Will they know its me?
Can I hide it if I flux it?
Its a nice massive crystal.
What should I expect in retaliation?
Thanks

They will know it's you, it shows up on their logs.
You can hide the planet but not you. Fluxing is a very good idea if it's a really nice planet.
In retaliation you should expect a zeroing and/or an alert. Some can take it to comm though.
Basically never avoid a planet because of retaliation. It lasts 2-3 days at most, and it doesn't really affect you in any way other than a few credits for repair, and a few minor resource points.

Will they know its me?

Yes. If you have to use any artifacts or are successful, they will get a message saying, "players name" used an artifact against "planet name" or "planet" was invaded by "players name"

Can I hide it if I flux it?

If you use a planet flux, and the person you take the planet from isn't online, then yes, they will no longer be able to see the planet. If they are online when you attack/invade, if they are able to alert the planet before you can flux it, then they will be able to see the planet for 12 more hours, regardless of whether or not you flux it.

Its a nice massive crystal.

How nice for you.

What should I expect in retaliation?

You should expect to have your actions at 0 / 0 / 0 for the next 2 days, as thats how long you'll stay on their ships log after performing an offensive action. Depending on the player and whether they have access to any of your planets, you may also expect to lose some of your own planets as retribution, although that doesn't happen too often these days.

As Epic stated, never be afraid to take a planet from anyone, if you do allow yourself to be intimidated then you'll not get anywhere, as most of the really great planets are owned by those who will kick your ship silly if they lose a planet.

I can't believe it. I get all ready to invade that planet and someone else took it !!

Then invade it from the new owner lol. Unless someone within your legion took it...

also another tip if you do take a planet alert it to your legion so that your legion players can guard it for awhile adding more attack defense, and cloak(doesn't much that much since they have it on there scans)

if a new owner took it wait for them to let there guard down from retaliation from the first owner

since im assuming a VERY low ranking player is asking, I really doubt that this person has the ability to defend a planet properly from 2 other players who have it on scan..

Legions and planet fluxes are all deciding factors. Besides he may be in a high research legion.