Neptune. Neptune is the last gas giants of the series beginning at Jupiter. Gas giants are these planets like Jupiter, or Saturn, which formed in the outer part of the solar system by accretion of gas around a small, solid core. 30,775 miles in diameter (49,528 km), Neptune has a strong weather activity. Some large spots, similar to those of Jupiter, may be observed. Neptune has the strongest winds in the solar system. Neptune was discovered in 1846 only, from orbital predictions made by Le Verrier. The planet was first visited by a manmade probe in 1989 when NASA's Voyager 2 reached there. Neptune is fairly far away from Earth, at 2.7 billion miles (4.3 billion km). It takes to this faraway planet 164 years to complete one orbit. The large spot seen at the center is as large as Earth, on its longest dimension, as the wispy "cirrus-type" clouds seen hovering in the vicinity are higher in altitude. picture JPL