It's one of those questions that is easy to ask, but hard to get a good answer.
I guess there are some questions that need to be answered first.
What is a plane?
How can we know that we are on it?
How does one perceive it?
Is there something besides mind that can perceive it?
Which means, by inference that there is something measurable.
If it is measurable what do you measure it with?
If it isn't measurable, then why?
Where did the astral plane get the common associations with the emotions that it has?
And if that isn't enough, what questions should be here that aren't?