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Astronomers have characterized the atmosphere of a young (20 Myr old) transiting exoplanet and found it to be unusually clear and puffy. By analyzing the planet's atmospheric features, they were able to precisely measure the planet's mass surpassing traditional dynamical techniques like radial velocity, which poorly perform with such active young stars. They found that V1298 Tau b is a proto-sub-Neptune, still hot and inflated from its recent formation.

The team, led by Saugata Barat (MIT, MA, US) and his Ph.D. supervisor Jean-Michel Désert (UvA, Netherlands) used the James Webb Space Telescope to study the very young planet, and their results are accepted for publication in the Astrophysical Journal and currently on the preprint server arXiv.

V1298 Tau b is just 10 to 30 million years old and has an unusually clear and puffy atmosphere. The astronomers detected strong absorption signals from molecules like water vapor, methane, carbon dioxide, carbon monoxide, and even hints of complex photochemical processes, such as tentative detections of sulfur dioxide (SOâ‚‚) and carbonyl sulfide (OCS).

Metal-poor

Despite the rich spectrum, the atmosphere is surprisingly metal-poor—around 100 times less enriched than mature sub-Neptunes—suggesting that it is still evolving and may become more metal-rich with time. Another surprise comes from the fact that the planet shows 100 times less methane than expected, which can be the result of extreme internal heat and strong vertical mixing in the atmosphere.

Such high internal temperatures are inconsistent with standard thermal evolution models for sub-Neptune-sized planets, challenging the current understanding of their formation and early evolution. These results strongly motivate further studies of young planetary atmospheres to explore how composition changes over time.

V1298 Tau b's atmosphere is probably not well-mixed and could have metallicity gradients, like those inferred in solar system giants. This could naturally explain both its low observed metallicity and its high internal heat, reconciling the observations with existing formation and evolution models.

Sub-Neptunes

Barat says, "We found that V1298 Tau b has a much clearer and metal-poor atmosphere and hotter interior than what observations of sub-Neptunes have previously revealed. This suggests that evolutionary processes are still unfolding and will potentially transform the atmospheric composition of this planet with age."

Barat continues, "The JWST observations were needed to precisely constrain the mass and atmospheric composition as it gave us access to multiple molecular absorption features (H₂O, CH4, SO2, CO₂, CO) in the near infrared."

More molecules

Based on the previous Hubble observations, the team hoped to see water vapor in the atmosphere of the planet and potentially methane. But with JWST they were not only able to measure the abundances of these two molecules but also other carbon bearing molecules (CO and CO₂), as well as photochemical products like SO2 and OCS. Désert says, "When this proposal was written none of these molecules apart from water had been confidently detected using low resolution space-based spectroscopy."

"Overall, these findings challenge our understanding of how sub-Neptune planets form and evolve. We showed that in their earliest stages, these planets possess atmospheres that differ significantly from those of their mature, billion-year-old counterparts," concludes Désert.

Following the results of V1298 Tau b, the team is now (as co-investigators) part of a large JWST Cycle 3 program which will look at seven young transiting planets between 20-200 million years old, to see how atmospheric composition evolves with age.