More astrophysics
I've just realized that I forgot a point I was going to make in my last post.
Earth has several moons, but Luna isn't one of them.
Yes, really.
Compare the Terra/Luna pair with any other moon & planet in the solar system, and you find a number of *glaring* differences.
First of all the center of mass of the Terra/Luna pair is *outside* of Earth. Not true for any other planet/moon pair. Well, maybe Pluto/Charon, but that may be another special case like Terra/Luna.
Second and far more telling is that if you plot the orbit of the planet with respect to the sun, and plot the path of the moon on the same plot, for all the other moons (again, with the possible exception of Charon) you'll see that the orbits of the various moons have a number of points where they are falling *away* from the sun. At those points the satellite's orbit is convex with respect to the sun rather than concave.
But you do this with Terra and Luna, and Luna's orbit is always concave with respect to the sun. at the points where it is closer to the sun than Terra, the orbit is flatter, but still concave.
Terra/Luna is a binary planet, *not* a planet and moon.
This has a nuimber of interesting effects. For one, Luna stabilizes Terra's axis. We still get things like precession of the equinoxes and other long term cycles, but they are both longer term and less drastic than they would be without Luna's influence.
Mars doesn't have such stabilizing factor and its axis has flip-floped a lot over the millennia, making it a lot less suitable for life.
Also, Luna has stripped of some of Earth's atmosphere , especially in the early days. This may be a reason we didn't wind up like Venus.
So, binary planets may be a *lot* more favorable to life than other planets. Which reduces the odds of finding life in other star systems, as binary planets in the habitable zone aren't likely. Not impossible, just a lot less likely.
So other habitable worlds are less likely to have huge "moons" in the the sky (much less multiple ones).
If binary planets are *necessary* for life, then that biases things in favor of a huge "moon". But lowers the number of planets suitable for humans (and other, similar lifeforms) a *lot*.
If habitable planets are common, then most won't have the huge "moons".
This entry was originally posted at https://kengr.dreamwidth.org/1148637.html. Please comment there using OpenID.
Earth has several moons, but Luna isn't one of them.
Yes, really.
Compare the Terra/Luna pair with any other moon & planet in the solar system, and you find a number of *glaring* differences.
First of all the center of mass of the Terra/Luna pair is *outside* of Earth. Not true for any other planet/moon pair. Well, maybe Pluto/Charon, but that may be another special case like Terra/Luna.
Second and far more telling is that if you plot the orbit of the planet with respect to the sun, and plot the path of the moon on the same plot, for all the other moons (again, with the possible exception of Charon) you'll see that the orbits of the various moons have a number of points where they are falling *away* from the sun. At those points the satellite's orbit is convex with respect to the sun rather than concave.
But you do this with Terra and Luna, and Luna's orbit is always concave with respect to the sun. at the points where it is closer to the sun than Terra, the orbit is flatter, but still concave.
Terra/Luna is a binary planet, *not* a planet and moon.
This has a nuimber of interesting effects. For one, Luna stabilizes Terra's axis. We still get things like precession of the equinoxes and other long term cycles, but they are both longer term and less drastic than they would be without Luna's influence.
Mars doesn't have such stabilizing factor and its axis has flip-floped a lot over the millennia, making it a lot less suitable for life.
Also, Luna has stripped of some of Earth's atmosphere , especially in the early days. This may be a reason we didn't wind up like Venus.
So, binary planets may be a *lot* more favorable to life than other planets. Which reduces the odds of finding life in other star systems, as binary planets in the habitable zone aren't likely. Not impossible, just a lot less likely.
So other habitable worlds are less likely to have huge "moons" in the the sky (much less multiple ones).
If binary planets are *necessary* for life, then that biases things in favor of a huge "moon". But lowers the number of planets suitable for humans (and other, similar lifeforms) a *lot*.
If habitable planets are common, then most won't have the huge "moons".
This entry was originally posted at https://kengr.dreamwidth.org/1148637.html. Please comment there using OpenID.