Does having an attack on a planet decrease invasion chances?

Yes.

It does? I always thought it was based solely from it's defense and invasion defense scores.

Nope, I asked Dan a couple weeks ago, and you can mouse-over "Attack" on a planet to see for yourself, if you wish.

EDIT: The mouse-over is on the "Colony" tab on a planet.

Nice, had not noticed that before. Sneaky mouse-overs... Think they're all scheming for better cheese or something. What would be nice is to know the invasion chance % before going in. Even if it was via an artifact.

Yeah, that is a pretty good idea. I've seen a few people post the chance they had, and their stats and the planet's stats, I'll try to find them, that would give some idea of what to expect at least. I'll post when I've found them.

Actually, finding them would take a while, I don't really remember. I think the one I saw was something like the invasion defense was 36% of the attack and there was a 64% of successful invasion, so I thought it was 1 - invasion defense/ship attack, but at lower ratios this didn't pan out, because this would mean you couldn't invade if the invasion defense is higher that your attack, but that was false.

Invasion % chance is invaders attack vs planets att/def total