This was indeed the prevailing wisdom for some time. However, more recent research is somewhat more optimistic. Even an atmosphere 10% as dense as that of earth can circulate heat to the dark side efficiently enough, given the right composition. The night side temperature could be within the parameters required to sustain some form of life.

Furthermore, tidal locking is dependent on oceans and atmosphere, and can take many billions of years to occur, or even never. Mercury has plenty of time to tidally lock but is in a 3:2 resonance with the sun. There are even other options; for example, moons of gas giants within the habitable zone would be tidally locked to their primary and thus have a day-night cycle.

Or a day-night-day-penumbra-umbra-penumbra cycle. The primary-facing side would get maximum light just before entering the penumbra, experience total darkness in the umbra, and get some light directly from the star and some reflected from the primary from the time it exits the umbra to the time it re-enters. Every day would have a full stellar eclipse at noon. Every night would see the primary cycle from waxing half through full to waning half. The space-facing side would have an ordinary day-night cycle with the same period as its orbital period around its primary.

"However, more recent research is somewhat more optimistic."

I am deeply skeptical of that "recent research", which AFAIK is "computer models, where we have zero capability to verify the computer models against even a single data point".

The incentives to keep kicking the computer model until it provides something publishable, with no countervailing force provided by real data to be explained by the model, are just too strong for me to take that too seriously.

Every new simulation is publishable. And every simulation that is more detailed than what came before will receive some attention after being published.

If physics has some bias they polish their models towards, it's on the direction of repeating the findings of the previous ones, not of changing them.

Physics models are often checked against other models (which might have had some confirmation) in simple cases, and there are a lot of other sanity checks you can make on them to ensure they are providing reasonable results.

Obviously, one shouldn't blindly accept them without experimental confirmation, but one shouldn't also be wildly pessimistic in believing calculations grounded in three centuries worth of validated equations.

Yes, I'm sure the simulations don't have results like making these atmospheres suddenly collapse into black holes due to errors in the simulation or something. They'd notice.

But these atmospheres have to stable across cosmological scales in a highly iterative, chaotic environment. There is a mathematical lower bound on the rate such simulations must leak bits as a result of those things. I don't think we have enough bits of real information to make up for that, as it would take rather a lot.

Against the mathematical argument, I set the human argument that, like I said, there's all sorts of incentives to report the one simulation that produced the result that there may be an atmosphere and maybe it could even sustain life! That's a lot of high-profile articles and probably a promotion for getting my department some public attention. Even if hundreds of other simulations all result in "Yeah, the atmosphere freezes out".

Between the math saying we can't really expect this sort of simulation to contain meaningful information and the human factors involved, I can't put a lot of confidence in a model which can't be validated against real data.

I do find it amazing how people who probably think they're really in favor of science and support it will jump up to defend a methodology that literally runs off of zero data. Can someone explain the scientific process to me clearly that involves having no data at any point?

well even our deep space observations are largely computed. nobody has "seen" these 2 planets at all ;)

what does seeing mean? does it have to involve photons? can gravity work as well? sensors and computation are always involved, nothing is directly seen.

How did we verify the earlier theories?

We check them against data.

I have no objection to computer models; I object to computer models that can't be checked against data.

Cosmologists run a lot of models, to do things like try to determine the effects of dark matter on the universe. Such models are intrinsically problematic since they have to have resolution on the order of hundreds or thousands of lightyears on a side, or have to work with universes much smaller than the real one, or something that means the simulation is by necessity literally several dozen orders of magnitude smaller than the real universe (consider both the timestep and spatial dimensions).

But they can check the result by looking up in the sky and seeing if it matches. This allows them to overcome the problematic nature of the models and use them to say real things.

One wonders what exciting cosmological papers proposing all sorts of amazing and outlandish theories as a result of some computer model of the universe have been suppressed by the fact that they didn't correspond to the sky at all. I guarantee you that some very amazing simulations have been run that produced incredible results of great interest, whose only crime was that they completely failed to match the universe.

We don't have a tidally locked planet that has an active atmosphere in our solar system to look at.

I always visualized that the tidal locking to be sort of like meshed gears, such that the one revolution around itself is same as a rotation around the star/planet if its a satellite.

