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Transmission spectra contain a wealth of information about the atmospheres oftransiting exoplanets. However, large thermal and chemical gradients along theline of sight can lead to biased inferences in atmospheric retrievals. In orderto determine how far from the limb plane the atmosphere still impacts thetransmission spectrum, we derive a new formula to estimate the opening angle ofa planet. This is the angle subtended by the atmospheric region thatcontributes to the observation along the line of sight, as seen from the planetcentre. We benchmark our formula with a 3D Monte-Carlo radiative transfer codeand we define an opening angle suitable for the interpretation of JWSTobservations, assuming a 10-ppm noise floor. We find that the opening angle isonly a few degrees for planets cooler than ca. 500 Kelvins, while it can be aslarge as 25 degrees for (ultra-)hot Jupiters and 50 degrees for hot Neptunes.Compared to previous works, our more robust approach leads to smaller estimatesfor the opening angle across a wide range scale heights and planetary radii.Finally, we show that ultra-hot Jupiters have an opening angle that is smallerthan the angle over which the planet rotates during the transit. This allowsfor time-resolved transmission spectroscopy observations that probe independentparts of the planetary limb during the first and second half of the transit.

All along the line of sight: a closer look at opening angles and absorption regions in the atmospheres of transiting exoplanets