Far, far from the sun, Uranus has a blue-green atmosphere that hints at its makeup. One of the two ice giants, the planets composition differs somewhat from Jupiter and Saturn in that it is made up of more ice than gas.

Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which the planet is tilted.
Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which the planet is tilted.
Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory

The surface of Uranus

Like the other gas giants, Uranus lacks a solid, well-defined surface. Instead, the gas, liquid, and icy atmosphere extends to the planet's interior. Were you to land — and hover — at the point where the atmosphere transitions to the interior, you would experience less of a gravitational tug than you might feel on Earth. Gravity on Uranus is only about 90 percent that of Earth; if you weigh a hundred pounds at home, you would only weight 91 pounds on Uranus.

Uranus is the second least dense planet in the solar system, indicating that it is made up mostly of ices. Unlike Jupiter and Saturn, which are composed predominantly of hydrogen and helium, Uranus contains only a small portion of these light elements. It also houses some rocky elements, equal to somewhere between 0.5 to 1.5 times the mass of Earth. But most of the planet is made up of ices, mostly water, methane, and ammonia. Ices dominate because the vast distance to Uranus from the sun allows the planet to maintain frigid temperatures.

A frigid core

While most planets have rocky molten cores, the center of Uranus is thought to contain icy materials. The liquid core makes up 80 percent of the mass of the planet, mostly comprised of water, methane, and ammonia ice, though it only extends to about 20 percent of the radius.

The internal heat of Uranus is lower than astronomers would expect. The planet's core heats up to 9,000 degrees Fahrenheit (4,982 degrees Celsius). This seems hot, but is in fact pretty cool when compared to the cores of other planets.

While other gas giants are powered by their cores, Uranus radiates almost no excess heat into space. One reason for this could be due to an impact soon after the planet's formation. The planet's current sideways rotation, spinning at a 90 degree angle compared to the other planets in the solar system, already indicates a collision. The impact could also have carved out a portion of the core, leaving it with a lower temperature.

A strange magnetic field

Movement inside of the core tends to drive a planet's magnetic field, but the field around Uranus is strange. Fairly weak, no indication of a field was recorded until NASA's Voyager 2 arrived at the planet in 1986.

Generally, a magnetic field shrouds the planet from its poles. On Earth, for instance, the geographic North Pole is very close to the magnetic North Pole. But Uranus, discovered in 1781, is tipped on its side, so that one pole or the other is pointed almost directly at the sun. The planet's magnetic field is offset from the poles by almost 60 degrees, creating a magnetic field that tends to be stronger at one pole than the other.

Although the magnetic field of Uranus is strange, it is not unique. Neptune, the other ice giant, boasts a similar magnetic field, leading astronomers to conclude that the core may not drive the fields.

Rocky rings

Like all gas giants, Uranus carries a set of rocky rings around its equator. The thin strips, most only a few miles wide, are made up of tiny bits of rock and ice smaller than a meter. The planet has at least 13 known rings in two systems.

— Nola Taylor Redd, SPACE.com Contributor