# 13 Earths - collision problem



## columbo1977 (Aug 6, 2010)

Hi Guys

I have posted here before about a book I am writing (Sci-fi) and the problem is, I need to have 13 Earths orbiting our sun, It was suggested that to get round the gravilty issue they could be slightly out of phase with our dimention. A friend has commented that how would they stay in an orbital path if gravity wasnt holding them. 

I also have the  problem of 13 Earths orbiting and because orbits are not circular sooner or later, planets are going to collide.

Can anyone giev any odeas to help?

Cheers

Graham


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## Cran (Aug 6, 2010)

13 Earths? 

You can have up to 6 Earths sharing one stable orbit - each at 60 degrees apart from the next -
after that, it gets a bit messy without resorting to some form of gravity-defying co-existences because, as you say:
planets are going to collide, or be thrown out of anything resembling an Earth-like orbit.


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## columbo1977 (Aug 7, 2010)

Unfortunately I need 13 for my story.

I will have to come up with some way of having them all there without cataclysmic events


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## garylawing (Aug 19, 2010)

The chances the earth will collide with mars, mercury, or mass a new study predicts that there is indeed a very slim possibility that such a cataclysm will rock our world, but notes that the possible collisions wouldn’t happen for more than 3 billion years, by which time humans may be long gone. Thank you


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## Cran (Aug 19, 2010)

actually, *colombo1977*, there is a way to have your 13 Earths ... if you are prepared to lose at least half of the Moons. 

At the six basic points of the annual orbit, the first 12 can exist in co-rotating orbits, similar to the Earth-Moon system, but with longer (and incrementally increasing) "days" and "months"; it's a stretch*, but one Moon could occupy the "Trojan" position with respect to two co-orbiting Earths; the 13th would have to be a "wanderer", slipping in and out of either the orbital plane or the space-time dimension in which the Earths have collected.

*it's a stretch because the mass ratios are not large enough in normal circumstances ...


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## WolfieReveles (Oct 2, 2010)

Do these 13 earths "appear" as if 13 dimensions suddenly joined? In that case it would work as long as (what Cran said) they all keep an equal distance, given they maintain the same speed. It would really just mess up the tides a bit, but considering the real proportions of our solar system, the moon would still have more influence over the tides and so on then the two closest earths.

If we are talking about 13 earths that were somehow always there it gets harder. The way planets are formed make it impossible practically for two to form at the same speed in the same orbit. The only way for that to work is artificial. If artificially created planets aren't an important part of your plot just remember what Horace said about resorting to _Deus ex Machina_. 

Either way, once they are formed you're fine. The earths orbit has a circumference of ca. 942 000 000 km. The avarage distance to the moon is over 384 000 km. Even when two moons were at their closest there would be more then 71 600 000 km of orbit between them.


There is however another thing you should take into consideration. Due to orbiting planets our sun wobbles, but wit 13 equally distributed earths they would cancel their tug on the sun... not that it makes much of a difference considering that the main pull isn't really the earth.


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## Richard Smith (Oct 3, 2010)

Now this is interesting... in my story, I need two planets orbiting the same sun, and by the simulations that I had done, they had to be 180 degrees apart or the orbits would become unstable, so I decided to use either a Gas Giant or Brown Dwarf in orbit around a sun-like star, and the two planets at Lagrangian points L4 and L5, that way we could have two rocky planets of similar earth mass, that would be of relatively negligible mass.  It may be stretching it a bit, but I can't do the calculations.  I hope it is believable enough that it will pass muster as long as I don't go into too much detail, and someone with more knowledge doesn't try to figure it out. 

I am curious to know why thirteen planets.  That is a whole lotta rock in one place, and highly unlikely to form naturally.


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## WolfieReveles (Oct 13, 2010)

*@Richard Smith*: I would still question the likelihood of two planets being caught in the same orbit... Perhaps you would consider a planetary duo so to say. Pluto and its moon Charon are so similar in size (the moon is 50% its size) that they practically orbit each other, and with two equally sized planets they would literally satellite each other.

If you have one side always face the other planet that would mess up their day/night cycle, freeze one side of the planet and burn the other leaving only a strip of habitable land.  Now on the other hand, as long as the two spin around their own axis as well as each other and the sun  the only difference would be colossal tides, which can be reduced by  having them farther apart. I'd say that your best option is the latter. Perhaps giving a tidal difference in water levels of 8-10 feet could work for your story, or just function as a funny quirk on this planet, or perhaps you separate them far enough to not have such a big influence and give them a tide of maybe 3-4 feet.

The best part is that this planetary constellation would be possible in theory, if as the solar system formed, two planetary orbits intersected and merged. Two equal bodies circle the sun, planet A has a stable orbit and planet B has a decaying orbit. over billions of years they come closer to each other and out of shear luck, instead of colliding they get locked in each others gravitational pull, adopting planet A's current orbit or causing a slightly new but stable one.

Here's an article on Pluto and it's moons that may prove interesting:
Pluto and Charon - Explore the Cosmos | The Planetary Society

"Gravity is a bitch" said the boy who fell


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## columbo1977 (Oct 14, 2010)

Hi

In my story they are not formed there, there is "our" Earth and then the other 12 are brought to out dimension by technology. I found a program (website) a while ago that simulated our solar system and as many Earths as you needed, it seemed to run fine with 13. There was the question of a collision billions of years int he future but that is way past what I need.


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## WolfieReveles (Oct 14, 2010)

Perfect. As long as they don't have to form there it shouldn't have to be a problem, the physics add up. Actually as long as the division between them is equal they would keep each other in place.


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