This is Neptune. It is the outermost planet of our solar system, full time or most of the time, depending on your view of the status of Pluto (Pluto has a very eccentric orbit, which partially resides closer sunward than Neptune’s orbit).
Neptune was discovered on 23rd Sept 1846, and completed an entire orbit back to the point of its discovery on 2nd July 2011. Right, we know of Neptune’s existence for slightly more than one Neptune-year.
Also, there is a 2:3 resonance between Pluto’s and Neptune’s Orbit, so for 3 Neptune-years there are exactly 2 Pluto-years.