c. 4.5 Billion BCE

Neptune

If Earth and Venus can be considered fraternal twin planets, Uranus and Neptune are more like identical twins. Both are denizens of the deep outer solar system, with Neptune, out at 30 astronomical units, taking approximately 165 Earth years to orbit the Sun once. Both ice giants are about the same size and mass (Neptune is slightly heavier, at 17 Earth masses), and both have a similar composition: about 80 percent hydrogen, 19 percent helium, and trace amounts of methane and other hydrocarbons. As it does on Uranus, methane gives Neptune its beautiful azure color.

Neptune, like Uranus, is another ice giant world with a modest number of icy satellites (13) and a system of dark icy rings. From telescopic measurements, data from the 1989 Voyager 2 flyby, and laboratory studies, astronomers have deduced that Neptune’s gaseous atmosphere extends about 10 to 20 percent of the way to the center of the planet. Then, as pressure and temperature increase, higher concentrations of water, ammonia, and methane form a hot liquid mantle. Astronomers refer to this zone as “icy” because the molecules there are thought to have originally come from the mostly icy outer Solar Nebula planetesimals that were part of Neptune’s original building blocks. Some astronomers even think of this zone as a water-ammonia ocean, and computer simulations suggest that a rain of diamonds fall through this ocean to the planet’s Earthlike core of rock, iron, and nickel.

It’s puzzling to astronomers that the ice giants reside in the far outer solar system, because there may not have been enough solar nebula material at those distances to form them. One explanation may be that they formed closer to the Sun and slowly migrated out, perhaps gravitationally nudged by Jupiter and/or Saturn. We think of the solar system today as stable, running like clockwork, but when the planets were forming, it was much more violent and chaotic.

Voyager 2 photo of Neptune’s Great Dark Spot, a smaller dark spot to the south, and fast-moving, wispy, cirrus-like white clouds. Winds in excess of 1,300 miles (2,100 kilometers) per hour were measured in these atmospheric storm systems.