A proper noun with an indefinite article -is it a rare case?

Question

Proper nouns don't take indefinite articles -simple and straightforward. But then, I came across a sentence while searching for some examples as I was pondering over this question.

I learned that at times, we 'capitalize' the letter 'e' in earth. The rule as described says:

We capitalize 'e' in 'earth' when it's used as a proper noun

The example follows:

This means its light has taken an astonishing 12.9 billion years to reach us here on Earth. [BBC News]

We don't capitalize 'e' if 'earth' is working like a noun

Shale gas is extracted from beneath the surface of the earth through a process called hydraulic fracturing, or “fracking.” - [CBC]

But...

from COCA, I got a few sentences:

Computer simulations suggest a that if a more massive object, like a brown dwarf (which contains the mass of dozens of Jupiters), orbited the Sun instead of Jupiter, it would have been harder for Earth and the other terrestrial planets (Mars, Venus, and Mercury) to have formed. So under these circumstances, there might not even be an Earth.

That's sufficient to detect a giant planet in a big orbit, or a small one if it's very close to its star, but not an Earth at anything like our Earth's 93-million-mile distance from its star.

An Earth without plastic bags would be a better place.

I'm not asking, why the word 'earth' is taking the indefinite article, I'm asking why the proper noun is taking indefinite article here in all.

Answer 1

These are all cases where the idea of having more (or less) than one Earth is considered. The other ones are of course hypothetical, but the indefinite article is used to indicate that there are other such hypothetical earths.

So, intuitively, you can consider this as 'an element from the set of hypothetical Earth-like planets' instead of the strict sense of a proper noun (although, of course, it still is a proper noun, but used as if it were a normal noun).

The same story could of course go for any other proper noun for which hypothetical alternatives could be envisioned. For example, one could write about 'a United States that never became independent of the British Empire'.

Answer 2

The genera rule is that we don't use an article with a proper noun.

But occasionally, a proper noun is used as a common noun. That is, we take the name of one specific thing, and use it to refer to a class of things, usually to say "other things of this sort".

That's exactly what the writer is doing in this case. He is using the word "Earth" to refer, not to our particular planet that is named Earth, but rather to a class of planets like the Earth. A more straightforward example would be something like, "We hope to find evidence of another habitable planet elsewhere in the galaxy, an Earth circling another star."

Another example would be if you were discussing names a names rather than as references to specific people. Like, "I work with Robert." I'm using Rober to identify the person, its a proper name. But, "You can see our diverse national background just by looking at the names of the people who work here: We have a Fred, a Venkata, a Jose, and a Ivanovna." Even though these names are normally proper nouns, here I'm using them as common nouns.

BTW When we say, "I dug a hole in the earth behind the shed", we are using an entirely different definition of "earth" that is not a proper noun at all. It means "dirt". The fact that this takes an article while "Earth" the planet does not is not very interesting. Just like "Bob" the name does not take an article but "bob" the verb meaning to bounce up and down isn't a noun so it's a moot point.