What Does A Habitable Planet Look Like?

Answer: All kinds of things.

Right now, we have a sample of one (and this, of course, is restricting "habitable" to "humans can live on it").

We know what Earth looks like - a beautifully chaotic system, burgeoning with life in all of its corners, with firm, distinct seasons and a short day-night cycle.

But what else could habitable worlds look like?

1. Worlds orbiting an M-Dwarf sun could be habitable. However, due to the narrow habitable zone of those stars, most would be tidally locked. That doesn't mean the classic trope of one side of the planet burns while the other freezes - as long as you have liquid water and an atmosphere there would be enough circulation to even things out. Such a world would also have an almost circular orbit (if they're knocked into a more elliptical orbit they'll end up like Venus - not tidally locked but with a very long day) - and a circular orbit means no seasons. So, how do you measure time on such a world? Maybe by distance or by the amount of stuff you get done.

2. Super-terrestrial worlds could easily be habitable. Worlds 3-5 times the size of Earth - but you might end up needing to use mechanical augmentation to handle higher gravity. What difference to society and technology would a larger world make?

3. Worlds in binary or trinary systems. It used to be believed that such systems rarely hosted planets - and if they did they wouldn't be in a stable enough orbit to be habitable. Math and observation have, though, proved that planets can indeed rest in stable orbits in systems with multiple stars. Many stories show how having more than one sun might affect the mythology and cosmology of people on the planet. And if you have never read Isaac Asimov's classic Nightfall - widely considered one of the best science fiction stories ever written - (I mean the original, although the novel with Silverberg isn't bad either).

4. Worlds with very short orbital periods. Helliconia speaks of a world with an extremely long orbital period, but such are unlikely to be habitable. But one of our lead candidates for a habitable exoplanet is Gliese 581g - and its year is 37 days. Such a short year would probably not have intense seasons, but even a small amount of variation on such a small timeframe could make such a world interesting indeed.

So far, most of our candidate worlds are super-terrestrials - but that's an accident of sampling. Smaller worlds will almost certainly be found - Kepler-186f is the first planet close to Earth's size to be found in the habitable zone.

So, by all means, vary your habitable worlds. This list is probably too short - who knows what might be out there.