[Semper911]

Earth: It is lowercase unless at the beginning of a sentence. ... depending on whether your English teacher insists.

I very much doubt an English teacher would insist that you use lower case for Jupiter, Saturn, or Mars. Likewise Earth.

When we say "the earth", we are really saying "the planet named Earth."

It is a minor point, though, and my proofreading correction really didn't belong on this physics thread. (But as we all know, there is no slack on FR.)

[RightWhale]

lower case for Jupiter, Saturn, or Mars

This is far more interesting and useful than ZPE. Jupiter, Saturn, Mars, Venus, Sol, Luna, Terra, Pluto are classical, formal, proper names for the planets and are capitalized. Informal terms such as earth, moon, and sun are common names and are not capitalized. But as another poster mentioned, your editor will make the final decision.