Key Ideas

Discovery of the Outer Planets: Uranus was discovered by chance, while Neptune was discovered at a location predicted by applying Newtonian mechanics.

Atmospheres of Uranus and Neptune: Both Uranus and Neptune have atmospheres composed primarily of hydrogen, helium, and a few percent methane.

• Methane absorbs red light, giving Uranus and Neptune their greenish-blue color.
• No white ammonia clouds are seen on Uranus or Neptune. Presumably the low temperatures have caused almost all the ammonia to precipitate into the interiors of the planets. All of these planets’ clouds are composed of methane.
• Much more cloud activity is seen on Neptune than on Uranus. This is because Uranus lacks a substantial internal heat source.

Interiors and Magnetic Fields of Uranus and Neptune: Both Uranus and Neptune may have a rocky core surrounded by a mantle of water and ammonia. Electric currents in these mantles may generate the magnetic fields of the planets.

• The magnetic axes of both Uranus and Neptune are steeply inclined from their axes of rotation. The magnetic and rotational axes of all the other planets are more nearly parallel. The magnetic fields of Uranus and Neptune are also offset from the centers of the planets.

Uranus’s Unusual Rotation: Uranus’s axis of rotation lies nearly in the plane of its orbit, producing greatly exaggerated seasonal changes on the planet.

• This unusual orientation may be the result of one or more collisions with planetlike objects early in the history of our solar system. Such collisions could have knocked Uranus on its side.

Ring Systems of Uranus and Neptune: Uranus and Neptune are both surrounded by systems of thin, dark rings. The low reflectivity of the ring particles may be due to radiation-darkened methane-ice.

Satellites of Uranus and Neptune: Uranus has five satellites similar to the moderate-sized moons of Saturn, plus at least 22 more small satellites. Neptune has 13 satellites, one of which (Triton) is comparable in size to our Moon or the Galilean satellites of Jupiter.

• Triton has a young, icy surface indicative of tectonic activity. The energy for this activity may have been provided by tidal heating that occurred when Triton was captured by Neptune’s gravity into a retrograde orbit.
• Triton has a tenuous nitrogen atmosphere.

Dwarf Planet: An object orbiting the Sun (but is not a moon) with enough mass to gravitationally pull itself into a spherical shape, yet not enough gravity to clear out planetesimals from its surroundings.

• This term was introduced in 2006 to help reclassify Pluto as a dwarf planet.

Trans-Neptunian Object: An object that orbits the Sun (other than planets and comets) and on average orbits at a distance greater than Neptune.

• More than a thousand icy worlds have been discovered beyond Neptune. Pluto and Charon are part of this population.
• Most trans-Neptunian objects lie in a band called the Kuiper belt that extends from 30 to 50 AU from the Sun. Neptune’s gravity shapes the orbits of objects within the Kuiper belt.