Astronomers have discovered 85 possible planets outside our solar system with temperatures potentially cool enough to sustain life.
These exoplanet candidates are similar in size to Jupiter, Saturn and Neptune, and were discovered using data from Nasa’s Transitioning Exoplanet Survey Satellite (TESS).
TESS enables scientists to observe dips in the brightness of stars, known as transits, caused by objects passing in front of them.
Typically, at least three transits need to be seen to discover an exoplanet in this way, in order to determine how long they take to orbit their star.
I think the idea here with not mentioning too cold is that rocky planets (at least those we can find) are usually relatively close to the stars they orbit (unless there was a realignment event in the solar system that knocks one to a larger orbit), and can have a lot of strong tidal forces and radiation that heat up the planet.
Within the range most rocky planets fall, too hot is much more common than too cold, and models and data indicate that even snowball exoplanets can have some far-from-ocean land regions that host liquid water, extending the range even further.
Just look at our own solar system for an idea of the possible spread, assuming we are relatively average (which so far seems to be the case). We have 2 uninhabitable hot planets, two not too hot and not too cold (mars may not have atmosphere, but there’s evidence of liquid water, maybe even somewhat recently, putting it in the “not too cold” category.) and none that are too cold (moons yes, rocky planets no). Where we would commonly get “too cold”, on the outer edge of the habitable zone for that star type, we usually see gas giants, just due to how solar systems form.
(When solar systems are forming, there is a cloud of dust and gas that starts to rotate, and collapse toward the center. The disk of material outside of the center will mostly become planets, and the rotational energy eventually turns into the orbital momentum. When the star finally amasses enough material to ignite, a wave of radiation goes out in all directions, pushing the gas, but not as much of the dust and rock, outward, to right around the edge of the likely habitable zone. Planetoids out where the gas wasn’t blown away form gas giants, those within the wave form rocky planets.)
I think it’s also fair to say that “too cold” is generally more livable than “too hot”. It’s quite a bit easier to generate and conserve heat than to ward it off, and even a planet that is so cold that its atmosphere has precipitated into snow could theoretically be survivable with habitat domes or the like, much like a proposed moon base. “Too hot,” on the other hand, can potentially be hot enough to melt basically anything we send there, which is why there’s a lot more focus on colonizing Mars right now than Venus.
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… :(