2025AJ....170..165B

We present the JWST/NIRSpec G395H transmission spectrum of the young (10-30 Myr) transiting planet V1298 Tau b (9.85 ± 0.35 R_⊕, T_eq = 670 K). Combined Hubble Space Telescope and JWST observations reveal a haze-free, H/He-dominated atmosphere with a large scale height (∼1500 km), allowing detection of CO₂ (35σ), H₂O (30σ), CO (10σ), CH₄ (6σ), SO₂ (4σ), and OCS (3.5σ). Our observations probe several scale heights (∼4.4 in the CO₂ 4.3 μm band and ∼3 in the 2.7μm water band). The planet's mass, inferred from atmospheric scale height using free retrieval and grid modeling, is 12 ± 1 M_⊕ and 15 ± 1.7 M_⊕, respectively, which is significantly lower than previous radial velocity estimates and confirms it as “gas-dwarf” sub-Neptune progenitor. We find an atmospheric metallicity (log Z = 0.6${}_{-0.6}^{+0.4}\times $ solar) and subsolar C/O ratio (0.22${}_{-0.05}^{+0.06}$). The atmospheric metallicity is low compared to mature sub-Neptunes by an order of magnitude. The CH₄ abundance ([CH₄] = -6.2${}_{-0.5}^{+0.3}$]) is ∼7σ lower than the equilibrium chemistry prediction. To adjust for the low methane abundance, the self-consistent grids favor a high internal temperature (∼500 K) and vertical mixing (K_zz ∼ 10⁷-10⁸ cm² s^–1). These internal temperatures are inconsistent with predictions from evolutionary models, which expect ∼100-200 K at the current system age. We estimate a gas-to-core mass fraction between 0.1% and 8%, with a core mass of 11-12 M_⊕, consistent with in-situ gas-dwarf formation. A deep atmospheric metallicity gradient may explain both the high internal temperature and low observable metallicity. Over time, mass loss from such an atmosphere could enhance its metallicity, potentially reconciling V1298 Tau b with mature sub-Neptunes.