But that shouldn't affect the day/night in this case right? Since we are talking about planets not satellites of planets and they are light sources not just reflecting light from the star. Can you please correct where I am wrong?

Tidal locking means that the denser side of the lighter body always faces the heavier body, and the less-dense side of the lighter body already faces away from the heavier one.

The moon is tidally locked to the Earth, which is why it has a dark side.

There is no dark side in the moon.

There is a far side.

You're right, but I'm pretty sure that "The dark side of the Moon" is a proper name at this point.

It's dark like the darknet is dark... Occluded.

So the Apollo "earthrise" photos are just "earth"?

No; they weren't taken from the moon; they were taken from the Apollo 8 spacecraft, which was orbiting, so the Earth did in fact 'rise'.

Yes, Earth does not move significantly on the moon sky, if i remember right. It does rotate though.

Edit: Archgoon is right, the photos were taken from the orbiting module, so a rise happened.

It follows from "one revolution around itself is same as a rotation around the star" that a planet tidally locked to its star presents the same face to the star at all times, so you have a 'Day' side and a 'Night' side. Neither side of which is likely to be particularly life friendly. Perhaps there might be some 'twilight zone' that could be though...

They could have a resonance orbit that would enable a (slow) day-night cycle. But some theories also suggest that M-class stars tend to flare for the regular radiation diet. Otherwise it would be awesome to have an M-class sun, since they can have lifespans multiple times longer than the current age of the universe.

This was my first thought, too, when I saw the article.

But our thoughts about the effects of tidal lock on the emergence of life is little more than philosophy, having only one datapoint to extrapolate from, at that one not being tidal locked.

I think there are other loopholes. Imagine a Jupiter sized planet that had an Earth like moon. The Jupiter size planet would probably be tidally locked with the star, but the moon could be tidally locked with the planet, not the star. Given that, it would have quite a varied and complicated day/night cycle.

Interresting setup you mention there, the earth-like planets d/n cycle would indeed be very irregular i imagine, depending which orbital speed it would have.

I'm sorry, I'm not at all educated in this subject and I might be misunderstanding what you are saying, but isn't Earth the perfect example to argue against this? We're a life-friendly planet that isn't tidally locked?

It has to do with the class of star. This star is so cool that the only way for the planet to be warm enough to support life is for the planet to be really close to the star. I don't really understand tidal locking but I think it happens vastly faster at closer orbits - because there is a stronger gravitational gradiant closer to the parent body.

Tidal lock is caused by differential gravitational attraction. The half of the Earth currently facing the sun experiences more force pulling it towards the sun then the part facing away - which manifests as the spin of the planet having to do work against gravity to keep the spin going.

As a planet gets closer and closer to it's star, this gets more and more pronounced and all the various deformations and instabilities begin to favor braking the planet rather then it continuing to spin.

I think this is just for red dwarf stars, not ones like sol.

Atmospheres are useful, but are they a requirement for calling a planet life-friendly? Life on earth first developed in liquid oceans, not in the atmosphere.

Wouldn't an ocean vaporise somewhat automatically to create an atmosphere of there wasn't one already?

I think so. I don't know of many liquids whose surface tension is strong enough to resist a vacuum. Also, I've never heard anyone propose mercury (the metal) as a particularly good substrate for life.

Well, there's the possibility of liquid water under icy moons.

Only if it's exposed to vacuum (or an atmosphere). It might be enclosed by ice or other (preferably but not nessesarily transparent) material.

Even if the ocean is exposed, we now have the time it takes for 1. the planet to become tidally locked due to tidal forces, 2. the atmosphere to be stripped away, 3. the ocean to evaporate. That might be plenty of time for higher lifeforms to develop.

Such planets might also have moons.

Not ones with 1.1g!

Why not?

Because then it would be a double planet.

It depends on how big the "main" planet is, I guess. But I think we are running into definitional issues here, and it gets technical very fast. First, IAU does not recognize the term "double planet". Second, a body which is large enough to reach hydrostatic equilibrium (i.e. a sphere) but has not yet cleared its orbit is technically not a planet. So I don't know where this leaves us. A Jupiter-sized planet with an Earth-sized satellite would be such a huge mass and volume difference (an even bigger difference than the Earth and Moon) that it seems to odd to term that a "double planet".

TFA said the planets were rocky and 1.1g.