A mysterious thing I am

So you say now I am not what you used to say I was? You’ve kicked me out of the group you used to say I was a member of? Well, without me the other members of the group are eaten, ungrammatically. But say what you wish; it makes no difference to me. I was here long before you ever existed, and I’ll probably be here long after you’re gone. To date I have been mostly unreachable for you, though lately you have been doing some probing into my business. Really, it’s been less than half a year that you’ve even known of my existence, though you would say it’s been 90.

I don’t mean to sound bitter about any of this; I’m really very ambivalent. If I sound cold, well, that’s because I am. Very.

We are distant and different, you know. What is bright and blinding for you is only a glimmer for me. And yet in a way, I am in your element. You named me after one thing and then named another thing after me; the second thing being the first in reverse.

Knowing me well, as I have said, would be difficult for you. And maybe you don’t want to dive deep into learning about me right now. But you can at least learn my name by solving this riddle; taking a partial plunge, to which you must add.

I am __________