Yeah, I had that in my comment originally (we have people living in Antarctica, but not too many volcano explorers) but it was getting a bit wordy and didn’t seem to fit terribly well because, I believe, “too cold” takes that into consideration already. It would be planets without any liquid water at all because it’s all completely frozen (at those temps it could be a real struggle to produce enough heat to prevent equipment seizing, depending on the heat source, and it may cause batteries and such to freeze up). But NASA believes even snowball planets completely covered in ice shells which are outside the habitable zone may have liquid oceans beneath the surface ice, as long as they have internal heating, like Europa and Enceladus do. And we can probably work with that the same way we do geothermal here.

When I was writing that comment I went looking for stats on rocky planets considered too cold, and there isn’t much. A few papers about land masses/liquid oceans/geysers on snowball planets, one paper about what makes a planet uninhabitably cold (spoiler, it’s a lack of atmospheric co2, to the surprise of absolutely no-one), and a ton of articles that have nothing to do with too-cold exoplanets, just the habitable zone generally. It’s sort of a weird gap, but I think it’s because snowballs tend not to be in the habitable zone (and were likely booted out of it, so less common) so we aren’t really looking for them for habitability, we can handle cold better, and we know how to warm a planet… :(