Neptune has completed only one orbit around the Sun since its 1846 discovery: a year there lasts just under 165 Earth years, and its first birthday after discovery came in 2011. It was the first planet found through mathematical prediction.
After Uranus was discovered in 1781, astronomers noticed it deviating from its predicted path and suspected another planet was influencing it. French astronomer Urbain Le Verrier calculated where the unseen body should be; Johann Gottfried Galle and student Heinrich Louis d’Arrest found Neptune in 1846 at the Berlin Observatory.
More than 200 years before its official discovery, Galileo recorded the object in his notebook as a star.
