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Sister Earth

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doom_shepherd

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Earthlike Double Planet Possible?

Is an Earthlike "Double Planet" possible?

By this, I mean two Earthlike planets orbiting a common center of gravity, still remaining in the "goldilocks zone" around their sun. I've heard of at least one SF book where this is speculated, but I'm wondering if the portrayal is accurate (the twin planets are described as being so close together they share an atmosphere.)

If our Earth were co-orbital with another Earth, how far away would the other Earth have to be for the orbits to be stable? What would it look like in the sky?
 
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Shpaget

Guest
Re: Earthlike Double Planet Possible?

uh huh...
Shared atmosphere? Considering that our atmosphere is roughly 100 km thick, another massive body and it's gravity pulling would pull it, let's say, twice that high, and than another planet's atmosphere begins, that would place two Earth like planets roughly 300-500 km apart (from one surface to another), and that is waaaay too close for something as big as another planet to be safely in orbit.

How would it look in the sky? It would stretch across entire sky.
There would be no real sunshine, and really dark darkness on the "near" side. On the "far" side it would look no different than what it's now.
I assume that planets would both be in, lets call it, geostationary orbit, always above exactly the same place, like the Earth is always seen from the nearside.

I'm not sure if two planets would be able to form like that in the first place, but it would rather become one single body.

Edit to add:
Orbital periods for two Earth sized planets at such close distances would be really small.
It would seriously affect my beauty sleep.
 
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doom_shepherd

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Re: Earthlike Double Planet Possible?

Thanks.

The "close earths" were in Robert Forward's "Rocheworld" books (went and looked it up.) They probably weren't Earth-sized after all.

Now I'm more interested in a "realistic" pair of Earthlike worlds, not so close as that. What they might really look like...

More like the way Charon and Pluto are a double-dwarf planet, or some folks used to call the Earth-Moon system an 'almost double planet.' Could an Earth-Earth system exist, and what would it be like? I assume both planets would be tidally locked to Each other.
 
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crazyeddie

Guest
Re: Earthlike Double Planet Possible?

doom_shepherd":24x65s2h said:
Is an Earthlike "Double Planet" possible?

By this, I mean two Earthlike planets orbiting a common center of gravity, still remaining in the "goldilocks zone" around their sun. I've heard of at least one SF book where this is speculated, but I'm wondering if the portrayal is accurate (the twin planets are described as being so close together they share an atmosphere.)

If our Earth were co-orbital with another Earth, how far away would the other Earth have to be for the orbits to be stable? What would it look like in the sky?
I don't think two Earth-mass planets could possibly orbit each other so closely that their atmospheres intermingled. The tidal forces between the two objects would tear them apart, unless they were somehow made of some material with incredible tensile strength. No natural planets would be strong enough.
 
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harper05

Guest
Re: Earthlike Double Planet Possible?

Keep in mind there would be an area of immense opposing gravitational forces in between the close part of the 2 super close planets. Creating kind of a neutral g zone if you will. This may allow the atmosphere to stretch further than a few hundred kilometers. So if they were say 1500-2000 km apart, it might be more feasible, they would have to orbit each other really fast though. And this motion of the g zone probably would tear apart the planets, talk about tidal forces... wow

Either way this would definitely make them unearth-like

I bet I would have like a 20ft vertical leap in this more nuetral g zone, anyone wanna play 1on1!! :D
 
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dryson

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Re: Earthlike Double Planet Possible?

I don't think two Earth-mass planets could possibly orbit each other so closely that their atmospheres intermingled. The tidal forces between the two objects would tear them apart, unless they were somehow made of some material with incredible tensile strength. No natural planets would be strong enough.
It would really depend on how much water was on each planet for the tidal forces to have any affect. If one planet had less water then the tidal forces would be less prevelant. Two Earth like planets orbiting each other would be interesting too see and would make a great backdrop for a sci-fi story based on The Tale of Two Cities, but would instead be called The Tale of Two Planet's.
 
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MeteorWayne

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Re: Earthlike Double Planet Possible?

Sorry, but this shows a very sorry understanding of the physics of gravity. Your ideas have no credibility whatsoever.
 
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doom_shepherd

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Re: Earthlike Double Planet Possible?

Who is this "your" of whom you speak? There's several participants in this discussion, so it would be helpful to specify WHOM you are insulting.

These aren't MY ideas. I'm just asking if a double planet system of two Earth-sized worlds in any kind of stable orbit is possible. Not if Rocheworld is.

(Though I thought Forward was a hard-SF writer, which meant that the ideas had to be at least theoretically possible.)
 
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MeteorWayne

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Re: Earthlike Double Planet Possible?

doom_shepherd":j6u4x9ez said:
Who is this "your" of whom you speak? There's several participants in this discussion, so it would be helpful to specify WHOM you are insulting.

These aren't MY ideas. I'm just asking if a double planet system of two Earth-sized worlds in any kind of stable orbit is possible. Not if Rocheworld is.

