Planets can be toxic, radioactive, aquatic, barren, or heavy gravity and still be colonizable - assuming you are playing Dark Avatar or Twilight of the Arnor. They will still show up with planet class greater than zero, but you cannot colonize them without researching tech first. All five types of planets have two levels of colonization tech, basic and advanced. Having both techs for a particular type of planet will give you production values the same as a standard planet. Having one of the two techs will give you 50% production on that type of planet. Having neither tech means you have 0% of the usual production on that planet - this is the situation you are in, the little production you have is coming from either asteroids or focus.
Normally, if you try to land a colony ship on them, you get a popup telling you that you cannot colonize this world yet. If you take the planet via any other means, you do not get any sort of popup, you simply see a planet with 0% production.
Note that the lack of production is not an absolute hinderance. Research is unaffected, and if you have high enough research capacity on the planet you can get a pretty good amount of production from focus. You can also still rush-buy structures on the planet, which means you don't even have to build those labs one point at a time. Another alternative is simply making it an economy planet, as population and economy buildings are unaffected as well.
Regarding tech adaptation, there is no such thing in Dark Avatar. In Twilight of the Arnor, any building you lack the technology to build yourself is destroyed when you take the planet (one per galaxy buildings excepted). The important thing to remember there is that you should always buy tech (specifically techs that unlock structures) before invading, flipping, or buying an empire's planets.
