11. Uranus and Neptune


Planetary twins

Saturn was the most distant planet known to the ancients. Uranus and Neptune are both so far away, and so faintly illuminated by the Sun, that telescopes were required to discover them. Uranus is about 19 times as far away from the Sun as the Earth is, and Neptune is about 30 times as distant. As a result, it takes 84.0 Earth years for Uranus to complete one revolution about the Sun and nearly twice that for Neptune. With an orbital period of 164.8 years, Neptune has not yet completed its first full orbit since discovery.

These two distant planets remain little more than dim, fuzzy spots of light in even the most powerful telescope. One can still infer enough about Uranus and Neptune from telescopic observations to know that they have similar physical properties. The size, mass, composition and rotation of Uranus and Neptune are in fact so similar that they are often called planetary twins. These parameters lie between those of the Earth and the giants Jupiter and Saturn.

Some comparisons of Uranus and Neptunea

Mass (Earth-mass)14.5317.14
Radius (Earth-radius)3.983.91
Density (kilograms per cubic meter)12401670
Rotation period (hours)17.2416.11
Orbital period (Earth-years)84165
Mean distance from Sun (AU)19.1930.06
Atmosphere83 percent hydrogen
15 percent helium
79 percent hydrogen
18 percent helium
Energy balanceless than 1.42.7± 0.3
Effective temperature59.3 degrees kelvin59.3 degrees kelvin
Temperature at one-bar level76 degrees kelvin73 degrees kelvin
Central temperature5,000 degrees kelvin5,000 degrees kelvin
Magnetic dipole moment.50 DE25 DE
Equatorial magnetic field strength0.23 x 10-4 tesla0.14 x 10-4 tesla
a The Earth’s mass is 5.9743 x 1024 kilograms, the Earth’s equatorial radius is 6.378 x 106 meters, the astronomical unit, denoted AU, is the mean distance between the Earth and the Sun with a value of 1.496 x 1011 meters. The energy balance is the ratio of total radiated energy to the total energy absorbed from sunlight, the effective temperature is the temperature of a black body that would radiate the same amount of energy per unit area, and a pressure of one bar is equal to the atmospheric pressure at sea level on Earth.

Uranus is tipped on its side and has no strong source of internal heat

Blue-green Uranus lies sideways, with its poles where its equator should be. This knowledge comes not from watching the small, featureless ball rotate, but instead from observing the orbits of its major moons. The orbits are all circular, and they lie in one plane, which is turned at right angles to the plane of Uranus’ orbital motion about the Sun. As a result, the moons form a bulls eye pattern, revolving around Uranus like a Ferris wheel. Since these satellites should be orbiting within the plane of Uranus’ equator, the entire planet has to be tipped on its side. One speculation is that Uranus was knocked sideways during a massive collision, perhaps when the planet was still forming.

When Voyager 2 arrived at Uranus, on 24 January 1986, the spacecraft’s infrared detectors found that the planet is radiating about as much energy as it receives from the Sun. This means that Uranus lacks a strong internal heat source, in contrast to Jupiter and Saturn that produce heat in their centers. These two giants each radiate away about twice as much energy as they receive from the Sun. But like Jupiter and Saturn, and unlike Uranus, the planet Neptune glows in the infrared with its own internal heat discovered when the hardy spacecraft flew past Neptune in August 1989, twelve years after launch.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University