The metal-poor atmosphere of a potential sub-Neptune progenitor

Saugata Barat, Jean Michel Désert, Allona Vazan, Robin Baeyens, Michael R. Line, Jonathan J. Fortney, Trevor J. David, John H. Livingston, Bob Jacobs, Vatsal Panwar, Hinna Shivkumar, Kamen O. Todorov, Lorenzo Pino, Georgia Mraz, Erik A. Petigura

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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 cm3 at a 3σ level). This is one of the lowest-density planets discovered so far. We retrieve a low atmospheric metallicity (logZ=−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.

Original languageEnglish
JournalNature Astronomy
StatePublished - 9 May 2024

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