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

TL;DR

This study presents the first detailed atmospheric characterization of a young, low-density exoplanet, revealing a primordial, metal-poor atmosphere with water vapor, challenging existing formation theories.

Contribution

We provide the first transmission spectrum of a young Neptune-sized exoplanet, showing a primordial, low-metallicity atmosphere and insights into its formation and evolution.

Findings

Detection of water vapor at 5σ significance

V1298 Tau b has one of the lowest densities among known exoplanets

Atmospheric metallicity is consistent with solar/sub-solar values

Abstract

Young transiting exoplanets offer a unique opportunity to characterize the atmospheres of fresh and evolving products of planet formation. 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 5$\sigma$ level of significance. We estimate a mass and density upper limit (24$\pm$5$M_{\oplus}$, 0.12gm/$cm^{3}$ 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), consistent 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 formation entropy of this planet. Our observations suggest that V1298 Tau b is likely to evolve into a Neptune/sub-Neptune type of planet.