A planetary year is defined as the time it takes for that planet to make one complete orbit around the sun. Neptune was discovered on September 23, 1846. In terms of planetary years, it turned one year old on July 12, 2011, when it completed its first full orbit since its discovery.

Pluto is even younger. It was discovered on February 18, 1930. It takes 248.09 Earth years to complete one orbit around the sun, so it will complete its first full orbit since discovery on Monday, March 23, 2178, just a few years after Neptune turns 2. Perhaps by then it will have officially regained its status as a planet.

Pluto and Neptune have a unique relationship. Pluto is usually farthest from the Sun, but for 20 years out of every 248, it intersects Neptune’s orbit and is closer to the Sun than its gas giant neighbor. Pluto last crossed inside Neptune’s orbit on February 7, 1979, and temporarily became the 8th planet from the Sun. It crossed back over Neptune’s orbit again on February 11, 1999 to resume its place furthest from the Sun for the next 228 years.