2024NatAs...8..899B

Young transiting exoplanets offer a unique opportunity to characterize the atmospheres of freshly formed and evolving planets. We present the transmission spectrum of V1298 Tau b, a 23-Myr-old warm Jupiter-sized (0.91 ± 0.05 R_J, where R_J is the radius of Jupiter) planet orbiting a pre-main-sequence star. We detect a mostly clear primordial atmosphere with an exceptionally large atmospheric scale height, and a water vapour absorption at a 5σ level of significance, from which we estimate a planetary mass upper limit (23 Earth masses, 0.12 g cm^–3 at a 3σ level). This is one of the lowest-density planets discovered so far. We retrieve a low atmospheric metallicity ($log⁡Z=-0.7-0.7+0.8solar\log{Z}=-0.{7}_{-0.7}^{+0.8}\,{\mathrm{solar}}$), consistent with solar/sub-solar values. Our findings challenge the expected mass-metallicity relation from core-accretion theory. Our observations can instead be explained by in situ formation via pebble accretion together with ongoing evolutionary mechanisms. We do not detect methane, which hints at a hotter-than-expected interior from just the formation entropy of this planet. Our observations suggest that V1298 Tau b is likely to evolve into a sub-Neptune.