Ice Giants

Abstract Uranus and Neptune, the “ice giants” in the solar system, represent a fascinating class of planets. They are the outermost planets in the solar system with intermediate masses and sizes, complex nonpolar magnetic fields, strong atmospheric winds, and not well-understood internal structures. Studying the interiors of Uranus and Neptune is vital for advancing an understanding of planetary formation and evolution as well as for the characterization of planets around other stars. The internal structures and compositions of Uranus and Neptune are derived from numerical models that fit the available measurements. Both planets are expected to be composed of rocks and ices and have H-He atmospheres of the order of 10% of their total masses. The rock-to-water ratios on Uranus and Neptune, however, are very uncertain. It is also unclear how the different materials are distributed within the interiors and whether distinct layers exist. While Uranus and Neptune are often viewed as being “twin planets,” it is in fact unclear how different the two planets are from each other, and whether they are indeed “icy” (water-dominated) planets. Regarding the challenges in constraining the formation paths of Uranus and Neptune, it is still unclear whether the planets formed at their current locations, or what the dominating processes that led to their formation (accretion rates, size of solids, etc.) were. It is also unclear what the role of giant impacts was shortly after their formation. Going forward, advanced modeling, future observations from space and the ground, lab experiments, and links with exoplanetary science can improve the current understanding of Uranus and Neptune as a class of astronomical objects that seems to be very common in the galaxy.