An extremely low-density and temperate giant exoplanet
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Transiting extrasolar planets are key objects in the study of the formation, migration, and evolution of planetary systems. In particular, the exploration of the atmospheres of giant planets, through transmission spectroscopy or direct imaging, has revealed a large diversity in their chemical composition and physical properties. Studying these giant planets allows one to test the global climate models that are used for the Earth and other solar system planets. However, these studies are mostly limited either to highly-irradiated transiting giant planets or directly-imaged giant planets at large separations. Here we report the physical characterisation of the planets in a bright multi-planetary system (HIP41378) in which the outer planet, HIP41378 f is a Saturn-sized planet (9.2 $\pm$ 0.1 R$_\oplus$) with an anomalously low density of 0.09 $\pm$ 0.02 g cm$^{-3}$ that is not yet understood. Its equilibrium temperature is about 300 K. Therefore, it represents a planet with a mild temperature, in between the hot Jupiters and the colder giant planets of the Solar System. It opens a new window for atmospheric characterisation of giant exoplanets with a moderate irradiation, with the next-generation space telescopes such as JWST and ARIEL as well as the extremely-large ground-based telescopes. HIP41378 f is thus an important laboratory to understand the effect of the irradiation on the physical properties and chemical composition of the atmosphere of planets.
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Cited by 6 Pith papers
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ASTEP confirmation of a pair of long-period Jupiter-sized planets with extremely low densities transiting TOI-791
Two extremely low-density Jupiter-sized planets on long-period orbits around TOI-791 were confirmed via ground-based photometry and TTV-derived masses.
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Observing a 542-day transiting giant with large TTVs: The 2025 transit of HIP 41378 f and new constraints on the outer system
New 2025 transit timing of HIP 41378 f shows a 7-hour early arrival consistent with TTVs; N-body modeling with TRADES refines ephemerides for planets d, e, and f.
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Observing a 542-day transiting giant with large TTVs: The 2025 transit of HIP 41378 f and new constraints on the outer system
New 2025 transit timing for HIP 41378 f confirms large TTVs and is combined with prior data on planets d and e in an N-body model to update ephemerides and predict future transits.
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Do Super-Puffs Defy Core Accretion? Population-Wide Interior Structure Constraints
Interior structure models show 28 of 34 cold super-puffs are consistent with core accretion while six require non-standard explanations such as impacts or exo-rings.
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A decade of monitoring the HIP 41378's planetary system
Decade-long RV data from multiple instruments confirms Pd=278 days for planet d, refines Pe=393 days for e, measures Mf=25 Earth masses confirming low density of 0.166 g cm^-3, and identifies candidate planet h at ~2600 days.
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Characterising transiting exoplanets at long orbital period: lessons learned for PLATO from 10 years of monitoring the HIP41378 system
Ten years of observations of the HIP41378 system provide practical lessons for characterizing long-period exoplanets with the upcoming PLATO mission.
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