Probing 2D Asymmetries Of An Exoplanet Atmosphere From Chromatic Transit Variation

We propose a new method for investigating atmospheric inhomogeneities in exoplanets through transmission spectroscopy. Our approach links chromatic variations in conventional transit model parameters (central transit time, total and full durations, and transit depth) to atmospheric asymmetries.

By separately analyzing atmospheric asymmetries during ingress and egress, we can derive clear connections between these variations and the underlying asymmetries of the planetary limbs. Additionally, this approach enables us to investigate differences between the limbs slightly offset from the terminator on the dayside and the nightside.

We applied this method to JWST’s NIRSpec/G395H observations of the hot Saturn exoplanet WASP-39 b. Our analysis suggests a higher abundance of CO₂ on the evening limb compared to the morning limb and indicates a greater probability of SO₂ on the limb slightly offset from the terminator on the dayside relative to the nightside.

These findings highlight the potential of our method to enhance the understanding of photochemical processes in exoplanetary atmospheres.

Positions of limbs observable during ingress and egress on the planet, as expected from the orbital parameters of WASP-39 b. Panel (a) is similar to Panel (b) of Figure 1, but the x_i–axis and y_i–axis are colored blue, and the x_e–axis and y_e–axis are colored orange. The blue and orange points indicate where the limbs intersect the x_i–axis or x_e–axis. The white-filled points represent those on the dayside of the northern hemisphere, while the filled points represent those on the nightside of the southern hemisphere. The limb observable during ingress is colored blue and the limb observable during egress is colored orange. Panels (b), (c), and (d) show the positions of the x_i–axis, y_i–axis (blue), x_e–axis, and y_e–axis (orange), and the limbs observable during ingress (blue belt) and egress (orange belt) on the planet. The finite width of each belt reflects the shift caused by the planet’s rotation during ingress or egress. The white and gray hemispheres represent the dayside and nightside of the planet, respectively. Panel (c) shows the view from the nightside, and Panel (d) shows the view from the north. The limbs observable during ingress and egress are shifted slightly away from the terminator. The yi–axis and ye–axis intersect the terminator. — astro-ph.EP

Shotaro Tada, Hajime Kawahara, Yui Kawashima, Takayuki Kotani, Kento Masuda

Comments: 38 pages, 31 figures, accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2503.08988 [astro-ph.EP] (or arXiv:2503.08988v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2503.08988
Focus to learn more
Submission history
From: Shotaro Tada
[v1] Wed, 12 Mar 2025 01:41:08 UTC (26,292 KB)
https://arxiv.org/abs/2503.08988
Astrobiology