OK, it's not Earth and "Counter-Earth"...

[FreeTrav]

http://www.newscientist.com/article/dn20160-two-planets-found-sharing-one-orbit.html

Unfortunately, the article doesn't go much beyond "Wow, they're in the same orbit, in the Trojan points!", but at least now there's something you can point to and say "Yes, you CAN have two worlds sharing an orbit, not just a planet and a bunch of little rocks.".

 

[hdan]

<insert obligatory "Journey to the Far Side of the Sun" reference>

 

[DFW]

A question here for any astrophysicists out there:

I you look at something like Pluto, how would two Pluto's directly opposite each other (180 degrees) work/not work and, why?

 

[rust]

I you look at something like Pluto, how would two Pluto's directly opposite each other (180 degrees) work/not work and, why?

The two planets in the same orbit would suffer different gravitational dis-
turbances from the other planets of the system, since these other planets
would rarely be at the same distance from both planets in the same orbit,
and this would soon (on an astronomical time scale) lead to instability, so
one of the two planets would leave the common orbit.

 

[DFW]

I you look at something like Pluto, how would two Pluto's directly opposite each other (180 degrees) work/not work and, why?

The two planets in the same orbit would suffer different gravitational dis-
turbances from the other planets of the system, since these other planets
would rarely be at the same distance from both planets in the same orbit,
and this would soon (on an astronomical time scale) lead to instability, so
one of the two planets would leave the common orbit.

Well, I was only considering that there would be the two planets...

 

[GJD]

Unless they were exactly the same mass they would orbit at different speeds, one would eventually catch up with the other and BAM! - right in the kisser.

 

[rust]

Well, I was only considering that there would be the two planets...

Ah, yes. I think if they would have the same mass and move in a very far
and circular instead of elliptical orbit (to keep the star's gravitational influ-
ence the same on both planets over the entire orbit), this could probably
remain stable for a comparatively long time - although in the end the gra-
vitational influence of other stars would still destabilize it.

 

[Solomani666]

I you look at something like Pluto, how would two Pluto's directly opposite each other (180 degrees) work/not work and, why?

The two planets in the same orbit would suffer different gravitational dis-
turbances from the other planets of the system, since these other planets
would rarely be at the same distance from both planets in the same orbit,
and this would soon (on an astronomical time scale) lead to instability, so
one of the two planets would leave the common orbit.

Give that ananlogy then so would a single planet in an orbit.
Granted that natural orbits are eliptical and not circular the planets are bound to colide, but that could take a very long time especially in a system void of gas giants and other significant planets.

.

 

[DFW]

Unless they were exactly the same mass they would orbit at different speeds, one would eventually catch up with the other and BAM! - right in the kisser.

It's coming back to me. I forgot about Newton's corrected version of Kepler's 3rd law of motion.

Thanks.

Also rust, that would be a problem eventually as it effects the common center of gravity in the star - planet pair.

Thanks guys!

 

[Solomani666]

Unless they were exactly the same mass they would orbit at different speeds, one would eventually catch up with the other and BAM! - right in the kisser.

Last I checked, an objects mass is not a factor in its orbital velocity.
And certainly not when orbiting something as massive as a star.


.

 

[rust]

Last I checked, an objects mass is not a factor in its orbital velocity.

Then better check again ...

Orbital velocity
The velocity of an object at a given point in its orbit. If the orbit is per-
fectly circular, the magnitude of the velocity is constant and given by

Vorb = √[G(M + m)/r],

where G is the gravitational constant, M is the mass of the primary gra-
vitating body, m is the mass of the orbiting object, and r is the radius of
the orbit. In this special case, orbital velocity is the same as circular ve-
locity.

One can ignore the orbiting object's mass if it is significantly smaller than
the orbited object's mass and use a simplified formula, but the result is
less precise.

 

[DFW]

Unless they were exactly the same mass they would orbit at different speeds, one would eventually catch up with the other and BAM! - right in the kisser.

Last I checked, an objects mass is not a factor in its orbital velocity.
And certainly not when orbiting something as massive as a star..

I has to do with the actual center of mass around which the planet orbits NOT being the center of the star but, being the combined mass of the planet and star. Thus, the center of mass around which both objects orbit is off centre from the exact middle of the star.

 

[GJD]

Unless they were exactly the same mass they would orbit at different speeds, one would eventually catch up with the other and BAM! - right in the kisser.

Last I checked, an objects mass is not a factor in its orbital velocity.
And certainly not when orbiting something as massive as a star..

I has to do with the actual center of mass around which the planet orbits NOT being the center of the star but, being the combined mass of the planet and star. Thus, the center of mass around which both objects orbit is off centre from the exact middle of the star.

That's right, the barycentre. A three body calculation gives a barycentre that moves as the bodies each orbit it, as the mass in the system is redistributed.

If a planet is sufficently small and a star sufficently large then the barycentre will be inside the star, and hence the planet seems to orbit it (the star).

G.

 

[GJD]

Unless they were exactly the same mass they would orbit at different speeds, one would eventually catch up with the other and BAM! - right in the kisser.

Last I checked, an objects mass is not a factor in its orbital velocity.
And certainly not when orbiting something as massive as a star.


.

Last I checked it was. Should I delete the thread now?

 

[DFW]

That's right, the barycentre. A three body calculation gives a barycentre that moves as the bodies each orbit it, as the mass in the system is redistributed.

If a planet is sufficently small and a star sufficently large then the barycentre will be inside the star, and hence the planet seems to orbit it (the star).

G.

Here is a good visual simulation. http://phet.colorado.edu/sims/my-solar-system/my-solar-system_en.html
Just click Start in upper right.
You can also change a lot of the variables including # of bodies.

Another: http://astro.unl.edu/naap/pos/animations/kepler.html

 

[Solomani666]

I stand corrected.