6 ms·

None of these "habitable" planets is really plausibly habitable. In particular, orbiting so close, they are all tidally locked 1:1 to their star, presenting al

None of these "habitable" planets is really plausibly habitable. In particular, orbiting so close, they are all tidally locked 1:1 to their star, presenting always the same face. If they have any water, it is frozen on the dark side. But they might have no water anyway because they (might!) have no magnetic field to fend off the solar wind that would take away all their hydrogen, as happened to Venus. Earth has its big moon helping it not get tidally locked, thus still spinning and generating its strong magnetic field. Probably Earth-size planets with a Moon-size moon are vanishingly rare. Dual planets might do better. Or maybe a dual moon of a gas giant, but the radiation belts of the gas giant would be a problem. So actually-habitable planets must be much, much rarer, among those announced, than astronomy departments' PR crew would have us think. Inside moons of Jupiter and Saturn might even present a better prospect. That is not to say actually habitable planets must be rare; rather, we can't spot them yet. But our big moon might be important.

bagels 4y ago
Mars and Venus are not tidally locked. Earth, Jupiter and Ganymede have magnetic fields, and Mars used to. Do we know that these exosolar planets don't have moons, don't have magnetic fields, and are tidally locked?
- moloch-hai 4y ago
  Mercury and Venus are very near to being tidally locked, despite being thousands of times farther from the sun than the planet cited is from its star. Mars is a good distance off. Earth got its moon via a nearly, but not quite, fatal collision. Probably most other planets did not. Ganymede's magnetic field is news to me. I don't think we know exactly what gives a body a magnetic field, or how it is maintained, but rotation seems to be important. Ganymede's is much, much weaker than ours, and some suggest it is a remnant frozen into rock.
  - irq 4y ago
    > I don't think we know exactly what gives a body a magnetic field, or how it is maintained, but rotation seems to be important. The magnetic field on Earth is caused by electrical currents flowing in the rotating molten metal of the outer part of our planet’s core. You’re right, rotation is an important factor.
    - mcv 4y ago
      I've long suspected that tidal forces from our moon may be a big factor in keeping the outer core liquid, and thereby keeping our magnetic field active. I have no idea if that's actually true, but if it is, both rotation and a large moon might be requirements for advanced life.
      - moloch-hai 4y ago
        Tidal forces from the sun are remarkably close in magnitude to those from our moon. Their chaotic interaction seems to me to interfere with any big, systematic effect on Earth's rotation. It seems possible that during certain times in the past, the lengths of the day and month (and year?) were in close enough resonance to accelerate loss of rotation for a while, until the change drove them back out of it .
rendall 4y ago
You're saying that all Earth-sized planets without moons are tidally locked?
- moloch-hai 4y ago
  All such planets orbiting so close to their star are certainly tidally locked. Even Venus is almost tidally locked, tens of millions of miles away from the sun. All moons in our solar system are locked. It is the norm.
  - rendall 4y ago
    Thanks. TIL. I have read a science fiction story or two set on tidally locked planets, and the humans occupy the slim twilight belt between the hemispheres facing towards and away from the sun, where the temperatures are less extreme. If it has an atmosphere, though, I imagine its expansion and compression cycle would make it quite stormy.
    - moloch-hai 4y ago
      Given the negligible variation in energy input to various parts of the planet, over time, I would expect it to be fatally placid.
nakedrobot2 4y ago
If they're one in a billion, there are still billions of them.
- danw1979 4y ago
  … but so far apart we’ll likely never meet or even hear from them ?
  - vkou 4y ago
    We're likely never to meet or hear from them even if they were a mere 5 light-years. It's easier for Michael Jordan to jump to the Moon, than for man to travel to the nearest star.
    - sph 4y ago
      Weird popsci analogy. Travel to the nearest star takes a very long time. Michael Jordan can train for thousands of years and will never leave atmosphere with his jump.
      - Insanity 4y ago
        Agree it is odd, but I guess they mean through successive jumps always starting from the previous height. (Like jumping up a large staircase).
        867-5309 4y ago
        they obviously didn't, choosing a person with extraordinary jumping ability implies they achieve it in one jump
      - mcv 4y ago
        And we have gone to the moon, despite it having been impossible for millennia. And we understand the basic physics of traveling to another star. The only problem is: it's going to take a ridiculous amount of time, and we don't want to do that for a myriad of reasons (money, risk, time), but if we really absolutely wanted to, we could, and there'd be a non-zero chance we'd succeed. Unlike Michael Jordan jumping to the moon.
