Here’s an interesting plot from a new paper by Michael Zhang et al.
The x-axis is the irradiation temperature for a sample of hot-Jupiter exoplanets; that is, how blasted the day-side of their atmosphere is by irradiation from the host star. This depends on the temperature of the star, its size, and the closeness of the orbit.
The heat of the day side of the planet is then transported to the night side by winds (the planets are phase-locked, so the same side always faces the star). The efficiency of this re-circulation of heat then determines whether the hottest regions of the planet are directly facing the star, or whether they are offset by some angle. This angle can by measured by looking at the “phase curve” radiation in the infra-red.
The y-axis then shows the observed offset angle as a function of the irradiation. The plot shows that the offset angle appears to be highest for cooler planets, and then decreases as irradiation increases, but then perhaps increases again for the very hottest planets such as WASP-33b.
There is, however, also a lot of scatter in the plot. The authors speculate that this might result from differing metallicities of the planets, which affects how well they form clouds, which can then determine the albedo of the planet, and thus how much irradiation is simply reflected.
WASP-43b is the “hot Jupiter” exoplanet with the orbit closest-in to its star, producing an ultra-short orbital period of only 20 hours. The dayside face is thus strongly heated, making it a prime system for studying exoplanet atmospheres.
Kevin Stevenson et al have pointed NASA’s Spitzer Space Telescope at WASP-43, covering the full orbit of the planet on three different occasions. Spitzer observed the infrared light from the heated face in two bands around 3.6 microns and 4.5 microns.
The three resulting “phase curves” are shown in the figure:
The 4.5-micron data from one visit are shown in red in the lower panel; the 3.6-micron data from the two other visits are in the upper panel. The transit (when the planet passes in front of the star) is at phase 1.0, and drops below the plotted figure. The planet occultation (when it passes behind the star) is at phase 0.5. The sinusoidal variation results from the heated face of the planet facing towards us (near phase 0.5) or away (near phase 1.0).
Intriguingly, the depth of the variation in the 3.6-micron data is clearly different between the two visits. Why is this? Well, Stevenson et al are not sure. One possibility is that the data are not well calibrated and that the difference results from systematic errors in the observations. After all, such observations are pushing the instruments to their very limits, beyond what they had been designed to do (back when no exoplanets were known and such observations were not conceived of).
More intriguingly, the planet might genuinely have been different on the different occasions. The authors report that, in order to model the spectra of the planet as it appears to be during the “blue” Visit 2 in the figure, the night-time face needs to be predominantly cloudy. But, if the clouds cleared, more heat would be let out and the infrared emission would be stronger. That might explain the higher flux during the “yellow” Visit 1. Here on Earth the sky regularly turns from cloudy to clear; is the same happening on WASP-43b?
A team led by Ian Wong of Caltech have announced observations of the hot Jupiters WASP-19b and HAT-P-7b, looking at infra-red light using the Spitzer Space Telescope. By observing the planets around their entire orbit they detect the transit, caused by the planet passing in front of the host star, the secondary eclipse, when the planet passes behind the star, and the “phase curve” caused by the changing visibility of the heated face of the planet.
The figure shows the infra-red light (“heat”) of the WASP-19 system in two pass bands (3.6 microns and 4.5 microns). The middle panels are expanded to show the phase curve, while the lowest panels show the residuals about a fitted model (the red line).
By fitting all three features, the authors can constrain the temperatures of the “day time” heated face of the planet (which faces towards us near the secondary eclipse) and of the “night time” face of the planet (which faces us near transit). From there they can estimate the “recirculation”, how efficient the planet is at redistributing heat from the day-time face to the night-time face.
Such short-period planets are phase-locked by tidal forces, and so always present the same face to the star. Thus redistribution of heat energy requires powerful winds circling the planet.
An interesting plot by Wong et al shows the recirculation in different hot Jupiters against the albedo (the fraction of energy that is reflected).
There appear to be two groups of hot Jupiters: ones with albedos near 0.4, such as WASP-19b, and ones with much lower albedos, such as WASP-14b and WASP-18b. So far there is no simple explanation for this difference.
Further, the recirculation efficiency also appears to be different in different systems. Wong et al suggest that the hot Jupiters experiencing the highest irradiation, such as WASP-19b, are least efficient at redistributing heat, while
longer-period, less-irradiated hot Jupiters such as HD209458b and HD189733b are better at redistribution.