The metal-poor atmosphere of a potential sub-Neptune progenitor
Fortney, Jonathan J.; Désert, Jean-Michel; Petigura, Erik A.; Line, Michael R.; Livingston, John H.; Baeyens, Robin; David, Trevor J.; Barat, Saugata; Vazan, Allona; Pino, Lorenzo; Todorov, Kamen O.; Panwar, Vatsal; Jacobs, Bob; Shivkumar, Hinna; Mraz, Georgia
Netherlands, Israel, United States, Japan, United Kingdom, Italy
Abstract
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 RJ, where RJ 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 ), 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.