Isn't it just our luck to be given a faulty one - an elliptical orbit, a 23 degrees tilt and it's not even round.
(Background - Calvin has met two aliens - Galaxoid and Nebular - while collecting leaves for a school project, and has agreed that they can have the Earth if they give him 50 leaves, which they do. Anyway, they've now realised the Earth doesn't meet the specification ..... )