Master of Orion/Solving problems

Can't colonize Radiated planets:

Often you won't be able to research Controlled Radiated Environment on your own behalf. (If Improved Terraforming +40 is among your options, it's time to stop hoping for it to come up.) Sometimes you will also be missing Controlled Toxic Environment and even Controlled Inferno Environment. How annoying this is will depend on how many planets of these types are in range, how rich they are, whether they would make a good stepping stone to other planets, and whether other races can colonize them.

Not only can't you colonize these planets. You also can't invade them. You can bomb them out of existence, which is a good way to deny the benefits to your enemy, but other races tend to object to this behaviour, and they often get recolonized anyway.

If no one can colonize them then they might as well be No Planet.

If you find that another race has this tech, it is often worth a great deal of effort to take it from them. If you're on friendly terms with them, and they have another tech higher in the Planetology tree, you may be able to get them to trade it to you. In more hostile situations, crank your spying up to max, and/or invade systems where they have plenty of factories.

Sakkra and Psilon opponents are the most likely to research Controlled Radiated Environment. Consider not picking on them until they've had a chance to research that level.

That's the straightforward approach. There are also two sneakier approaches. One is to gift "Atmospheric Terraforming" to the race who is colonizing the Radiated planets; once they have started applying this to those planets, they'll switch to Minimal (Hostile), which is enough for you to route soldiers to the planet. (Silicoids cannot use this tech.)

The other is to research Hyperspace Communications. This is a very high level Computer Tech, but it opens up a new option. Get a team of colonists/soldiers into space by routing them to some planet more than one turn away. (Something 'behind' the target planet is often a good option.) Once they're in space, you can re-route them to the Radiated planet!