Exoplanets & Exomoons

Evidence for Morning-to-Evening Limb Asymmetry on the Cool Low-Density Exoplanet WASP-107b

By Keith Cowing
Status Report
June 22, 2024
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Evidence for Morning-to-Evening Limb Asymmetry on the Cool Low-Density Exoplanet WASP-107b
A5 Apparent chromatic offsets in WASP-107b’s time of conjunction with wavelength due to limb asymmetry. Shown are the best-fit times of conjunction as a function of wavelength when fitting our JWST/NIRCam F322W2 data by itself. We fit the data using a uniformlimb transit model, described in Section 3.4.2, fitting only for tc, the planet-star radius ratio, and a linear flux vs. time ramp model. The black points are the result from fitting each of our thirty spectroscopic light curves. The purple line is the result from fitting the broadband light curve, with the shaded region representing the 1-σ uncertainty. We show the difference between these results and the joint-fit time of conjunction derived by fitting other multi-wavelength data sets simultaneously, whose uncertainty is shown as the green shaded region. The observed difference is due to the significant evening-morning limb asymmetry on WASP-107b at these wavelengths. — astro-ph.EP

Transmission spectroscopy has enabled unprecedented insights into the makeup of exoplanet atmospheres. A transmission spectrum combines contributions from a planet’s morning and evening limbs, but these limbs may have different temperatures, compositions, and aerosol properties due to atmospheric circulation.

High-resolution ground-based observations have detected limb asymmetry on several ultra-hot (>2000 K) exoplanets, but space-based investigation into limb asymmetry is in its infancy, and limb asymmetry’s prevalence in the broader exoplanet population remains unexplored.

We find evidence for limb asymmetry on the exoplanet WASP-107b via transmission spectroscopy from 2.5 to 4.0 micrometers with JWST/NIRCam. This is one of the first low-resolution space-based measurements of limb asymmetry and is unique because, at 770 K, WASP-107b is in a relatively cool regime where planetary terminators are expected to be homogeneous.

These observations imply a difference in temperature and cloud properties between WASP-107b’s limbs, challenging our models of limb asymmetry in this cooler regime.

Dynamic spectrum and broad-band light curve from our JWST/NIRCam F322W2 transit observation of WASP-107b. The top panel shows our spectroscopic transit data at full spectral and temporal resolution, with wavelength on the y-axis and time on the x-axis in terms of the sequence of integrations. Each column has been normalized by its median value, so the color-coding illustrates the corresponding relative flux values. In our analysis, we only use dispersion columns 56 – 1594, or wavelengths from 2.45 – 3.95 µm, in order to avoid excess noise visible on either edge of the detector. The bottom panel shows our broad-band light curve integrated over those wavelengths, with no additional light curve detrending done. Overall, this light curve displays no strong non-linear systematic trends or starspot crossing events. — astro-ph.EP

Matthew M. Murphy, Thomas G. Beatty, Everett Schlawin, Taylor J. Bell, Michael R. Line, Thomas P. Greene, Vivien Parmentier, Emily Rauscher, Luis Welbanks, Jonathan J. Fortney, Marcia Rieke

Comments: This preprint has been submitted to and accepted in principle for publication in Nature Astronomy without significant changes. 9 figures, 3 tables, 39 pages
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2406.09863 [astro-ph.EP] (or arXiv:2406.09863v1 [astro-ph.EP] for this version)
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Submission history
From: Matthew Murphy
[v1] Fri, 14 Jun 2024 09:21:51 UTC (2,760 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