The first letter of a proper noun is normally capitalized (Susan, Denver, Honda), whereas common nouns are written in lowercase (woman, city, company). For a word such as earth, which can be used as a proper noun or as a common noun, your decision whether or not to capitalize the e will depend upon the way the word is being used.

In a list with other planets, Earth would be capitalized:

• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune

The Style Guide of the National Aeronautics and Space Administration (NASA) says the following about the names of astronomical bodies:

Capitalize the names of planets (e.g. “Earth,” “Mars,” “Jupiter”). Capitalize “Moon” when referring to Earth’s Moon, otherwise, lowercase “moon” (e.g. “The Moon orbits Earth,” “Jupiter’s moons”). Capitalize “Sun” when referring to our Sun but not to other suns. Do not capitalize “solar system” and “universe.” (1)

Style guides disagree, however; some will tell you not to capitalize sun and moon, for example.

“You’re like the sun; you bring light into my life.”

“I’m over the moon; I couldn’t be happier!”

In our solar system, nine planets revolve around one sun.

Here are some general guidelines to follow if no particular style is specified.

Capitalize Earth when referring to the planet:

The mission to Mars was launched from Earth early this morning. (When used with the names of other planets, capitalize Earth as well.)

“Infinity to Earth,” said the commander of the space station Infinity.

“People who live on Earth are called Earthlings.”

Compare:

“People who live in New York are called New Yorkers.”

“People who live on Mars are called Martians.”

Do not capitalize earth when referring to soil or land:

The earth felt solid beneath my feet after I emerged from the swimming pool. (Here, ground could substitute for earth.)

I’ve been blessed with the opportunity to visit some of the most beautiful spots on earth.

Some people believe the earth’s resources can and should be exploited for profit.

Do not capitalize words containing earth that are used as adjectives and adverbs (earthy, down-to-earth, earthly) or the verb unearth.

Capitalize Earth when the planet is personified or deified:

Ancient peoples regarded the spirit of Earth as a living being. (Some people refer to Earth as Gaia, for example; as such, it would be capitalized in the same way the names of gods, such as Zeus, are capitalized.)

Stop polluting Mother Earth!

In honor of Earth Day, volunteers donated their time to cleanup efforts.

The cult of Sol, the Roman sun god, was popular in ancient times.

Do not capitalize heaven and earth (or adjectives like solar and lunar).

God created heaven and earth.

“She’s brought me a little bit of heaven right here on earth.”

(A title, of course, would follow normal rules of capitalization: On Earth and in Heaven: A Soul’s Journey. The first word of a sentence is always capitalized: Earth’s atmosphere is changing.)

Disagreement exists in the gray zones; use your judgment and be consistent.

Solar flares occurring on the sun can affect communication systems on earth. (If sun were capitalized, then earth would be also.)

Tides on Earth are influenced by the Moon’s gravitational pull.

Tides on earth are influenced by the moon’s gravitational pull.

Could go either way . . .

My recommendation: Unless the use you intend falls into a category that clearly requires capitalization, use lowercase.

Reference:

(1) Style Guide for NASA History Authors and Editors,

http://history.nasa.gov/printFriendly/styleguide.html (accessed April 18, 2019)