Stargate Atlantis and Stargate Universe aren't "occasionally on space ships". That's fundamental to their entire plot.
From Wikipedia, though: "Space opera is a subgenre of science fiction[1] and science fantasy[2] that emphasizes epic outer space adventures set in a universe in which faster-than-light travel has become common."
Stargate, even without the 2 series above, absolutely fits that definition. Very little of SG1 takes place on Earth. It's absolutely about aliens and crossing distances faster than light, and the epic adventures and social interactions that result.
Edit: Also, there's pretty heavy discussion that Dr Who is space opera.
From Wikipedia, though: "Space opera is a subgenre of science fiction[1] and science fantasy[2] that emphasizes epic outer space adventures set in a universe in which faster-than-light travel has become common."
Stargate, even without the 2 series above, absolutely fits that definition. Very little of SG1 takes place on Earth. It's absolutely about aliens and crossing distances faster than light, and the epic adventures and social interactions that result.
Edit: Also, there's pretty heavy discussion that Dr Who is space opera.