Hot-Jupiters are a class of planets that orbit very close to their host stars. As a result, these planets receive extremely intense irradiation and are expected to be tidally-locked, with one side experiencing permanent day and one side experiencing permanent night. The strong temperature contrast between the permanent day and night sides on a hot-Jupiter drives a powerful west-to-east circulation that goes around the planet.
If the orbit of the hot-Jupiter is observed edge-on, it will appear to pass behind its host star on every orbit. Such an event is known as an eclipse. The planet’s evening-side is more visible pre-eclipse (i.e. during the first half of the orbit), while the planet’s morning-side is more visible post-eclipse (i.e. during the second half of the orbit). As the planet circles around its host star, some fraction of its illuminated hemisphere will be visible depending on the position of the planet in its orbit. This generates a light curve where the combined brightness of the star and planet varies in a small and periodic fashion every orbit. The variation is small because the star is orders of magnitude brighter than the planet.
Figure 1: Artist’s impression of a hot-Jupiter that is glowing red hot.
Using data from NASA’s Kepler space telescope, a study by Esteves et al. (2015) uncovered weather cycles on 6 hot-Jupiters. The brightest spot on each planet appears offset from the substellar point. Basically, the substellar point on a hot-Jupiter is the spot on the planet where the planet’s host star is directly overhead and it is where the stellar irradiation is most intense. The 2 hotter planets (Kepler-76b and HAT-P-7b) appear brightest pre-eclipse (i.e. the brightest spot on the planet is east of the substellar point, towards the evening-side); while the 4 cooler planets (Kepler-7b, Kepler-8b, Kepler-12b and Kepler-41b) appear brightest post-eclipse (i.e. the brightest spot on the planet is west of the substellar point, towards the morning-side).
On these 6 hot-Jupiters, winds blow eastward from the substellar point towards the evening-side, around the night side and back towards the morning-side, before returning to the substellar point. The 4 cooler planets (Kepler-7b, Kepler-8b, Kepler-12b and Kepler-41b) all have temperatures under ~2,300 K. The best explanation why they appear brightest on the morning-side (i.e. post-eclipse) is because cool temperatures on the night side allow clouds to form by condensation and the west-to-east atmospheric circulation brings these reflective clouds to the morning-side, causing the morning-side to be the brightest region on the planet. Subsequently, these clouds continue on towards the substellar point where they dissipate due to the increased irradiation.
In contrasts, the 2 hotter planets (Kepler-76b and HAT-P-7b) have temperatures exceeding ~2,700 K and both planets appear brightest on the evening-side (i.e. pre-eclipse). The best explanation is that the brightness is dominated by thermal emission from a hot-spot that has been shifted east from the substellar point, towards the evening-side. Furthermore, the high temperatures make it difficult for clouds to form, resulting in little or no clouds on the morning-side that can reflect a sufficient amount of incoming stellar radiation to contribute significantly to the planet’s brightness.
For the 4 cooler planets, the brightest spot is shifted west (i.e. towards the morning-side) from the substellar point by over ~25° since the clouds are probably thickest and most reflectively near the day-night terminator on the morning-side. For the 2 hotter planets, the brightest spot is shifted by a much smaller amount of less than ~8°, this time the shift is eastwards (i.e. towards the evening-side). The brightest spot is closer to the substellar point for the 2 hotter planets due to the rapid re-emission of absorbed stellar energy after leaving the substellar point. This study supports the correlation between the position of a planet’s brightest spot and the planet’s temperature. The brightness of a hotter planet is likely to be dominated by thermal emission from a hot-spot that has been shifted east of the substellar point (i.e. evening-side), while the brightness of a cooler planet is likely to be dominated by reflected light from clouds west of the substellar point (i.e. morning-side).
Figure 2: Light curve of Kepler-8b, a hot-Jupiter with 1.42 times the radius and 0.59 times the mass of Jupiter, in a 3.52-day orbit around its host star. The left and right panels contain, respectively, the transit light curve (i.e. drop in the star’s brightness when the planet passes in front of its host star) and phase light curve (i.e. combined brightness of the star and planet as the planet orbits the star). On the right panel, the dip in the middle denotes the eclipse, whereby the planet passes behind its host star and is obscured. Esteves et al. (2015).
Figure 3: Light curve of Kepler-12b, a hot-Jupiter with 1.75 times the radius and 0.43 times the mass of Jupiter, in a 4.44-day orbit around its host star. Esteves et al. (2015).
Figure 4: Light curve of Kepler-41b, a hot-Jupiter with 1.04 times the radius and 0.56 times the mass of Jupiter, in a 1.86-day orbit around its host star. Esteves et al. (2015).
Figure 5: Light curve of Kepler-8b, a hot-Jupiter with 1.36 times the radius and 2.01 times the mass of Jupiter, in a 1.54-day orbit around its host star. Esteves et al. (2015).
Esteves et al. (2015), “Changing Phases of Alien Worlds: Probing Atmospheres of Kepler Planets with High-Precision Photometry”, arXiv:1407.2245 [astro-ph.EP]