Glossary · Thiên văn học
Ice giant
The name often given to Uranus and Neptune. They are usually described as made mostly of ice-forming materials such as water, methane and ammonia, rather than being dominated by hydrogen and helium like Jupiter and Saturn — though current data do not directly show that they are water-rich.
For Uranus, NASA states that 80% or more of the mass is a hot, dense fluid of "icy" materials — water, methane and ammonia — above a small rocky core; near the core it heats up to about 5,000 °C (9,000 °F). "Icy" here refers to ice-forming materials, not frozen ice in the everyday sense; Uranus's atmosphere, by contrast, is mostly hydrogen and helium. A 2020 review (Helled and colleagues) notes that water-rich models are not a unique solution: all the observed parameters can also be reproduced if the innermost regions are about 82% rock by mass, with the rest hydrogen and helium. The authors consider that extreme case implausible, since water is also expected to be present, but it shows that the data do not require the two planets to be "icy" worlds.
The two planets have masses of about 14.5 and 17 Earth masses — far more massive than the terrestrial planets — sizes about four times that of Earth, sit at about 20 and 30 AU from the Sun, and have much colder atmospheres; compared with Jupiter and Saturn they are significantly smaller. Despite similar masses and radii, they differ noticeably in atmospheric composition, axial tilt and thermal emission: Neptune's measured heat flux implies that it is still cooling, while Uranus is near equilibrium with the sunlight it receives.