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

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.