A planet which is warm enough so that it lies above the gas/liquid critical point and above the solidification line of its main constituent is a fluid planet. This is the case of Jupiter and Saturn, and of all planets in the Universe that are dominated by hydrogen, due to its very cold critical point (33K) and low melting temperature. Planets mainly made of other elements than hydrogen and helium, such as water, may also be fluid, if they are close enough to their star. Hydrogen, helium and oxygen being the most abundant elements in the Universe, fluid planets hold some of the keys to understand our origins.
Being fluid implies that these planets have low viscosities and few barriers to convection and mixing. They are governed by the same hydrostatic equations as stars. The progressive transition between the gaseous atmosphere and the fluid interior allows one, in principle, to infer bulk composition from the atmospheric one. Behind their apparent simplicity, complications arise rapidly: How do these planets rotate? How efficiently would they mix other elements, particularly in the presence of condensation and clouds, but also when this is energetically unfavored? What is the effect of intense irradiation on the global heat transfer?
Over the past 30 years, solutions or partial solutions to these questions have been provided thanks to a combination of theoretical studies and observations. Simple theories of the evolution and atmospheric properties of exoplanets have proven relatively successful. Advances in gravitational sounding of Jupiter and Saturn have provided the basis to understand and predict how fluid planets rotate. Constraints on the structure of the planets and on the presence of primordial dilute cores have been provided.
Yet, recently, thanks to the Juno and Cassini observations, evidence of imperfect mixing and stable regions have arisen both in the deep atmosphere and interior of Jupiter and Saturn. Observed latitudinal and temporal variability in composition, lightning or general atmospheric properties have remained unaccounted for. Modeling atmospheric properties of fluid planets based on Earth parameterizations, not fully accounting for their abyssal nature and moist convection inhibition has failed. Fluid planets are more complex and challenging than previously envisioned.
Progress will come from the continuation of a combination of studies: theoretical and numerical studies to understand heat transfer and mixing in the presence of condensation, observations of a large variety of exoplanets and measurements in solar system fluid planets. But the next milestone lies in the outer solar system, with the exploration of Uranus and Neptune. These planets are only partially fluid and may have a solid interior. This increased complexity matches what is to be expected for other planets in the Universe. Their atmospheres, made of hydrogen and helium and large amounts of methane, are laboratories to test models of heat and element transport in abyssal hydrogen atmospheres. An international mission with an orbiter and a probe would allow for the direct measurements that we need in order to interpret with confidence the great wealth of data awaiting us with the more distant exoplanets.
