Too cool to be true: Atmospheric constraints on the strange super-puff HIP 41378 f in high-resolution

Le résumé fourni par la source

While most atmospheric characterisation efforts have focused on short-period planets, the atmospheres of long-period, temperate planets remain largely unexplored. These planets occupy a fundamentally different physical regime, where lower equilibrium temperatures and longer radiative timescales give rise to atmospheric compositions, winds and structures distinct from those seen in hot, close-in worlds. HIP 41378 f is a particularly intriguing case: a transiting long-period planet with an orbital period of approximately 1.5 years that has previously been reported to possess an unusually low bulk density of 0.17 g/cm³, placing it in the class of so-called “super-puff” planets. This characterisation is unexpected, given that it belongs to an evolved system (1.2 Gyr) and has a low equilibrium temperature of ~300 K, unlike the remaining planets of the same class. This raises fundamental questions regarding the physical origin of the planet’s inferred low density. HST observations revealed a featureless transmission spectrum, suggesting a high-metallicity atmosphere or a high-altitude cloud deck. Exciting hypotheses, such as the existence of a ring system, have also been proposed to explain its apparent radius, however, the true nature of HIP 41378 f remains to be uncovered. In this work, we present the first high-resolution optical transmission spectroscopy analysis of HIP 41378 f, using HARPS-N observations. Due to the rarity of this planet's transits, we simulate the next transit in detail with ESPRESSO and NIRPS to study the feasibility of constraining its atmospheric properties, and with ANDES to study the capabilities of the next generation of instruments to characterise atmospheres on low-velocity temperate planets.

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