How do I get the research to develop inhospitable planets?

If you're playing Dark Avatar, it's down the Xeno Biology tree. I think (I'm trying to remember off the top of my head) it's Xeno Biology, some general colonization tech, and then the different types of inhospitable planets you encounter, so you can pick your poison.

In TA, I have no idea. Haven't played it yet - waiting for the release.

in TA, as in DA, it is still under Xeno Biology--->Extreme Colonization--->Planet Extreme Type 1---->Planet Extreme Type II

Note that they are very expensive technologies. Most newer players to the game, or making the exchange from Dread Lords to Dark Avatar/Twilight make the mistake of thinking they need to rush to these techs right off the bat to beat the AI to them. Not so. In general, even at the highest difficulty levels, you usually have at least a year before the AI will start going after extreme worlds (the AI likes to capture all the "normal" worlds first) so you can plan your own colonization efforts based on their actions.

If you see the available supply of normal worlds dwindling, its a good idea to start on the extreme techs and try to grab them before the AI does. Just grab the 50% (Tech I) to colonize, you can come back for the Full (Tech II) later.

::EDIT:: My first post I mixed uninhabitable with inhospitable...

See above They both have excellent advice.