This is from http://grammar.ccc.commnet.edu/grammar/capitals.htm It talks about what words take capital letters. I had to look it up. 4. Names of celestial bodies: Mars, Saturn, the Milky Way. Do not, however, capitalize earth, moon, sun, except when those names appear in a context in which other (capitalized) celestial bodies are mentioned. "I like it here on earth," but "It is further from Earth to Mars than it is from Mercury to the Sun.