2024A&A...692A.198B

The V1298 Tau system is a multi-planet system that provides the opportunity to perform comparative exoplanetology between planets orbiting the same star. Because of its young age (20-30 Myr), this system also provides the opportunity to compare the planet's early evolutionary properties, right after their formation. We present the first atmospheric comparison between two transiting exoplanets within the same multiple planet system: V1298 Tau b and V1298 Tau c. We observed one primary transit for each planet with the Hubble Space Telescope (HST), using Grism 141 (G141) of Wide Field Camera 3 (WFC3). We fit the spectroscopic light curves using state-of-the-art techniques to derive the transmission spectrum for planet c and adopted the transmission spectrum of planet b obtained with the same observing configuration and data analysis methods from previous studies. We measured the mass of planet b and c (8_–2⁺⁴, 17_–6⁺¹³ M_⊕ ; respectively) from the transmission spectrum and found the two planets to have masses in the Neptune or sub-Neptune regime. Using atmospheric retrievals, we measured and compared the atmospheric metallicities of planet b and c (logZ/Z_⊙=-2.04_–0.59^0.69, logZ/Z_⊙ = -0.16_–0.94^1.15, respectively), and found them to be consistent with the solar or sub-solar, which is low (at least one order of magnitude) compared to known mature Neptune and sub-Neptune planets. This discrepancy could be explained by ongoing early evolutionary mechanisms, which are expected to enrich the atmospheres of such young planets as they mature. Alternatively, the observed spectrum of planet c can be explained by atmospheric hazes, which is in contrast to planet b, where efficient haze formation can be ruled out. Higher haze formation efficiency in planet c could be due to differences in atmospheric composition, temperature and/or higher UV flux compared to planet b. In addition, planet c is likely to experience a higher fraction of mass loss compared to planet b, given its proximity to the host star.