Neptune

This year marks the 165th Earth year since the human discovery of the planet Neptune. That comes out to one Neptunian year.

Neptune is the first planet discovered after a deliberate attempt to find it. Using Newton's Laws to analyze the orbit of Uranus, two mathematicians independently concluded that another large planet was gravitationally acting on Uranus, and they worked out where in the sky that planet should be. Astronomers pointed telescopes in that direction, and quickly found Neptune. It was a stunning demonstration of the power of science and mathematics to understand the universe.

Nearly a century later, similar reasoning led to the search for Planet X, which culminated in the discovery of Pluto by Clyde Tombaugh in 1930. Pluto, however, never really measured up, and it was eventually decided that there were in fact no unexplained irregularities in the motion of Neptune. Serendipity also plays a part in scientific discovery.