Today’s NASA announcement, of multiple tidally-locked Earth-sized planets found in the habitable zone of a small nearby star, made me think of the possibility of life on such planets. Without an atmosphere, it’s explained that a tidally locked planet would be scorched on one side and frozen on the other. With an atmosphere, there might be enough mixing for there to be some temperate zones in the dawn/dusk zones on the edges.
That makes sense, but couldn’t those dawn/dusk zones be the right temperature with or without an atmosphere, and even if the planet is inside the star’s habitable zone? If it’s scorching hot on one side and frozen on the other, won’t part of the planet’s surface where those sides meet be “just right”, at least some of the time, no matter where the planet is in relation to the star?
If so, this makes me wonder if tidally locked planets might be more likely to harbor life, and the habitable zone would actually be much, much larger, than non-tidally-locked planets.
If there is no atmosphere, if something is in permanent shadow, it gets pretty darn cold. There are craters on the moon colder than Pluto, and even ice in craters on Mercury.
Right – but what about the dawn/dusk zone, where the day side meets the night? Somewhere the temperature has got to be in between scorching and freezing.
Another problem is that the atmosphere and water might escape to space on the hot day side and freeze/condense out on the cold night side. This might be prevented by atmospheric heat transfer if the initial atmosphere is thick enough (I’m not sure how thick is “thick enough”).
Another possibility might be a planet a bit further out, so that the day side is generally at the right temperature – that way, the “escape into space” issue doesn’t come up any more than it would for a planet with a day/night cycle, and the only remaining show-stopper is the “everything freezes out on the dark side” issue.
I’m not sure what you mean by “dawn/dusk zone”. Any point on the planet is either in permanent sunlight or permanent shadow (assuming no libration). If it’s in sunlight, it’s hot. If it’s not in sunlight, it’s cold.
A tidally locked planet much inside the habitable zone won’t be able to keep an atmosphere. Half will freeze out on the dark side and half will evaporate into space on the hot side.
This also neglects how the planet came to be tidally locked. Planets start with some rotation and become locked only very slowly over time. The star-relative rotation rate asymptotically approaches zero.
So there might be a period of many thousand Earth-years where the planet rotates at just one rotation per thousand Earth-years. Followed by another period of many thousand Earth-years where the planet rotates at one rotation per *ten *thousand Earth-years. etc.
Whatever ecosystem may have existed at higher spin rates it’ll be pretty thoroughly disrupted on the way to full tidal locking.
@markn+: The star probably isn’t a point source at that distance. So you’ll have a very narrow band of longitudes that are twilight. If there’s any libration due to other planets’ perturbations that’ll also smear the twilight out a bit.
That could make an interesting story. A nearly tidally locked planet with a several (earth equivalent) year rotation that requires every organism to migrate around the planet as it rotates. Fall behind and freeze/burn to death. :eek:
You could even have to completely different ecosystems on opposite sides of the planet. One chasing the day, the other chasing the night.
The important factor in this respect is the angle of the sun to the ground surface. If the sun is at a low angle, each watt of power coming from the sun is spread out over a larger area when it hits the ground.
But there are many other factors to consider - what is the atmospheric composition? Is the atmosphere thick enough to transfer heat to the dark-side?
And don’t forget the sidereal rotation period. It is very easy to forget that a tidally-locked world actually does rotate- the planets around Trappist-1, for instance, rotate on a timescale of a few days. (1.5 to 20 days, to be more precise). This fairly rapid rotation would cause some Coriolis effects and mixing even on a tidally locked world).
Even if there is no atmosphere the angle of the sun dictates the temperature of the ground. At an solar elevation of 30 degrees the ground receives only half the energy it would get from a sun directly overhead. In the ‘twilight zone’ on a tidally-locked planet this would be much less.
One benefit to this is that tidal locking will never be complete, as no orbit is ever completely circular. I believe that’s Libration. So on a planet tidally locked to its sun, you’d still see the sun “wobble” back and forth in the sky over the course of the year, and near the terminator this would actually lead to some very slow sunrises and sunset.
Fun fact: You know how an ellipse has two foci, and the Sun is at one, and the other one is just empty? Well, to first order at least, a tidally-locked planet doesn’t actually always face the Sun… It instead always faces that empty focus.
This would happen fairly early in the planetary system’s history. At that point, it’s more “primordial chemical soup” than “ecosystem”. If the planet stabilizes with an atmosphere and a reasonably large supply of liquid water (which, as described above, I think is more likely if it’s at the outer fringe of the usual “habitable zone”, so that the dayside is moderately warm), biogenesis and evolution can proceed from there.
Tres cool. Neat idea.
A few years ago we did a thread about ecosystems on a cubical planet with ordinary rotation rates. Naturally a cubical planet can’t really exist. Nothing is strong enough to hold that shape against its own gravity trying to sphericalize it. But we can pretend.
One of the consequences of the shape of the gravity field of such a planet would be that each cube face held a separate bubble of air with an ocean in the middle and dry land around it. Farther out from the center of each face the terrain “rose” against the gravity vector until the cube’s edges and corners were so “high” they were outside the atmosphere.
So 6 utterly disconnected ecosystems and eventually 6 unrelated intelligent societies could evolve and exist on the planet. Each necessarily completely ignorant of the other 5 until they invented first satellites then human sub-orbital travel.
I think that the most ‘habitable’ planet among the worlds surrounding TRAPPIST-1 is the ‘g’ planet. It is at the outermost edge of the habitable zone, but has a high gravity so is more likely to hang on to volatiles. One problem with planets orbiting young red dwarf stars is that the frequent flares could strip the atmospheres and liquids away; this planet is a bit heavier, and could potentially hold on to enough atmosphere to create a greenhouse effect and allow liquid water. But wit a bit of luck the d, e, and/or f worlds might be volatile rich too- it depends on how much volatiles they started with, among other things.
On a tidally-locked planet with an atmosphere, in theory, the twilight zones could be habitable/favourable for life, but in practice, there’s going to be a tendency for volatile substances (such as water) to condense on the cold side and remain locked up there forever, steadily reducing their availability in the theoretically habitable zone.
Yes, but on a planet with a relatively short year, the atmosphere (if any) can be displaced by rotation, so that warm air reaches the cold side. Most people forget that tidally-locked worlds actually do rotate- and these rotate quite fast. The three worlds in the TRAPPIST-1 system that are in the habitable zone rotate with periods of 4,6, and 9 days respectively.
