A Closer Look At Opening Angles And Absorption Regions In The Atmospheres Of Transiting Exoplanets

The angle over which a tidally locked planet rotates during its transit, plotted as a function of its equilibrium temperature 𝑇eq (assuming zero albedo and full heat redistribution) and the effective temperature 𝑇eff of the host star. The white bullets show all systems for which the stellar radius and the semi-major axis of the planet orbit are currently known (data acquired from https://www.astro.keele.ac.uk/jkt/tepcat/; Southworth 2011). The red and yellow curves show for which (𝑇eq, 𝑇eff) the planet’s rotation angle is equal to the opening angle. Here, we assume two different planet radii (one Jupiter radius and one Neptune radius), and two different values for 𝛽 (see equation 10). To the right of the curves, the rotation angle is larger than the opening angle.

Transmission spectra contain a wealth of information about the atmospheres of transiting exoplanets. However, large thermal and chemical gradients along the line of sight can lead to biased inferences in atmospheric retrievals.

In order to determine how far from the limb plane the atmosphere still impacts the transmission spectrum, we derive a new formula to estimate the opening angle of a planet. This is the angle subtended by the atmospheric region that contributes to the observation along the line of sight, as seen from the planet centre. We benchmark our formula with a 3D Monte-Carlo radiative transfer code and we define an opening angle suitable for the interpretation of JWST observations, assuming a 10-ppm noise floor.

We find that the opening angle is only a few degrees for planets cooler than ca. 500 Kelvins, while it can be as large as 25 degrees for (ultra-)hot Jupiters and 50 degrees for hot Neptunes. Compared to previous works, our more robust approach leads to smaller estimates for 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 smaller than the angle over which the planet rotates during the transit. This allows for time-resolved transmission spectroscopy observations that probe independent parts 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

Joost P. Wardenier, Vivien Parmentier, Elspeth K.H. Lee

Comments: 10 pages, 10 figures, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2111.11830 [astro-ph.EP] (or arXiv:2111.11830v1 [astro-ph.EP] for this version)
Submission history
From: Joost Wardenier
[v1] Tue, 23 Nov 2021 12:51:22 UTC (2,870 KB)

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