is neptune the third largest planet
1 Neptune is the third largest planet in the Solar System, much smaller than the real giants, Jupiter and Saturn, and only a little bigger than Uranus. 2  Neptune has a diameter of 29,297 miles, or 47,150 kilometres. 3  The Earth has a diameter of 7928 miles (12,760 kilometres).4  Neptune is one of the four âgas giantsâ. They named it Neptune after the Roman God of the Sea. 2  Neptune is the third largest planet in the Solar System, much smaller than the real giants, Jupiter and Saturn, and only a little bigger than Uranus. 3  Neptune has a diameter of 29,297 miles, or 47,150 kilometres.