A high internal heat flux and large core in a warm neptune exoplanet

Abstract

Interactions between exoplanetary atmospheres and internal properties have long been hypothesized to be drivers of the inflation mechanisms of gaseous planets and apparent atmospheric chemical disequilibrium conditions¹. However, transmission spectra of exoplanets has been limited in its ability to observational confirm these theories due to the limited wavelength coverage of 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 WFC3, JWST NIRCam and MIRI. From this spectrum, we detect spectroscopic features due to H₂O (21σ), CH₄ (5σ), CO (7σ), CO₂ (29σ), SO₂ (9σ), and NH₃ (6σ). The presence of these molecules enable constraints on the atmospheric metal enrichment (M/H is 10–18× Solar³), vertical mixing strength (log₁₀K_zz = 8.4–9.0 cm²s⁻¹), and internal temperature (>345 K). The high internal temperature is suggestive of tidally-driven inflation⁴ acting upon a Neptunelike internal structure, which can naturally explain the planet’s large radius and low density. These findings suggest that eccentricity driven tidal heating is a critical process governing atmospheric chemistry and interior structure inferences for a majority of the cool (<1,000K) super-Earth-to-Saturn mass exoplanet population.