If you're talking about planet "Earth", then yes, it should be capitalised. In other cases, the word shouldn't be capitalised. Here's an excerpt from the Longman Dictionary of Contemporary English:
earth 1 S2 W2 / ɜːθ $ ɜːrθ / noun
1 world ( also Earth ) [ singular, uncountable ] the planet that we live on : the planet Earth
the earth The earth revolves around the sun.
the earth’s surface/atmosphere/crust etc 71% of the earth’s surface is sea.
on earth the origin of life on Earth
2 soil [ uncountable ] the substance that plants grow in SYN soil soft/bare/damp etc earth footprints in the wet earth
a lump of earth
3 land [ uncountable ] the hard surface of the world, as opposed to the sea or air SYN ground : The earth shook.
They watched the kite fall back to earth.
Edit: According to the link below, what I said wasn't exactly right:
http://grammar.ccc.commnet.edu/grammar/capitals.htmNames of celestial bodies: Mars, Saturn, the Milky Way. Do not, howver, 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.
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