Are Opening Angles in Exoplanet Transmission Spectroscopy Overestimated?

Transmission spectroscopy has been integral to the atmospheric characterisation of transiting exoplanets – constraining their chemical abundances and temperature profiles. However, transit spectra are often interpreted using 1D models of the atmosphere, and this can lead to biased inferences in retrievals. Especially on tidally locked planets, which have a permanently irradiated dayside and a permanently dark nightside, atmospheric variations along the line of sight can be substantial. In order to assess whether a 1D model suffices, or whether a 3D model is needed to interpret observational data, one requires an estimate of the planet’s opening angle. This is the angle subtended by the atmospheric region that contributes to the observation along the line of sight, as seen from the centre of the planet. In this talk, we show that the opening angles that have been computed in previous works are likely an overestimation of the true opening angles. This is because the maximum and minimum pressure probed by transit observations do not lie on the same transit chords. Using two different methods, we re-evaluate the opening angles for a large number of atmospheres with different scale heights and planetary radii. We also present a new analytical formula to estimate the more realistic size of the opening angle.