defrost 4y ago
Tidal locking doesn't equate to "always presenting the same face". For example, > Mercury .. is tidally locked with the Sun in a 3:2 spin–orbit resonance, meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. > As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. > An observer on Mercury would therefore see only one day every two Mercurian years. https://en.wikipedia.org/wiki/Mercury_(planet) https://en.wikipedia.org/wiki/Mercury_(planet)
- moloch-hai 4y ago
  Mercury is a very interesting case. But it is much farther from the sun than announced planets are from theirs.
  - defrost 4y ago
    (again) Spin–orbit resonance means that "Tidally-locked" doesn't equate to "always presents the same face. which leads to: > But it is much farther from the sun than announced planets are from theirs. a) So? b) Is it really? I mean sure, in an absolute sense - but what about in an appropriately scaled sense, allowing for the 1/sixth decreased sun mass and the inverse square force relationships, etc. c) Even so, see a) - How does this alter the fact that tifally locked doesn't equate to "always presents the same face". To be clear, I'm just highlighting the fragile and dubious nature of your earlier assertion about tidal locking.
    - moloch-hai 4y ago
      Literally all moons in the solar system are tidally locked, 1:1, to their parent body. That is the norm for tidal locking. Mercury, lacking ocean or even atmosphere, and mostly frozen solid, is accorded leeway a putatively-habitable planet does not get. Word games are tiresome. Mercury's 2:3 spin-orbit resonant "lock" is interesting but off topic.
      - 4y ago
winReInstall 4y ago
Habitable also needs a saturn class planet outside, defending against meteorites and comets. Else you can have life on a planet, but it gets "reset" every nthousand years. Lucky us, there have only been 7 to 20 of these events so far. https://en.wikipedia.org/wiki/Extinction_event https://en.wikipedia.org/wiki/Extinction_event
maxamillion2020 4y ago
Yes, but our planet detecting tools can only currently detect planets that are 1. close to their stars and 2. smallish, so that self-selects for tidally locked planets.
- moloch-hai 4y ago
  As I said.
pantalaimon 4y ago
> So actually-habitable planets must be much, much rarer, among those announced, than astronomy departments' PR crew would have us think That’s not the conclusion you can draw from that. With the methods we currently have we can only really spot exoplanets if they are very close to their star and also reasonably big in relation to the star. That means super-earths close to red dwarfs. There are very likely many more types of planets, they are just much harder to see from afar.
- moloch-hai 4y ago
  As you quoted yourself, "among those announced". Life arose here, on a planet not among those announced, or like them.
exitb 4y ago
There's probably much more complexity to it, but I wonder if a tidally locked planet could have a habitable ring of mild weather along the terminator.
- anovikov 4y ago
  Certainly it must have, more than that, it helps widening the definition of habitability as slightly too cold planet must be still habitable further out to the sunny side, and slight too hot one must still be habitable deeper into the twilight zone.
- moloch-hai 4y ago
  Anyway if the water is not all locked up in ice on the dark side. There could be so damn much water that enough sublimates to rain on the hot side or the terminator. Still, not a place I would start looking for life.
rich_sasha 4y ago
I was reading about "eyeball planets", which are possibly suitable for life, and match this description (vaguely, IANAAstrophysicist). Basically, the idea is that a planet of right size and roughly in the right distance to its star is tidally locked in a 1:1 resonance, so one side is always pointing at the star, and one always poiting away. Now, the perpetual noon point is very hot, and the perpetual midnight point is very cold, but you get a consistent temperature gradient inbetween. Somewhere there, it's about 20 deg C ~all the time. It's also somewhat tunable. If the planet is closer to the star, perhaps only the perpetual midnight is a happy spot, and the rest too hot. Or maybe the planet is further out, and only the perpetual noon is nice. And so on. Of course you also make the point about magnetic field requiring rotation, which is not addressed by this, but the temperature side at least maybe is OK.
- moloch-hai 4y ago
  The water remains frozen on the dark side. Maybe there is so much water that enough sublimates to rain out at the terminator? Still, it is poor pickings.
- koheripbal 4y ago
  The problem is that over time the atmosphere would vaporize into space from the hot side so you will get a barren rock like Mercury. There's always a liiiiitle rotation, so even any frozen gases on the cold side would eventually rotate into the sun and be heated to escape velocity.
  - moloch-hai 4y ago
    There is always a little sublimation, anyway.
deleted 4y ago
[deleted]
lamontcg 4y ago
> None of these "habitable" planets is really plausibly habitable. They can plausibly have life, though, but it may not get anywhere near as far as life on Earth has gotten. That is still scientifically interesting.

