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 (\(\log{Z}=-0.{7}_{-0.7}^{+0.8}\,{\mathrm{solar}}\)), 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.

年轻的凌日系外行星提供了一个独特的机会来描述新形成和演化的行星的大气特征。我们展示了 V1298 Tau b 的透射光谱，这是一颗年龄为 23 迈里的温暖木星大小（0.91 ± 0.05  R _J，其中R _J是木星的半径）行星，绕着一颗前主序恒星运行。我们检测到了一个基本清澈的原始大气，具有异常大的大气尺度高度，并且水蒸气吸收达到了 5 σ的显着水平，据此我们估计了行星质量上限（23 个地球质量，0.12 g cm ^{- 3}在 3 σ水平）。这是迄今为止发现的密度最低的行星之一。我们检索到低大气金属丰度（\(\log{Z}=-0.{7}_{-0.7}^{+0.8}\,{\mathrm{solar}}\)），与太阳/亚太阳一致太阳能值。我们的发现挑战了核心吸积理论中预期的质量-金属丰度关系。相反，我们的观察结果可以通过卵石堆积的原位形成以及持续的进化机制来解释。我们没有检测到甲烷，这暗示着这颗行星的内部温度比预期的要高。我们的观察表明，V1298 Tau b 很可能演化为亚海王星。