One of the most extensively used observational techniques to study the atmospheres of exoplanets is transmission spectroscopy, the study of how absorption in an exoplanet’s atmosphere modifies star light transmitting through it while the planet transits in front of its host star [1]. The resulting transmission spectrum, the planet’s absorbing cross-sectional area relative to its star’s disk area as a function of wavelength, probes the planet’s atmosphere in an annulus around its limb. The longitudinal extend of this annulus around the equator (the ‘opening angle’) varies from single degrees in cold planets to tens of degrees in hot ones [2]. Since exoplanets rotate while they transit, the region in their atmospheres that is being probed at any point during their transits steadily changes in longitude as the planet’s orbital phase changes. Subsequently, their transmission spectra can change as a function of phase during transit. These changes are observable through phase-dependent changes of the planet’s absorbing cross-sectional area [3], which we refer to as rotational transits. Thus, measuring rotational transits would open up the possibility to constrain transiting exoplanets’ atmospheric temperatures and chemical compositions as functions of longitude [4]. Some of the hottest known exoplanets, the ultrahot Jupiters, gas giants with equilibrium temperatures exceeding ∼2000 K, rotate by more degrees during their transits than their opening angles [5]. Thus, the star light transmitting through the planet at the end of the transit probes a longitudinal segment of the atmosphere that does not overlap with the segment probed at the transit’s beginning. Therefore, ultrahot Jupiters are expected to exhibit strongly phase-dependent absorption signals in transit, delivering leverage into studying their atmospheric temperatures and chemical composition as functions of longitude. To that end, the longitudinal structures of these planets have been studied extensively with ground-based telescopes at high spectral resolution. In the ultrahot Jupiter WASP-76 b, phase-dependent Fe absorption [6–8] has revealed the planet’s atmosphere to be asymmetric about the sub-stellar point. This asymmetry might be driven by Fe condensation in the nightside [9, 10], cloud condensation around the leading limb [10–12], higher temperatures in the trailing compared to the leading half of the planet [13] or a combination of any of these factors.

In WASP-121 b, resolving the absorption signals of Na I, Fe I and Hα which probe the planet’s atmosphere at different heights has enabled to vertically and longitudinally resolve its atmospheric flow pattern with a deep day-to-night circulation and an equatorial jet in the upper atmosphere [14]. These past achievements have been accomplished with spectroscopic observations of atoms and ions at optical wavelengths, where the absorption fraction of star light in the Earth’s atmosphere is low. Similar observations at longer, infrared wavelengths targeting molecules suffer from increased telluric absorption, leading to distortions of the signals that can cause observations to not repeat in different observing nights [15]. Thus, space-based observations in the infrared can compliment ground-based observations by revealing the longitudinal structure of molecular abundances in exoplanets from transit observations. From the ground, constraints on an exoplanet’s three-dimensional atmospheric structure are derived from measuring Doppler shifts of absorption lines driven by the planet’s rotation and winds in its atmosphere. These measurements are possible thanks to high spectral resolving powers that are not achievable in space. Therefore, measuring heterogeneity in exoplanet atmospheres using their rotations during transits observed with space telescopes requires analysis approaches that are distinct from line shift measurements from the ground.