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Abstract The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5μm transmission spectrum of the 17 Myr, Jupiter-size (R∼10R_⊕) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneousg-band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%–50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b’s mass is <20M_⊕irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0M_⊕. This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm⁻³, HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H₂O and CO₂(≥7σ), a modest detection of CO (3.5σ), and weak detections of H₂S and SO₂(≃2σ). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3 × 10 solar due to the amplitude of the SO₂feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03M_⊕Myr⁻¹), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence. 

Abstract We obtained ultraviolet and optical spectra for nine M dwarfs across a range of rotation periods to determine whether they showed stochastic intrinsic variability distinguishable from flares. The ultraviolet spectra were observed during the Far Ultraviolet M-dwarf Evolution Survey Hubble Space Telescope program using the Space Telescope Imaging Spectrograph. The optical observations were taken from the Apache Point Observatory 3.5 m telescope using the Dual Imaging Spectrograph and from the Gemini South Observatory using the Gemini Multi-Object Spectrograph. We used the optical spectra to measure multiple chromospheric lines: the Balmer series from Hαto H10 and the CaiiH and K lines. We find that after excising flares, these lines vary on the order of 1%–20% at minute-cadence over the course of an hour. The absolute amplitude of variability was greater for the faster rotating M dwarfs in our sample. Among the five stars for which we measured the weaker Balmer lines, we note a tentative trend that the fractional amplitude of the variability increases for higher-order Balmer lines. We measured the integrated flux of multiple ultraviolet emission features formed in the transition region: the Nv, Siiv,and Civresonance line doublets, and the Ciiand Heiimultiplets. The signal-to-noise ratio of the UV data was too low for us to detect nonflare variability at the same scale and time cadence as the optical. We consider multiple mechanisms for the observed stochastic variability and propose both observational and theoretical avenues of investigation to determine the physical causes of intrinsic variability in the chromospheres of M dwarfs.