The Campaign Builder's Guild

Actually, something else that's interesting is that if the planets were on exact opposite sides of the same orbit, they probably wouldn't even know about each other until the advent of modern technology, as the other would always be eclipsed by their mutual star.  There are plenty of ways to get around that now, but when all you've got are standard telescopes, I assume you'd have absolutely no idea your twin existed.  The less advanced planet might be very surprised when they invent the radio and discover that the airwaves are already filled with alien transmissions.

Quote from: Polycarp!Actually, something else that's interesting is that if the planets were on exact opposite sides of the same orbit, they probably wouldn't even know about each other until the advent of modern technology, as the other would always be eclipsed by their mutual star.  There are plenty of ways to get around that now, but when all you've got are standard telescopes, I assume you'd have absolutely no idea your twin existed.  The less advanced planet might be very surprised when they invent the radio and discover that the airwaves are already filled with alien transmissions.

Wouldn't the movements of the other planets be affected by the 'twin' as well, therefore making the other planet observable indirectly? Granted, I have no idea how precise astronomical observations would have to be for that to work.

Planets can run into each other, though it generally only happens in early solar systems when there is still a lot of junk floating around. Scientists believe that our moon formed when the first Earth was hit by another planet at such an angle that a molten chunk was knocked out. Some of their reasoning for this is that the moon's iron core is disproportionately small; if it was made of the same cloud as earth, it would have the same proportions. If it was a captured body, it would have to be a lot smaller.

I like the idea of two planets sharing the same orbit on opposite sides of the star. Reminds me of a Planet X type deal I read about in a "what if" type science book in elementary school.

QuoteWouldn't the movements of the other planets be affected by the 'twin' as well, therefore making the other planet observable indirectly? Granted, I have no idea how precise astronomical observations would have to be for that to work.

In our world, if such a planet existed, we wouldn't have known until past like newton probably. Any inconsistencies caused by the other planet would probably be explained away in extra epicycles or something.

Remember, like, even in keplar's time, the relationship between things falling and planetery orbits wasn't realized; we were just inventing increasingly accurate equations to predict planetary orbits. So yeah, it'd probably be pretty hard to find. Of course, the OP's idea I think involved a spacefaring civilization on one side. That might get pretty obvious real fast-like. :)

Quote from: brainfaceOf course, the OP's idea I think involved a spacefaring civilization on one side. That might get pretty obvious real fast-like. :)

The threads actually been taken off my original track: I never intended that one or both planets wouldn't know about each other.  It's just a question of having multiple Earth-like, or nearly-Earth-like, planets in the same solar system without them orbiting a gas giant, orbiting different stars of a binary, orbiting each other, or crashing, so they'd be obviously close enough that one could send a rescue call to the other.

Another question, as long as everyone is here: Could several (let's say at least three) Earth-sized planets orbit a central empty point which itself orbits a star?  I assume no, you'd have invoke magic, technology-seems-like-magic, or break the laws of physics as we know them.  It's not terribly important that I know a way to do it, I just like to explore the scientific examples.

Quote from: http://en.wikipedia.org/wiki/File:Orbit2.gif[/url]

In our Solar System, the barycenter (that red cross in the middle) is in the Sun, which wobbles a bit around it.  Other systems could have it wherever the masses and gravities balance it out to.

The problem with several Earth-sized planets orbiting around a barycenter which orbits around a star would be that to make it work for any length of time, they would need to find some kind of balance.  It's quite possible, just not likely as it's hard to get that kind of system stable.

You could do two pretty easily, they would orbit each other as moons, and the pair would then orbit the sun. More than that and you run into the three-body problem...

Pluto and Charon do it. Anytime the barycenter is outside the surface of both worlds you get a binary planet system like that. Three planets though wouldn't be stable for very long. Your best bet would be to again look at the Pluto system which is comprised of two planetoids (pluto and charon) and 2 moons (nix and hydra). So then you could have a binary planet system with a moon (though a moon large enough to support life might be an issue without making the two planets too large to support life).

Quote from: Prone To WanderingPluto and Charon do it. Anytime the barycenter is outside the surface of both worlds you get a binary planet system like that. Three planets though wouldn't be stable for very long. Your best bet would be to again look at the Pluto system which is comprised of two planetoids (pluto and charon) and 2 moons (nix and hydra). So then you could have a binary planet system with a moon (though a moon large enough to support life might be an issue without making the two planets too large to support life).

Couldn't you have a binary planet system, which is in binary with another binary planet system, and that whole mess rotating around a sun?

You can have a Binary in binary with another single planet, a binary with a binary, etc. I had seen a page it had all the different possible stable orbits for varying # of suns (planets could easily be subbed into it), for 1-4. It was incredibly neat, I have however lost it.

Has anyone else seen this page? It would be a good thing to share in this thread.

Double Post in the Name of Good!

 Here it is, found it after some googling. So I guess any planets in that kind of configuration would work?

I should probably have added a "most likely wouldn't be stable" into my previous post :P

Llum brings up a viable way to make a trinary or quadrary system. And yes stargate you could... which would be really awesome