  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.