bagels 4y ago

Mars and Venus are not tidally locked. Earth, Jupiter and Ganymede have magnetic fields, and Mars used to. Do we know that these exosolar planets don't have moons, don't have magnetic fields, and are tidally locked?

moloch-hai 4y ago
Mercury and Venus are very near to being tidally locked, despite being thousands of times farther from the sun than the planet cited is from its star. Mars is a good distance off. Earth got its moon via a nearly, but not quite, fatal collision. Probably most other planets did not. Ganymede's magnetic field is news to me. I don't think we know exactly what gives a body a magnetic field, or how it is maintained, but rotation seems to be important. Ganymede's is much, much weaker than ours, and some suggest it is a remnant frozen into rock.
- irq 4y ago
  > I don't think we know exactly what gives a body a magnetic field, or how it is maintained, but rotation seems to be important. The magnetic field on Earth is caused by electrical currents flowing in the rotating molten metal of the outer part of our planet’s core. You’re right, rotation is an important factor.
  - mcv 4y ago
    I've long suspected that tidal forces from our moon may be a big factor in keeping the outer core liquid, and thereby keeping our magnetic field active. I have no idea if that's actually true, but if it is, both rotation and a large moon might be requirements for advanced life.
    - moloch-hai 4y ago
      Tidal forces from the sun are remarkably close in magnitude to those from our moon. Their chaotic interaction seems to me to interfere with any big, systematic effect on Earth's rotation. It seems possible that during certain times in the past, the lengths of the day and month (and year?) were in close enough resonance to accelerate loss of rotation for a while, until the change drove them back out of it .

rendall 4y ago

You're saying that all Earth-sized planets without moons are tidally locked?

moloch-hai 4y ago
All such planets orbiting so close to their star are certainly tidally locked. Even Venus is almost tidally locked, tens of millions of miles away from the sun. All moons in our solar system are locked. It is the norm.
- rendall 4y ago
  Thanks. TIL. I have read a science fiction story or two set on tidally locked planets, and the humans occupy the slim twilight belt between the hemispheres facing towards and away from the sun, where the temperatures are less extreme. If it has an atmosphere, though, I imagine its expansion and compression cycle would make it quite stormy.
  - moloch-hai 4y ago
    Given the negligible variation in energy input to various parts of the planet, over time, I would expect it to be fatally placid.

nakedrobot2 4y ago

If they're one in a billion, there are still billions of them.

danw1979 4y ago
… but so far apart we’ll likely never meet or even hear from them ?
- vkou 4y ago
  We're likely never to meet or hear from them even if they were a mere 5 light-years. It's easier for Michael Jordan to jump to the Moon, than for man to travel to the nearest star.
  - sph 4y ago
    Weird popsci analogy. Travel to the nearest star takes a very long time. Michael Jordan can train for thousands of years and will never leave atmosphere with his jump.
    - Insanity 4y ago
      Agree it is odd, but I guess they mean through successive jumps always starting from the previous height. (Like jumping up a large staircase).
      - 867-5309 4y ago
        they obviously didn't, choosing a person with extraordinary jumping ability implies they achieve it in one jump
    - mcv 4y ago
      And we have gone to the moon, despite it having been impossible for millennia. And we understand the basic physics of traveling to another star. The only problem is: it's going to take a ridiculous amount of time, and we don't want to do that for a myriad of reasons (money, risk, time), but if we really absolutely wanted to, we could, and there'd be a non-zero chance we'd succeed. Unlike Michael Jordan jumping to the moon.

