Three papers this week arrived on arXiv about the ultra-hot-Jupiter WASP-121b. All three report similar findings (and the near-simultaneous arrival on arXiv presumably reflects the teams being aware of the competition). Cabot et al analyse spectra of WASP-121b from the ESO 3.6-m/HARPS spectrograph, Bourrier et al also analyse HARPS data, while Gibson et al analyse data from UVES on ESO’s 8-m VLT.

All three teams then apply velocity shifts to correct for the orbital motion of the star, in order to try to detect features from the planet. The result is a plot looking like (this is the one from Bourrier et al):

As in the plot for WASP-107b, just below, this shows the spectra as a function of time, through transit. The extra absorption during transit (delineated by dashed lines) is from the atmosphere of the planet absorbing starlight while it is projected against the star’s face. The faint diagonal feature (marked by the green line) is the signal from the planet’s atmosphere, moving with the planet’s orbital velocity.

A schematic of WASP-121b as it transits its hot star in a near-polar orbit (from Bourrier et al).

The three papers report the detection of lines from neutral iron in the planet’s atmosphere, and discuss the possible role of iron absorption in producing an atmospheric temperature inversion. The papers also report a blue-shift of the iron absorption, of order 5 km/s, which could be produced by strong winds running round the planet. That is expected in phase-locked planets, where heat from the irradiated face must be transported round to the night side.