Lazy, lazy Neptune.
Short Sharp Science: Cheers, Neptune, on your first 'anniversary'
Dog years are nothing next to Neptune years. The distant planet has finally completed the first entire orbit of the sun since its discovery in 1846.
The planet is actually about as old as the solar system, which is 4.6 billion Earth-years old. But it makes sense to celebrate the "anniversary" of its discovery, since it was a veritable feat of astronomical deduction.
Astronomers predicted Neptune's existence and location based on the observed motion of Uranus, which was discovered in 1781. Astronomer Johann Gottfried Galle spotted Neptune from the Royal Observatory in Berlin right at the predicted location on 23 or 24 September 1846 (it was close to midnight and there is some uncertainty over the exact time). Galle is credited with its discovery, although Galileo appears to have seen it way back in 1612 and mistaken it for a star.
One full Neptune-year, which lasts 164.79 Earth-years, has now passed since its discovery. Figuring out the exact timing of this milestone is surprisingly complicated – it depends on the uncertain time of the discovery, and on exactly how you define "one Neptune orbit", since there are slightly different ways to do that.