Exoplanets and Stellar Physics

Abstract:

Spectroscopic phase curves of hot Jupiters measure their emission spectra at multiple orbital phases, thus enabling detailed characterization of their atmospheres. Precise constraints on the atmospheric composition of these exoplanets offer insights into their formation and evolution. We analyse four phase-resolved emission spectra of the hot Jupiter WASP-43b, generated from a phase curve observed with the Mid-Infrared Instrument/Low Resolution Spectrometer onboard the JWST, to retrieve its atmospheric properties. Using a parametric 2D temperature model and assuming a chemically homogeneous atmosphere within the observed pressure region, we simultaneously fit the four spectra to constrain the abundances of atmospheric constituents, thereby yielding more precise constraints than previous work that analysed each spectrum independently. Our analysis reveals statistically significant evidence of NH$_3$ (4$\sigma$) in a hot Jupiter’s emission spectra for the first time, along with evidence of H$_2$O (6.5$\sigma$), CO (3.1$\sigma$), and a non-detection of CH$_4$. With our abundance constraints, we tentatively estimate the metallicity of WASP-43b at 0.6$-6.5\times$ solar and its C/O ratio at 0.6$-$0.9. Our findings offer vital insights into the atmospheric conditions and formation history of WASP-43b by simultaneously constraining the abundances of carbon, oxygen, and nitrogen-bearing species.

Abstract:

Interactions between exoplanetary atmospheres and internal properties have long been proposed to be drivers of the inflation mechanisms of gaseous planets and apparent atmospheric chemical disequilibrium conditions¹. However, transmission spectra of exoplanets have been limited in their ability to observationally confirm these theories owing to the limited wavelength coverage of the Hubble Space Telescope (HST) and inferences of single molecules, mostly H₂O (ref. ²). In this work, we present the panchromatic transmission spectrum of the approximately 750 K, low-density, Neptune-sized exoplanet WASP-107b using a combination of HST Wide Field Camera 3 (WFC3) and JWST Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI). From this spectrum, we detect spectroscopic features resulting from H₂O (21σ), CH₄ (5σ), CO (7σ), CO₂ (29σ), SO₂ (9σ) and NH₃ (6σ). The presence of these molecules enables constraints on the atmospheric metal enrichment (M/H is 10-18× solar³), vertical mixing strength (log₁₀K_zz = 8.4-9.0 cm² s^-1) and internal temperature (>345 K). The high internal temperature is suggestive of tidally driven inflation⁴ acting on a Neptune-like internal structure, which can naturally explain the large radius and low density of the planet. These findings suggest that eccentricity-driven tidal heating is a critical process governing atmospheric chemistry and interior-structure inferences for most of the cool (<1,000 K) super-Earth-to-Saturn-mass exoplanet population.