An outline of the paper: Constraining High Speed Winds in Exoplanet Atmospheres Through Observations of Anomalous Doppler Shifts During Transit, by Kempton et al. (2011)
Dynamical 3D-models of the atmospheres of Hot Jupiters predict ferocious supersonic winds at the 1-10 km/s level. These winds are a result of a temperature gradient caused by the planets being tidally locked. There are several 3D-models out there (see references) all which differ in their complexity and underlying physics. Despite this, most of the models seem to agree on a few key aspects as stated by Kempton et al.:
- “Winds at pressures of ∼1 bar set up an equatorial jet that moves in the direction of the planet’s rotation”
- “The hottest area on the planet is shifted away from the substellar point in the direction of the planet’s rotation”
- “Winds higher in the atmosphere at ∼mbar pressures tend to flow directly from the hot day side to the cooler night side of the planet”
These high altitude winds create doppler-shifts in the transmission spectrum which can then be studied. Kempton et al. use 3D atmospheric dynamics models coupled with a transmission spectroscopy radiative transfer code to study these effects. Depending on whether magnetic drag is accounted for or not, the models predict blueshifts of the order of 2 ± 1 km/s, which are consistent with the observations of HD 209458 b by Snellen et al. (2010). Snellen et al. took 51 spectra of a single transit event using CRIRES spectrograph and studied the Doppler shifts of the CO absorption line. It is worth mentioning that a blueshift could also be possible due to a slight eccentric orbit Montalto et al. (2011).
It really is amazing how we now are able to say something about the weather conditions on exoplanets many lightyears away. The study of high speed winds in exoplanet atmospheres is in my opinion destined to be a sub field of intense study, especially with the advent of detailed exoplanet circulation models.