Neptunes celebrating today, having completed one orbit around the Sun since its discovery in 1846. Neptune’s year is 164.8 Earth years long, so it has taken until now for it to make one full circle of the Solar System. Neptune was the first planet to be found via a mathematical prediction. Astronomers had noted that Uranus—the next planet inwards to the Sun—was not following its predicted path, and the gravitational pull of an as-yet-undiscovered planet was thought to the culprit. Predictions were made for where in the sky this mystery planet might be found, and sure enough, there it was—Neptune.
The story of the prediction and discovery has lots of twists and turns—read more about it here. And the story of how the planet then got its name is equally complex—and you can read more about that here. Neptune orbits the Sun at an average distance of 30.1 AU (one AU being the distance between Earth and Sun), or about 4.5 billion kilometres. Neptune is the fourth-largest planet, with a radius at the equator of 24,764 kilometres—about four times wider than Earth. The giant blue world is the most distant Solar System body visited by a spacecraft. NASA’s Voyager 2 probe flew past Neptune in 1989. Story by Jonathan Nally. Images courtesy NASA.
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