Space Facts

Planet Neptune

At the Edge of the Solar System

Neptune from Voyager 2 at 4.4 million miles distance. The picture shows the Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. Image Credit: NASA/JPL

Neptune is planet number eight of the Solar system. The planet is named for the Roman god of the sea (comparable to the Greek god, Poseidon). This seems appropriate for the planet's deep blue color.

The planet Neptune has many characteristics similar to that of its near-twin, Uranus. Its atmosphere is largely hydrogen and helium, but it also contains many "ices" such as methane, ammonia and water. In fact, it is the methane that is responsible in part for the planet's blue color. Neptune is -218 °C (55 °K) cold, which is a little warmer than Uranus, despite being farther from the sun. The additional warmth seems to come from the heat of compression of the planet's original formation roughly 4.5 billion years ago. Uranus curiously lacks the expected quantity of internal heat.

The equatorial diameter of Neptune is about 49,530 kilometers (30,780 miles). One day on Neptune is about 16 hours. The Neptunian year is 168.95 Earth years long. That's a very long time between birthdays. Neptune also has a ring system, albeit light-weight compared to those of the other gas giants. Only one sizable moon exists in Neptune's family — Triton at 2,706 kilometers (1,682 miles) diameter. Twelve other moons add to the family, ranging from 440 kilometers (273 miles) down to 40 kilometers (25 miles) in diameter.

Size comparison of gas giant planets with Earth (left to right): Earth, Jupiter, Saturn, Uranus, and Neptune.		The last four planets of the Solar system are called "gas giants" because the bulk of each such planet is gaseous. The other type of planet is called "terrestrials" because of their rough similarity to Earth. The Solar System Facts table gives more comparisons between the planets.

Could Neptune Ever Sustain Life?

As the last full planet of the Solar system, Neptune is frigid — far too cold to support life as we know it. The thick atmosphere would create deadly pressures on any solid surface, if it had one — thousands of kilometers below the tops of Neptune's clouds. If Earth (and perhaps Mars and even Venus) are full and humanity needs extra space, it will likely have to look to one of our neighboring star systems for a more hospitable planet.

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