The architectures of compact multi-planet systems encode clues about their formation and evolution. The 20-30 Myr V1298 Tau system represents a rare captured moment enabling insights into planet assembly and atmospheric evolution. V1298 Tau d is a 750 K sub-Saturn with a radius of 6 R⊕, currently losing mass on its way to becoming a sub-Neptune. Its extended evaporating atmosphere, implied by its low mass, large radius, and incident irradiation, offers a direct probe of photoevaporation timescales. Planet d exhibits transit timing variations, which we have modeled to constrain its mass and enable precise scheduling of observations to characterize its atmosphere. We will measure its evaporating atmosphere via He absorption, as well as H2O to constrain its formation location. As a young sub-Saturn destined for sub-Neptune status, V1298 Tau d represents a singular chance to study multi-planet system architecture and envelope evolution, and test theories of intra-system uniformity. Current and future HST/JWST observations of this and other planets in the system provide a high degree of synergy, making these important legacy measurements.
This document was translated from LATEX by HEVEA.