Neptune was discovered by means of mathematics before being seen through a telescope. Astronomers had noticed that Uranus, which they thought was the most distant planet, was not always in the position they predicted for it. The force of gravity of some unknown planet seemed to be influencing Uranus.
Neptune is one of the two planets that cannot be seen without a telescope. The other is Pluto. Neptune is about 30 times as far from the sun as is Earth. Pluto is the only planet farther from the sun than Neptune. But every 248 years Pluto moves inside Neptune's orbit for about a 20-year period, during which it is closer to the sun than Neptune. Pluto last crossed Neptune's orbit on Jan. 23, 1979, and remained within it until Feb. 11, 1999. Neptune's diameter at the equator is 30,775 miles (49,528 kilometers), or almost 4 times that of Earth. It is about 17 times as massive (heavy) as Earth, but is not so dense as Earth. Neptune has 11 satellites (moons) and several rings around it. Neptune travels around the sun in an elliptical (oval-shaped) orbit. Its average distance from the sun is about 2,793,100,000 miles (4,495,060,000 kilometers). Neptune goes around the sun once about every 165 Earth years, compared with once a year for Earth. As Neptune orbits the sun, it spins on its axis, an imaginary line through its center. Neptune's axis is not perpendicular (at an angle of 90 degrees) to the planet's path around the sun. The axis tilts about 28 degrees from the perpendicular position. Neptune spins around once in about 16 hours and 7 minutes.
Neptune has 11 known satellites. Triton, Neptune's largest satellite, is about 1,681 miles (2,705 kilometers) in diameter and about 220,440 miles (354,760 kilometers) from Neptune. It is the only major satellite in the solar system that orbits in a direction opposite to that of its planet. Triton has a circular orbit and travels once around Neptune every six days. Triton may once have been a large comet that traveled around the sun. At some point, Neptune's gravity probably captured the comet, and it became a satellite of Neptune. Scientists have discovered evidence that volcanoes on Triton once spewed a slushy mixture of water and ammonia. This mixture is now frozen on Triton's surface. Triton has a surface temperature of -390 degrees F (-235 degrees C), the coldest known temperature in the solar system. Some volcanoes on Triton remain active, shooting crystals of nitrogen ice as high as 6 miles (10 kilometers) above the moon's surface.
Neptune has three conspicuous rings and one faint ring. All of these rings are much fainter and darker than the rings of Saturn. They appear to consist of particles of dust. Neptune's outer ring is unlike any other planetary ring in the solar system. It has three curved segments that are brighter and denser than the rest of the ring. Scientists do not know why the dust is spread unevenly in the ring.
