Figure 1: Artist’s impression of the planet HD189733b in front of its host star. A belt of wind races around the equator of the planet at over 2000 m/s from the heated day side to the night side. The day side of the planet appears blue due to scattering of light from silicate haze in the atmosphere. The night side of the planet glows a deep red due to its high temperature from winds that deliver heat from the hotter day side. Image credit: Mark A. Garlick/University of Warwick.
By studying data from a transit observation of HD189733b made with the High Accuracy Radial Velocity Planet Searcher (HARPS) on the European Southern Observatory’s 3.6 m telescope at La Silla, Chile; Louden & Wheatley (2015) present wind velocity measurements on the leading limb (90 degrees west of the substellar point) and trailing limb (90 degrees east of the substellar point) of HD189733b.
As parts of the atmosphere of HD189733b move towards or away from Earth, the Doppler Effect causes light from the planet’s host star to be red or blue shifted as it passes through the planet’s atmosphere. By measuring the wavelength of the sodium absorption feature in the planet’s atmosphere, the planet’s leading limb is observed to be moving away from the observer (i.e. red shifted) at a velocity of roughly 2.3 km/s and the planet’s trailing limb is observed to be moving towards the observer (i.e. blue shifted) at a velocity of roughly 5.3 km/s.
Since HD189733b is tidally-locked, its rotation period is the same as its orbital period of 2.22 days. As a result, based on rotation alone, both leading and trailing limbs will have symmetric red and blue shifts of 2.9 km/s. When these rotational velocities are subtracted from the measured velocities, the planet’s leading limb has a westward excess velocity of roughly 0.6 km/s and the planet’s trailing limb has an eastward excess velocity of roughly 2.4 km/s.
This study shows that the atmosphere of HD189733b has a strong eastward motion, with winds racing at over 2000 m/s from the intensely heated day side to the night side. Such an eastward motion also explains why the region of peak temperature on HD189733b is displaced 30 degrees east of the substellar point.
Figure 2: The posterior distributions of atmospheric velocities from the analysis. The leading limb (left) is red shifted by roughly 2.3 km/s and the trailing limb is blue shifted by roughly 5.3 km/s. The average velocity (middle) is found to be blue shifted with a velocity of roughly 1.9 km/s. The strength and direction of the velocity sets are consistent with a combination of tidally-locked rotation and an eastward equatorial jet that is seen crossing from the day side to the night side of the planet on the trailing limb. Louden & Wheatley (2015)
Louden & Wheatley (2015), “Spatially resolved eastward winds and rotation of HD189733b”, arXiv:1511.03689 [astro-ph.EP]