Is there a go-to/search planet?
Look in the lower right corner of the screen. There is a small bar with several icons, one of them, I think the magnifying glass, may also be the question mark is search planet. Just enter the name of the planet in the window popping up.
Alternatively, use the tried and true paradox hotkey: F to find.
Pressing F will bring up the search window. The results however only show the name of the System that contains the planet that you searched for.