Okay, this is take two! Last time, my browser decided it was time for another error with boards.straightdope.com.
Alright,I’m getting ready for the Physics know-it-alls to shoot me down here…
Alright, I have an idea I want for a hypothetical solar system - could it work, or is it nuts? (To use a scientific term
I want co-orbital Earths around a Sun – Two Earth mass planets are in orbit around one solar mass star. Trouble is, both of these suckers are going to be 1 AU from the star, +/- a couple million kilometres. Now, I’d like to get life “as-we-know-it” going on one, preferably both of them, so is there a way I can set this 3-body system so that:
a) The Earths don’t crash into each other or fall into the sun,
b) The Earths don’t tear each other apart with gravitational stress?
I tried to look this sort of thing up on Google, but I kept getting info about this movie called “Journey to the Far Side of the Sun”. I’ve heard that the science is bad in this movie, now I wanna hear why!
The easiest way I can think of to get your system working is make sure both earth’s are exactly 1 AU out, not +/- any distance. Then put the planets on the opposite side of the Sun from each other and you would never know the difference. If the twin Earth’s are in just slightly different orbits, eventually one will catch the other and it would be ugly. They need to be exactly the same orbit or significantly different orbits. Just a little will be bad news.
There are other solution as well, try looking up “Klemperer Rosettes”, that should get you lots of ideas to try.
IIRC, Lagrange points 4 and 5 are stable, points 1 through 3 aren’t… which means, if any other gravitational influence disturbs your planetary system, the planets will move out of alignment, and will continue to move further and further out. (Again IIRC, in your setup, each planet is in the L3 point of its counterpart.)
The idea of a “Counter-Earth”, on the opposite side of the sun from us, has been mooted before… the snag is that our solar system has lots of other gravitational influences, which would soon move “Counter-Earth” out of position. I read somewhere that the US Naval Observatory had calculated that, if there were such a planet, it would be pulled out of position so fast, it would be visible from Earth in only thirty years.
(Disclaimer: I’m an arts graduate, I don’t remember where I pick half this stuff up [most likely from cheap SF novels], I can’t guarantee its accuracy.)
“Journey to the Far Side of the Sun”, also known as “Doppelganger”, was the first live-action SF feature by Gerry Anderson. As an unregenerate fan of UFO and Space: 1999, I am prepared to concede that, while Mr Anderson is inestimably wonderful as a film and TV producer, he is… somewhat wanting… when it comes to strict astrophysical accuracy.
Three possibilities: First, put the two planets 60 degrees apart in their orbits. They’ll stably stay that distance apart. Second, you can go with a true double planet like Mangetout suggests. Third, and perhaps most interestingly, you can use an arrangement like Saturn’s satellites Janus and Epimetheus. They’re in slightly different orbits, so close that the difference in orbits is less than the diameter of either moon. Since one is a little closer than the other, it’s moving faster in its orbit. But wait, you say, won’t they collide? What actually happens is that whenever they get close, their gravitational interaction causes them to swap orbits: The closer, faster one shifts to the higher orbit, and starts to fall behind again, while the further, slower one shifts to the lower orbit, and starts drawing away again to “lap” the other one. When it catches up, the pattern repeats.
There is actually at least one asteroid which shares Earth’s orbit in this manner, but being much smaller than the Earth, almost all of the orbital change is in its orbit, not ours.
That’s really cool; I’d never heard of that. Here’s some more information, because I’m interested. Apparently they orbit at 150,000 km from Saturn’s center. Their orbital distances differ by about 50 km. They orbit in 0.7 days, and they swap once every four years or so.
