Ultrahot giant exoplanets receive thousands of times Earth'
) terminators could, however, be revealed as an asymmetric absorption signature during transit(4,7,15). Here we report the detection of an asymmetric atmospheric signature in the ultrahot exoplanet WASP-76b. We spectrally and temporally resolve this signature using a combination of high-dispersion spectroscopy with a large photon-collecting area. The absorption signal, attributed to neutral iron, is blueshifted by -11 +/- 0.7 kilometres per second on the trailing limb, which can be explained by a combination of planetary rotation and wind blowing from the hot dayside(16). In contrast, no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there. We conclude that iron must therefore condense during its journey across the nightside.
Absorption lines of iron in the dayside atmosphere of an ultrahot giant exoplanet disappear after travelling across the nightside, showing that the iron has condensed during its travel.
