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.
But it is much farther from the sun than announced planets are from theirs.
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.
Mercury, lacking ocean or even atmosphere, and mostly frozen solid, is accorded leeway a putatively-habitable planet does not get.
Looking at the moons in our solar system is evidence but not definitive.
Mercury is literally the only body in the whole solar system in any sort of more complicated "lock". So, no, it really is about word games.
How would you even begin to demonstrate this is true? There are so many bodies in the solar system, and we're still fairly frequently discovering new minor planet moon-systems..
The original topic, so cleverly but pointlessly derailed, was the viability of planets we can just barely detect in extremely close orbit around a dim star, in 1:1 tidal lock, with permanently hot and a cold sides, and, likely, no magnetic field. That merited discussion.
> hundreds of moons
> literally the only body
They are all moons, not planets, and not in different star systems. That's not enough to make conclusive statements about exoplanet spin.
That statistic is more relevant to the word games but I'm not here for word games either. It doesn't matter if "lock" implies 1:1.
Agreed, what matters is what percentage of these planets are 1:1 locked.
> All these exoplanets are certainly so locked.
How do you know? Just citing moons in our system isn't enough.
Moons in our solar system orbit closer to planets than planets to the sun, so get locked much more quickly. Venus and even Mercury have not quite got there. Earth is in a more complicated arrangement, which helps us avoid it. That might even have been essential for life.
You may read up elsewhere on what determines the tidal forces, friction, and rotational momentum and energy that determine how long it takes to get into a locked state. That would be smarter than word games.
that doesn't change the argument being made by much. that is still pretty terrible for consistent temperature variations. an extremely long day-night cycle is just about as bad.
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?
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.
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.
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 .
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.
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.
It's easier for Michael Jordan to jump to the Moon, than for man to travel to the nearest star.
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.
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.
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.
Still, not a place I would start looking for life.
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.
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.
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.
In particular, they are all tidally locked to their star, presenting always the same face. If they have any water, it is frozen on the dark side. But they probably have no water because they 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 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 than astronomy departments' PR crew would have us think.