You can still invade the planet, but you will be unable to have any social or military production on it (until you research the base extreme tech which will then allow you to get 50% social/military, advanced extreme tech enables 100%).  Research still works fine though as does population growth.  Its actually not a bad way to get economic worlds- since the maintenance is relatively low, you just let them build up and pay you in tax money. win-win

Not to mention that invading such planets gives a reasonable chance of stealing those techs.

Beg pardon, but I just gotta say that this one still seems like a bug to me. I never do it because it just *feels* wrong.

If your troops don't have the tech to do even 50% physical work on the world, just how are they supposed to be able to kill all those defenders? Seems like at the minimum, invading a world without the right enviro tech should give you a very large combat penalty.

There are methods, but these ideas are so innovative and good that the game couldn't be coded to convey their :awesome:ness.

For the first, you need to find some rocks.  There are usually a few asteroids around, or you can grab a few old satillites in a pinch.  Then, you take those rocks, and you- this is the important bit -drop them.  From orbit.  Right on their noggin-equivalents.

If you're really evil, you can use the Spore ship alternate tactic- have all your troop transports stop by the closest starbase to your target that has a Taco Bell, load up, wait about an hour, and Hyperspace towards the target planet.  The tricky part is getting everyone to flush just as the ships pull up.