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.
Best Answer
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'.