Uranus

Introduction

Uranus is the seventh planet from the sun in our solar system. This huge, ice giant is covered with clouds and is encircled by a belt of 11 rings and 22 known moons. Uranus’ blue color is caused by the methane (CH₄) in its atmosphere; this molecule absorbs red light.

Rotational Axis

Uranus’ rotational axis is strongly tilted on its side (97.9°). Instead of rotating with its axis roughly perpendicular to the plane of its orbit (like all the other planets in our Solar System), Uranus rotates on its side (along its orbital path). This tipped rotational axis gives rise to extreme seasons on Uranus. For more information on Uranus’ extreme seasons, click here.

Because of its almost-perpendicular axis orientation, there is a debate over which of Uranus’ poles is its north pole. This debates leads to yet another: Is Uranus spinning in a retrograde orbit (like Venus) or not (like the other planets)?

Size

Uranus is about 31,690 miles (51,118 km) in diameter. This is about 4 times the diameter of the Earth.

This gas giant is the third-largest planet in our Solar System (after Jupiter and Saturn).

Mass and Gravity

Uranus’ mass is about 8.68 x 10²⁵ kg. This is about 14 times the mass of the Earth. The gravity on Uranus is only 91% of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet’s surface is proportional to its mass and to the inverse of its radius squared).

A 100-pound person on Uranus would weigh 91 pounds.

Length of a Day and Year

Each day on Uranus takes 17.9 Earth hours. A year on Uranus takes 84.07 Earth years; it takes 84.07 Earth years for Uranus to orbit the sun once.

Orbit Distance

Uranus is over 19 times as far from the Sun as the Earth is; it averages 19.18 A.U.

At aphelion (the farthest point in its solar orbit) it is 1,850,000,000 miles (3,003,000,000 km) from the Sun. At perihelion (the closest point in its solar orbit) it is 1,700,000,000 miles (2,739,000,000 km) from the Sun.

Temperature

The mean temperature on the surface of Uranus’ cloud layer is -350°F (59 K). Uranus radiates very little heat in comparison with the other gas giants, Jupiter, Saturn and Neptune).

Planetary Composition and Atmosphere

Uranus is a frozen, gaseous planet with a molten core. Uranus’ atmosphere consists of 83% hydrogen, 15% helium and 2% methane.

For more information on Uranus’ composition, click here.

Rings

Uranus has a belt of 11 faint, narrow rings composed of rock and dust. They circle Uranus is very elliptical orbits. These rings are only a fraction of the size of Saturn’s rings, and were only discovered in 1977.

For more information on Uranus’ rings, click here.

Moons

Uranus has 5 large moons (2 were discovered by Wm. Herschel in 1781, 2 were discovered by Wm. Lassell in 1851, and one by G. Kuiper in 1948) and many small moons (which were discovered much later).

For more information on Uranus’ moons, click here.

Discovery

Uranus was discovered by the British astronomer William Herschel on March 13, 1781. Herschel also discovered two of the moons of Uranus (Titania and Oberon) and some of the moons of Saturn.

Uranus-Earth Comparison

Name and Symbol

This planet was originally named in 1781 by the British astronomer William Herschel - he called it Georgium Sidus (meaning “the Georgian planet”) to honor the King George III of England. The name was later changed to Uranus, the ancient mythological god of the sky, Ouranos. The name Uranus was suggested by the German astronomer Johann Elert Bode.

Spacecraft Visits

Uranus has been visited by NASA’s Voyager 2, whose closest approach was on January 24, 1986.

Activities and Links

• Uranus coloring page
• Find It!, a quiz on Uranus.
• An interactive puzzle about Uranus
• Uranus Cloze Printout: A fill-in-the-blanks activity on the planet Uranus.
• How to write a report on a planet - plus a rubric.