The metal-poor atmosphere of a Neptune/Sub-Neptune planet’s progenitor

Abstract

Young transiting exoplanets offer a unique opportunity to character- ize the atmospheres of fresh and evolving products of planet for- mation. We present the transmission spectrum of V1298 Tau b; a 23 Myr old warm Jovian sized planet orbiting a pre-main sequence star. We detect a primordial atmosphere with an exceptionally large atmospheric scale height and a water vapour absorption at 5σ level of significance. We estimate a mass and density upper limit (24±5M⊕, 0.12gm/cm3 respectively). V1298 Tau b is one of the lowest density planets discovered till date. We retrieve a low atmospheric metallicity (logZ=−0.1+0.66 −0.72 solar), con- sistent with solar/sub-solar values. Our findings challenge the expected mass-metallicity from core-accretion theory. Our observations can be explained by in-situ formation via pebble accretion together with ongoing evolutionary mechanisms. We do not detect methane, which hints towards a hotter than expected interior from just the for- mation entropy of this planet. Our observations suggest that V1298 Tau b is likely to evolve into a Neptune/sub-Neptune type of planet