"a:1:{i:0;s:12:\"Earth\";}"
This is a PHP serialized value. If you pass it to unserialize you’ll get back the array you’re expecting. (And it looks like you’ve edited it because “Earth” is not 12 characters long: that should be s:5:"Earth".)
However I’m surprised that get_user_meta doesn’t already do this for you: it passes the value it’s fetched from the database through maybe_unserialize() before it’s returned to you, which ought to do this automatically for you. Is there any chance you’ve double-serialized this before storing the string in the database? (And note also the warning on the documentation page about unserializing arbitrary user-provided strings.)
To answer the general question though, everything you can get from /users/me is available with normal API calls and you don’t need to use the REST API from inside WordPress itself.