defrost 4y ago

Tidal locking doesn't equate to "always presenting the same face". For example, > Mercury .. is tidally locked with the Sun in a 3:2 spin–orbit resonance, meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. > As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. > An observer on Mercury would therefore see only one day every two Mercurian years. https://en.wikipedia.org/wiki/Mercury_(planet) https://en.wikipedia.org/wiki/Mercury_(planet)

moloch-hai 4y ago
Mercury is a very interesting case. But it is much farther from the sun than announced planets are from theirs.
- defrost 4y ago
  (again) Spin–orbit resonance means that "Tidally-locked" doesn't equate to "always presents the same face. which leads to: > But it is much farther from the sun than announced planets are from theirs. a) So? b) Is it really? I mean sure, in an absolute sense - but what about in an appropriately scaled sense, allowing for the 1/sixth decreased sun mass and the inverse square force relationships, etc. c) Even so, see a) - How does this alter the fact that tifally locked doesn't equate to "always presents the same face". To be clear, I'm just highlighting the fragile and dubious nature of your earlier assertion about tidal locking.
  - moloch-hai 4y ago
    Literally all moons in the solar system are tidally locked, 1:1, to their parent body. That is the norm for tidal locking. Mercury, lacking ocean or even atmosphere, and mostly frozen solid, is accorded leeway a putatively-habitable planet does not get. Word games are tiresome. Mercury's 2:3 spin-orbit resonant "lock" is interesting but off topic.
    - 4y ago

winReInstall 4y ago

Habitable also needs a saturn class planet outside, defending against meteorites and comets. Else you can have life on a planet, but it gets "reset" every nthousand years. Lucky us, there have only been 7 to 20 of these events so far. https://en.wikipedia.org/wiki/Extinction_event https://en.wikipedia.org/wiki/Extinction_event

maxamillion2020 4y ago

Yes, but our planet detecting tools can only currently detect planets that are 1. close to their stars and 2. smallish, so that self-selects for tidally locked planets.

moloch-hai 4y ago
As I said.

pantalaimon 4y ago

> So actually-habitable planets must be much, much rarer, among those announced, than astronomy departments' PR crew would have us think That’s not the conclusion you can draw from that. With the methods we currently have we can only really spot exoplanets if they are very close to their star and also reasonably big in relation to the star. That means super-earths close to red dwarfs. There are very likely many more types of planets, they are just much harder to see from afar.

moloch-hai 4y ago
As you quoted yourself, "among those announced". Life arose here, on a planet not among those announced, or like them.

exitb 4y ago

There's probably much more complexity to it, but I wonder if a tidally locked planet could have a habitable ring of mild weather along the terminator.

anovikov 4y ago
Certainly it must have, more than that, it helps widening the definition of habitability as slightly too cold planet must be still habitable further out to the sunny side, and slight too hot one must still be habitable deeper into the twilight zone.
moloch-hai 4y ago
Anyway if the water is not all locked up in ice on the dark side. There could be so damn much water that enough sublimates to rain on the hot side or the terminator. Still, not a place I would start looking for life.

rich_sasha 4y ago

I was reading about "eyeball planets", which are possibly suitable for life, and match this description (vaguely, IANAAstrophysicist). Basically, the idea is that a planet of right size and roughly in the right distance to its star is tidally locked in a 1:1 resonance, so one side is always pointing at the star, and one always poiting away. Now, the perpetual noon point is very hot, and the perpetual midnight point is very cold, but you get a consistent temperature gradient inbetween. Somewhere there, it's about 20 deg C ~all the time. It's also somewhat tunable. If the planet is closer to the star, perhaps only the perpetual midnight is a happy spot, and the rest too hot. Or maybe the planet is further out, and only the perpetual noon is nice. And so on. Of course you also make the point about magnetic field requiring rotation, which is not addressed by this, but the temperature side at least maybe is OK.

moloch-hai 4y ago
The water remains frozen on the dark side. Maybe there is so much water that enough sublimates to rain out at the terminator? Still, it is poor pickings.
koheripbal 4y ago
The problem is that over time the atmosphere would vaporize into space from the hot side so you will get a barren rock like Mercury. There's always a liiiiitle rotation, so even any frozen gases on the cold side would eventually rotate into the sun and be heated to escape velocity.
- moloch-hai 4y ago
  There is always a little sublimation, anyway.

deleted 4y ago

[deleted]

lamontcg 4y ago

> None of these "habitable" planets is really plausibly habitable. They can plausibly have life, though, but it may not get anywhere near as far as life on Earth has gotten. That is still scientifically interesting.