Inhomogeneous terminators on the exoplanet WASP-39 b

TL;DR

This study uses JWST transmission spectroscopy to detect and characterize inhomogeneous terminators on the exoplanet WASP-39 b, revealing temperature differences and atmospheric asymmetries between the morning and evening sides.

Contribution

First direct detection of inhomogeneous terminators on an exoplanet using near-infrared spectra, confirming model predictions of atmospheric asymmetry.

Findings

Evening terminator is hotter by about 177 K

Transit depths are larger in the evening by approximately 405 ppm

Spectra suggest a cloudy morning and clearer evening terminator

Abstract

Transmission spectroscopy has been a workhorse technique over the past two decades to constrain the physical and chemical properties of exoplanet atmospheres. One of its classical key assumptions is that the portion of the atmosphere it probes -- the terminator region -- is homogeneous. Several works in the past decade, however, have put this into question for highly irradiated, hot ($T_{eq}\gtrsim 1000$ K) gas giant exoplanets both empirically and via 3-dimensional modelling. While models predict clear differences between the evening (day-to-night) and morning (night-to-day) terminators, direct morning/evening transmission spectra in a wide wavelength range has not been reported for an exoplanet to date. Under the assumption of precise and accurate orbital parameters on WASP-39 b, here we report the detection of inhomogeneous terminators on the exoplanet WASP-39 b, which allows us to retrieve its morning and evening transmission spectra in the near-infrared ($2-5\ \mu$m) using JWST. We observe larger transit depths in the evening which are, on average, $405 \pm 88$ ppm larger than the morning ones, also having qualitatively larger features than the morning spectrum. The spectra are best explained by models in which the evening terminator is hotter than the morning terminator by $177^{+65}_{-57}$ K with both terminators having C/O ratios consistent with solar. General circulation models (GCMs) predict temperature differences broadly consistent with the above value and point towards a cloudy morning terminator and a clearer evening terminator.