"Earth is a pretty planet."
Translation:Die Erde ist ein schöner Planet.
Same goes for the other planets: die Venus, der Mars, der Jupiter, der Saturn...
Asteroids like Ceres, moons (except that of Earth), comets, TNOs (like Eris, Sedna) and suchlike don't need an article.
The fact that I might be caught saying other planets' names without an article ("Merkur geht auf") is surely my mistake (I must've read too much astronomical English), and as for stars, it seems to me that you can use an article for the most important ones if you like ("Der Sirius geht auf"), but feel free to correct me...
By the way: Mercury = Merkur, Neptune = Neptun.