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Generally speaking, I would write earth when referring to soil, the ground, or land as opposed to sea. I would capitalize Earth when referring to the planet (and even more specifically, our planet; there are many earths out there). In short: if it's a proper noun, it should be capitalized.

1. A handful of earth.


2. The earth under this house.


3. The earth beneath my feet.

I agree that earth should not be capitalized here. However, I could come up with contexts in which I would capitalize it even in these sentences, though they would be rather poetic or sci-fi (read: far-fetched).

 4. What on earth?

I would capitalize Earth here. To me, this question is equivalent to "What on this planet?", "What in this world?", "What on Mother Earth?", and not to "What on soil?" or "What on land?". (If you want a comprehensive, highly scientific study, then I am happy to report that out of the first 30 Google results for "What on Earth", Earth is capitalized in 25 cases.)

 5. The heavens above and the earth below.

Soil, ground, land as opposed to the sky. No need to capitalize.

Edit: as Neil Fein points out in the comments, this could refer to the planet in certain contexts, in which case I would capitalize it (cf. 1, 2, and 3).

 6. The earth moved.

It depends. Did the Earth (proper noun) move around the Sun (proper noun), or did the earth (soil, ground) move because there was an earthquake?

 7. We returned quickly to earth.

I agree with you that here it should be capitalized.

According to the Wikipedia Manual of Style, it appears your examples should be as follows:

1. A handful of earth.


2. The earth under this house.


3. The earth beneath my feet.


4. What on Earth?


5. The heavens above and the Earth below.


6. The Earth moved.


7. We returned quickly to Earth.

Also, I believe 5 and 6 could have either capitalization, depending whether you mean the planet Earth, or just the dirt below your feet.

So, if you can replace earth with 'dirt' or 'land' or any common noun - lower case.
If you can replace Earth with Duluth or Mars or any proper noun then capitalize it.

I would write Earth when referring to the planet where we live because it is a proper noun (like Mars, and Venus, for example).

Looking at the New Oxford American Dictionary, it's reported that

The earth is the third planet from the sun in the solar system, orbiting between Venus and Mars at an average distance of 90 million miles (149.6 million km) from the sun, and has one natural satellite, the moon.

The definition for earth as a noun starts with

1. (also Earth) the planet on which we live; the world: the diversity of life on earth.

As for the example phrases you wrote, I would write them as

A handful of earth.

The earth under this house.

The earth beneath my feet.

What on earth?

The Heavens above and the Earth below.

The earth moved.

We returned quickly to Earth.

I wrote earth on the fourth phrase because it's a way of saying, which has a different meaning from What is happening on Earth? I am actually not 100% sure on how I would wrote earth in that case.

In the earth moved I wrote earth because the word is not referring to the planet (I suppose it is the sentence you say when there have been a quake).

Earth is capitalized when it's a name (proper noun of our planet), and not when it's a noun (referring to a planet, its soil, its land...)

Planet Earth, I was back to Earth after two years, I'm not on this earth to be a slave, the gravity close to (the) earth is not the same than in altitude.

When earth is preceded by the, this is a priori a noun.

1. A handful of earth.


2. The earth under this house.


3. The earth beneath my feet.


4. What on Earth?


5. The heavens above and the earth below.


6. The earth moved (if this is the land, but Earth moves in space)


7. We returned quickly to Earth.

Source: Earth (capitalization)

Style guides vary on the subject, for example the Chicago Manual of Style says that we usually lowercase "sun", "moon", and "earth", but 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." On the other hand, NASA suggest that "Earth", "Sun" and "Moon" be always capitalized when referring to the celestial bodies and recommend against using the definite article "the" in those cases (see http://history.nasa.gov/styleguide.html).

In most cases, I would says that if it is preceded by the definite article "the" it is not usually capitalized (the same when referring to the sun or the moon).

When discussing of Earth as a planet or celestial body, it is usually capitalized when it refers to its proper name.

When the meaning is soil or ground, or part of a common saying such as "What on earth?" it is not capitalized.

So in the sentences 1 through 6 above, earth should NOT be capitalized.

"7. We returned quickly to earth" depends on whether we actually went to space or we came back to reality. If we came back from space, it should be capitalized otherwise it is not.