Tidal locking isn't always 1:1. Mercury, the lone tidally locked planet in our Solar System, is locked at a 3:2 resonance. For simple thermodynamic reasons one would expect that in most cases the planets locked at an "offset" ratio are far more likely to be habitable than the ones locked at 1:1. This happens when there are outlying large planets that disrupt the orbit:

http://en.wikipedia.org/wiki/Mercury_%28planet%29#Spin.E2.80...

I though Mercury was 3:2 solely because of its elliptical orbit? The fast rotation at the closest orbit continues during the long eccentric part, where it has time to make an extra half turn. Just geometry.

> For simple thermodynamic reasons

Thermodynamic?

Someone already calculated in 1950 that such planet is very unlikely to have an atmosphere. Temperature difference is about 300 degrees celsius and most gasses would either froze on cold side, or escape to space on warm side.

In best cases you get permanent hurricane (Venus)

I have seen more recent depictions of tidally locked planets that have thick enough atmospheres that winds suffice to do enough cooling & heating to sustain the system.

Aren't many of those planets gas giants though? Which won't be able to harbor life.

Wouldn't there be a layer somewhere deep where water is liquid? It'd be boiling up in clouds to eventually freeze (which I guess would be a looong way up) or sink and turn to steam? Freeze?

Right, that idea that water would flow constantly across the terminator is nuts. The water would blow to the cold side and freeze there.

Also, give all that was said, probably no life there at all. Our Moon may have been instrumental in generating life. Tides and rotation meant constant motion of water on shores, combined with day/night heat/cold wet/dry. Every grain of sand on every beach was a different biochemical petri dish for billions of years, resulting ultimately in the cell.

A tide-locked no-day-night planet would likely be sterile.

> The water would blow to the cold side and freeze there.

You missed the part where the glaciers on the night side get big enough to flow despite being frozen.

Depends on how much water.

I think there's a lot more room for speculation.

We haven't been able to observe nearly enough tidally-locked planets to make general rules about them.

Such new types of planets are likely to exhibit all sorts of phenomena that act as counterexamples to the patterns seen in our own solar system.

> Every grain of sand on every beach was a different biochemical petri dish for billions of years

It didn't take billions of years for life to appear on Earth.

Right; it took 1 billion (for cells)

Planet Earth was formed 4.6 billion years ago. And while the interpretation of the earliest fossils of life is debatable, we can say that some 3.6 billion years old remains probably are fossils of bacteria.

But right after the formation, the surface was still molten lava, and there was heavy meteorite bombardment [1]. So from the time the planet had cooled down, and the solar system had calmed down, to the beginning of life (which could have happened earlier than there are fossils left) could have been anything from mere 100 million years to over 600 million years, for all we know.

[1] http://en.wikipedia.org/wiki/Hadean

I was at an astrobiology conference this week, and one of the ideas that we discussed is that life on Earth is good at finding and exploiting gradients for energy harvesting---thermal, chemical, even electrical. I'm not sure how to weight the whole "the atmosphere will freeze out" argument, but it does seem like having a constant, predictable thermal gradient, coupled with lower average high energy radiation (not much UV at sunset), should be conducive to life, all else being equal.

Conducive to existing life adapting. But what about life forming in the first place?

"Tidally locked"

There, was that so hard? Extraordinary effort here to coin a neologism for something that's already an established phrase.

This is also a blogpost promoting a paper about cloud formation in tidally locked planets, which it appears someone forgot to link, inside the phrase "Gory details here".

There's also a thousand variations on the theme of tidally locked planets in science fiction: from Larry Niven's (of Ringworld fame) world Jinx which was tidally locked to a gas giant, right back to Stapledon's Star Maker from 1937, which features a habitable planet tidally locked around a red dwarf.

The whole time I was reading this was "selection bias".

> The easiest planets to find are those that orbit close to their stars.

> Tides drive the planet’s obliquity to zero, meaning that the planet’s equator is perfectly aligned with its orbit. The planet will also be “tidally locked”

Seems likely that the first planets we'll find life on are the planets that are easy to find.

Yeah. That's the premise of the article. Given the kind of planet that's easy to find, what sort of conditions might support life?

I'm surprised that tidal locking happen for a planet with liquids on the surface, except in a jury-rigged toy scenario. I'd expect that the energy would cause fluid/molden volumes to circulate. This would create motion relative to the energy source.

It might be possible to use this same kind of circulation effect to have a self-contained space-craft. It would consume solar energy, and then strategically pump volumes of fluid from one end to the other, to deliberately change its momentum.

Pumping fluid wouldn't give long term motion to the planet (can't travel further than it's own length). That would violate momentum conservation no matter how it's powered. You need to give it momentum to make it move, and then you just get a solar sail.

I don't know enough about tides and atmospheres to say whether or not this sort of arrangement would be more conducive towards generating life.

However, there's plenty of evidence that our rotating Earth gives a lot of benefits. There's the jet stream, which helps to spread moisture, there's the tides themselves, which helped create thriving transition zones between aquatic and terrestrial life (tide pools, etc.)

I'm also thinking of Jared Diamond's Guns Germs and Steel. In our own case of planet Earth, we had pretty big geographic obstacles that both enabled and prevented the spread of certain forms of life (Deer can easily migrate within the larger temperate zones, enabling their species to spread easily. Viruses can't easily cross oceans, making it unlikely that they wipe out an entire species.)

Aside from the obvious problems with keeping an atmosphere, can tidally locked planets have a magnetosphere?

Sure, that comes from a molten core IIRC.

A spinning liquid metallic outer core, with a certain composition, but yes (we think.) Even Earth's magnetosphere isn't fully understood.

I wonder if the earth would be tidally locked without having bumped into the moon at some point.

I imagine Game of Thrones is set on such a planet :)

Surely there are day and night scenes in the same place?

True. So a little tilt then, enough so the sun sets just below the horizon - a day and night in the habitable zone. But when "winter comes" the tilt gets out of whack for a while.

A couple problems - when there's even a tiny wobble the goldilocks zone will alternate from extreme freezing to extreme heat. The other problem is that the atmosphere will eventually fully condense on the cold side of the planet.

Further, early cells depended on an ocean environment (algae) to protect their fragile forms. No ocean on these weird planets.

What about the icy eyeballs, where the substellar point is the center of a large ocean?

There, no land, no beach. Just deep water.

Even then life will be on the "Earth like" part of the planet.