Watery Surprise of a Young Super-Earth express
Astronomers used the JWST telescope to peer into the atmosphere of the young planet V1298 Tau c. This super-Earth is only 23 million years old — in cosmic terms, it has just formed and is still hot, like a pie fresh from the oven. As the planet passed in front of its star, starlight pierced its air, and molecules left colorful fingerprints in the spectrum. Scientists detected a clear signal of water, but no methane or carbon dioxide yet. In astronomy, all elements heavier than hydrogen and helium are called metals, like spices in cooking. The atmosphere of V1298 Tau c turned out to have 15 times more of these 'spices' than the Sun. However, that's noticeably less than in similar but more mature planets. It seems planets, like slow-cooked meals, gradually accumulate heavy elements over time. Astonishingly, at such extreme temperatures, the water didn't break apart, and the world itself is still puffed up from the heat of its birth. This hints that planetary evolution is a slow 'ripening' process, and JWST lets us glimpse the earliest recipes.
Original abstract
as written by the authors arXiv:2606.03740
While recent JWST observations of mature super-Earths and sub-Neptunes have frequently revealed featureless transmission spectra, their inflated progenitors offer a unique window into understanding their primordial compositions. As part of the Keys to Revealing the Origin and Nature Of sub-neptune Systems (KRONOS) JWST program, we present the NIRISS/SOSS transmission spectrum of V1298 Tau c, a $$\sim$$23 Myr super-Earth progenitor orbiting a young Solar analog. We detect H$$_2$$O in V1298 Tau c's atmosphere with a $$\log_{10}$$ volume mixing ratio of $$-1.83^{+0.68}_{-0.77}$$, but no additional molecules from these data alone. We find consistent results for the planetary atmospheric properties in both retrievals with and without informed priors on stellar heterogeneities based on the observed stellar spectrum. We infer an atmospheric metallicity [O/H] of $$14.8^{+56.0}_{-12.28}\times$$ the solar value. This metallicity is similar to literature measurements for other young planets, including its massive outer companion V1298~Tau~b. In contrast, this measured metallicity is systematically lower than the metallicities of mature planets of similar mass and temperature. Altogether, these results provide tentative but growing evidence that the exoplanet mass--metallicity relation evolves with planetary age.
Water vapor has already been found on dozens of exoplanets, but V1298 Tau c is the youngest of them all, making it a unique laboratory for studying the evolution of worlds.