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JWST observes a strong variability in eclipse depths of 55 Cancri e

Presentation #609.02 in the session Earths and Super-Earths.

Published onApr 03, 2024
JWST observes a strong variability in eclipse depths of 55 Cancri e

The nature of a close-in rocky planet 55 Cnc e is puzzling despite having been observed extensively. Its optical and infrared eclipse depths show temporal variability, in addition to the phase curve variability observed in optical. Although the thermal phase curve of the planet observed with Spitzer is consistent with an efficient heat circulation on the day side of the planet which could be a result of a thick atmosphere, most other observations have failed to detect a significant atmosphere on the planet. Both of these phenomena can naturally be explained by the planet being in a 3:2 spin-orbit resonance. The observed variability could then be a result of the planet showing different sides during eclipses because of 3:2 resonance. The thermal inertia from the heating of the surface or atmosphere can shift the hottest location from the sub-stellar point mimicking the observed heat re-distribution. We aim to test this hypothesis with JWST. We observed four secondary eclipses, of which three were observed within a week, of the planet with NIRCam/JWST around 2 and 4.5 micron. While the former is a photometric observation, the latter contains spectroscopic information. This analysis was complemented with an archival eclipse observation collected with the same observing mode. We find that the white-light eclipse depths around 4.5 micron change from ~8 ppm to 123 ppm within a week. Similarly, the photometric eclipse depths at 2.1 micron vary between -5 and 44 ppm in the same period. Interestingly, both variations are not related to each other and do not support the 3:2 spin-orbit resonance scenario. The eclipse depth spectra between around 4.5 micron are also variable and do not show any significant spectral feature. A thin transient atmosphere on the planet made up of CO2 or CO, balanced by outgassing from the surface and escape, could potentially explain the observations. Alternatively, the nature of variability, which is observed in infrared as well as in optical in the past, could also be a result of an inhomogeneous grey source of opacity in the form of circumstellar dust torus.

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