Investigating potential causes for high inferred intrinsic temperatures of warm transiting Neptune and Sub-Neptune planets

Abstract

Atmospheric characterization of a warm 10-30 Myr old sub-Neptune progenitor (V1298 Tau b) and a $\sim$3.4 Gyr old warm super-Neptune (WASP-107b) using JWST have revealed extremely low methane abundance. Atmospheric forward models find higher than expected inferred intrinsic temperatures (450~K and 500~K) --- inconsistent with theoretical planet formation and core-envelope evolution predictions (70K and 150K respectively) assuming a convective interior. We explore three hypotheses to reconcile the high intrinsic temperatures with evolutionary models --- tidal heating, silicate rainout and heat trapping due to deep cloud condensation. Tidal heating requires high planetary obliquity ($\gtrsim70^{\circ}$) and low reduced tidal quality factor ($\sim$100) for V1298 Tau b, implying an extremely short obliquity damping timescale ($<$10$^{4}$ years). Although high obliquity may be maintained by spin--orbit resonant locking, it requires fine-tuning and is therefore unlikely. Silicate rainout can increase the temperature in the deep atmosphere of the young V1298 Tau b (0.1-1 bar) by $\sim$700K, without requiring extremely high intrinsic temperature, but is unlikely to explain WASP-107b, as silicate rainout timescales are expected to be much shorter than its age. Including cloud condensation in self-consistent atmospheric models reduces the inferred intrinsic temperature for V1298 Tau b from 500K to 300K, still exceeding the core-envelope prediction (150K). Future measurements of thermal emission from young transiting planets such as V1298 Tau b, combined with methane abundances as a function of age, will be needed to distinguish between these scenarios.