To be "in heaven", uses estár ... but is it not permanent?
I see that to say "they are in heaven", the verb estar is used instead of Ser. And I understand that you could argue it is a location -- I guess?Who really knows? But because it is more of a permanent condition, it would seem Ser should be used. But I just want to check if estar is correct - regardless of what seems to make sense here. So, Ellos están en el cielo.
is correct?
3 Answers
Yes, estar is correct there, as you say - because you're speaking about someone being in a location.
Just a word of warning, although even many text books will encourage you to think of estar as being for temporary things and ser for permanent things this really is not the best explanation.
For instance in Spanish you say 'está muerto' (he's dead / fairly permanente) but 'soy estudiante' (I'm a student) quite likely to change.

Perhaps this lesson will be useful
Ser vs Estar
Another example of how the "permanent vs. temporary" rule doesn't work--
Neuva York está en los Estados Unidos (location)
However, remember that events take place in a location, but we use "ser," e.g., La fiesta es en el parque.
What Kiwi told you is correct. Whenenver you are confused by ser/estar, and let's use your phrase as an example, carefully analyse it and it could, most of the times, give you the answer:
Están en el cielo = They're in heaven, the bolded preposition makes it clear they're talking about location, and when you need to say where you are located, you used estar. If you used ser, then you'd be saying that they are heaven