the "so-called planet" remark is a way of saying that where they are isn't even worth being called a planet. Anyway, correct or not, just because a character makes a strange remark it isn't really a slip-up.
One of my top five favorite movies, by the way.
No Altair...Alpha Aquila, is a single star. Of interest it was recently found to be the flattest star yet discover. Seems it has a huge rotation rate. Spins once every 10 hours or so.