I don't know any bands like Planet X, but there's Mogwai. Pretty mellow, but they can really rock out at the same time. LOVE them
Probably my favorite song of theirs:
Then there's Sigur Ros, an Icelandic group. They're even more mellow, almost etherial, but beeeautiful. Some of their stuff is instrumental, but they have lyrical stuff too.
This is my favorite Sigur Ros song, Hoppipola. The video is so fun/cute too!
http://www.youtube.com/watch?v=_EyI4p0yjDQ
another favorite: