has been picked up and reported on several dozen science-related websites.
Credits: Artwork: NASA, ESA, and Z. Levy (STScI); Science: NASA, ESA, and G. Bruno (STScI)
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.”
The hot-Jupiter exoplanet WASP-67b is a curiosity, being the only known exoplanet with a grazing transit, such that not all of the planet transits the host-star’s disc. This means that the characteristic “second contact” and “third contact” points are missing from the transit lightcurves. These are the points where, usually, the whole planet is now in front of the star, and the transit is then flat-bottomed, apart from the relatively small effects of stellar “limb darkening”.
The four “contact” points of a planetary transit, illustrated for Mercury transiting our Sun.
In WASP-67b’s grazing transit the planet is never completely in transit and thus the transit lightcurve has a continuously varying V shape. The grazing nature of WASP-67b was confirmed by a detailed study of new transit lightcurves by Mancini et al (2014), who used the GROND instrument on ESO’s 2.2-m telescope at the La Silla observatory.
The transit of WASP-67b from Mancini et al. (2014)
The lack of second and third contact makes the system parameters hard to tie down, and thus obtaining a secure estimate of the planet’s radius and density requires Mancini et al’s high-quality lightcurves. WASP-67 is also notable for being in one of the target fields of the revamped Kepler spacecraft’s K2 mission, and thus we can expect ongoing detailed study of this system.