https://history.nasa.gov/styleguide.html "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.” Another note on usage: “Earth,” when used as the name of the planet, is not preceded by “the”; you would not say “the Neptune” or “the Venus.” When “earth” is lowercased, it refers to soil or the ground, not the planet as a whole. Do use “the” in front of “Sun” and “Moon” as applicable."
"When Merriam-Webster indicates that a term is “capitalized” or “usually capitalized,” the MLA capitalizes the term in its publications. When Merriam-Webster indicates that a term is “often capitalized,” our practice varies. We usually lowercase sun, moon, and earth, but, following The Chicago Manual of Style, when the does not precede the name of the planet, when earth is not part of an idiomatic _expression_, or when other planets are mentioned, we capitalize earth:
The earth revolves around the sun.
The astronauts landed on the moon.
The space shuttle will return to Earth next year.
The four planets closest to the sun—Mercury, Venus, Earth, and Mars—compose the inner solar system."
(Chicago style guide:)
"In nontechnical contexts, the word “earth,” in the sense of our planet, is usually lowercased when preceded by “the” or in such idioms as “down to earth” or “move heaven and earth.” When used as the proper name of our planet, especially in context w
th other planets, it is capitalized and “the” is usually omitted."
"Earth
when talking about the planet, but earth in such idioms as down to earth, what on earth? etc"
Currently we don't capitalize moon, sun, earth in the absolute majority of entries containing phrases "the moon" or "the sun" (some exceptions: 太陽潮, 採火, 星. We have 43 entries containing the phrase "the earth" (which NASA recommends against).
I'd be happy to follow NASA's style guide. (except not capitalizing them in idioms as per the Guardian/Observer, etc.)
Best,
Marcus