[PaulHoule]

If your definition of habitable is "has liquid water" probably most of the habitable places in the universe are like Ceres, the moons of Saturn, even deep inside Pluto. Between the effects of pressure, geothermal energy, and gravitational tides you find liquid water in those places more than the inner solar system where water gets evaporated and blown away.

(The trouble is that those places don't have sunlight and probably fewer energy sources overall.)

It could be the answer to why we don't meet aliens: they couldn't care less about dry places like the Earth. You could travel the stars hopping comet to comet, but after spending 10,000 years like that you might be comfortable enough in your lifestyle that you wouldn't want to stop at a star. (All contingent on solving the energy problem, either you get lucky and find a lot of uranium or you invent D-D fusion.)

[Symmetry]

My pet theory is that life is pretty common in the universe, it happened on Earth almost as soon as the oceans formed. But photosynthesis, mitochondria, and multi-celularity each took a lot longer and probably represent the biggest barriers to intelligent life.

Plus, as you say, most liquid water is in the deep dark under ice sheets far from the nearest star which makes photosynthesis difficult.

[Chris2048]

> it happened on Earth almost as soon as the oceans formed

But that might have been the final item in a long list of requirements. Maybe life form quickly (on a planetary-scale of quickness) once they are met, but who knows how long that list is.

TBH, I think people fixate on "life" because of some star-trekkian fantasy about aliens, and specifically, other space-faring aliens. There are lots of interesting life-forms on earth that are ignored. We have a lot yet to learn about chemistry, let alone the biochemistry of bacteria, algae, mould. Dolphins and octopi have yet to build boats, let alone spaceships (arguably the monkeys are ahead on this game).

TBH, I'm more interested in what kind of interesting materials and phenomena might exist on other planets. As for life, I think its more interesting to think about what kind of alien environments we can seed with life (and what would form) rather than the moon-shot of actually finding life we are exited about.

Also keep in mind: life or not, it might be more likely we find something else interesting or dangerous. e.g. some kind of adaptable self-replicating enzyme, but made of something more durable than protein (and reproducing in the environmental). Consider the rubber-eating contaminant in https://en.wikipedia.org/wiki/The_Andromeda_Strain

[gus_massa]

You are not so far from the current models. Take a look at https://en.wikipedia.org/wiki/Timeline_of_the_evolutionary_h...

(I think multicelularity of eucaryotes is easier in comparison with the other two steps, it has evolved like 20 times. But in the timeline, it took a long time...)

[ncmncm]

Rocky planets are interesting only to extreme primitives. Once you leave the nest, Oort cloud objects are the most useful. That is where to look for visitors.

[082349872349872]

"But Moooooom, I hate having a few seconds lag to all my friends."

"If all your friends descended to the bottom of a gravity well, you would too? After all, we're not savages."