If you are referring to the planet to which he went into voluntary exile, where Luke Skywalker found him, then then answer is Dagobah.

http://en.wikipedia.org/wiki/Dagobah

Yoda

the planet Dagobah

Dagobah was the name of the star around which the planets, including Yoda's planet, revolved.

The planet Yoda exiled himself to was an unnamed and unexplored planet. Yoda selected this planet because of it's remoteness and lack of population. The planet's star was named Dagobah, so all the planets revolving around the star were part of the Degobah system.

The planet of Dagobah