(Though I thought Forward was a hard-SF writer, which meant that the ideas had to be at least theoretically possible.)
Sorry, my bad. My reply was to the post immediately above mine. User dryson often derails topics with such goofy science that they wind up in the Unexplained. I should have specified, and will try and do so in the future. Your original topic is just fine, and I apologize for the confusion.

Wayne
 
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doom_shepherd

Guest
Re: Earthlike Double Planet Possible?

Okay, then. :)

I know that I see a lot of Sci-Fi art with people on a planet, and a huge moon looming in the sky. It's highly pretty, but I can't imagine from what little I know that such a thing could really exist.

I've always wanted to speculate on what effect looking up and seeing another viable world so close would have had on the course of human history, but first I'd like to know if it's a realistic scenario, you know?
 
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neilsox

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Re: Earthlike Double Planet Possible?

We should never say never, but two Earth like planets orbiting each other is likely extremely rare. If they are much closer than moon distance, then the extreem tides would make them not very Earth like. At moon distance, they would appear about three times larger = 9 times the disc area. Neil
 
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crazyeddie

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Re: Earthlike Double Planet Possible?

Consider the fact that it was not until about half a billion years ago that life first appeared on land on our planet. Prior to that time, it existed only in the oceans as single-celled organisms. This period also coincided with the time that our moon orbited considerably closer to it's primary. The ocean tides, soon after the ocean first precipitated out of the primordial atmosphere, must have reached staggering proportions, 1,000 times higher than they are today. These humongous tides plunged miles inland and withdrew every three hours (remember, the day was only six hours long). As they moved over the land, the awesome volumes of water scraped and pounded the primeval rock, removing and pulverizing a considerable amount of it. Every time the tide retreated, it dragged this material back into the ocean. It was not until the moon retreated and the tides became more manageable that life was able to colonize the land surface. Assuming that two closely-orbiting Earth-mass planets are possible, it's conceivable that complex life could not develop on such worlds until they achieved synchronicity in their respective rotations.
 
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kg

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Re: Earthlike Double Planet Possible?

There are objects called "contact binarys" that are two asteroids that orbit each other so closely that they touch and become a single object. http://en.wikipedia.org/wiki/Contact_binary_(asteroid)
I'm guessing that there is a well defined upper limit to how large such an object can be before gravity smooshes it more into a single asteroid.
 
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Fallingstar1971

Guest
Re: Earthlike Double Planet Possible?

I think it would be more interesting to put them on opposite sides of their parent star and have them discover each other by accident through their independent space programs.......If they were the same mass you might be able to pull it off.

For the orbit to be stable, each planet would have to be in one of the others Lagrange points.

I dont know about sharing atmospheres though, however, if you piled enough gas onto said planets they could emit tails like comets leaving trails in their shared orbit. This would allow them to occupy each others "tails" thus technically sharing atmospheres. Or give them moons made mostly of ice. That would cause some cool observations.

Come to think of it, that kind of extra gas in the planets atmosphere would cause great pressures on said planets, I dont know if life could evolve under that kind of pressure. An icy moon though, the size of our moon.........


Star
 
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crazyeddie

Guest
Re: Earthlike Double Planet Possible?

Fallingstar1971":1j20ubzg said:
I think it would be more interesting to put them on opposite sides of their parent star and have them discover each other by accident through their independent space programs.......If they were the same mass you might be able to pull it off.

For the orbit to be stable, each planet would have to be in one of the others Lagrange points.
Planets "on opposite sides of their parent stars" could never maintain such orbits. Minute gravitational perturbations from other objects in the system would eventually pull them out of this configuration, making them easily visible to each other. They would slowly drift towards each other along their shared orbital path and eventually collide.

I'm not quite sure what you mean regarding the Lagrange points comment.
 
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Fallingstar1971

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Re: Earthlike Double Planet Possible?

I meant exactly what I said. Any object that wants to share Earth(A)s orbit and not be disturbed by Earth(B) would have to reside at one of Earth(A)s Lagrange points. At the same time, Earth(A) would have to reside in one of Earth(B)s Lagrange points.

Just used more words..............

Im not factoring in the rest of the solar system here, one star, two planets on opposite sides of the star. Of course it would not work in our solar system. But thats a lot different than saying that it cant exist in ANY solar system.

Star
 
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MeteorWayne

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Re: Earthlike Double Planet Possible?

Which doesn't change the fact that unless they are the only two objects in the system other than the star, the orbits are unstable.
 
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Fallingstar1971

Guest
Re: Earthlike Double Planet Possible?

Your ability to restate the stated is simply amazing Wayne! Do you actually READ what is typed?

Star
 
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neilsox

Guest
Re: Earthlike Double Planet Possible?

Hi Star: I also often restate the stated. It is a way of saying, I agree and making it clear what i agree about. I think meteorwayne is doing an amazing job as a moderator plus more than a dozen posts per day. Give MW a break. No I don't reread all the posts in a long thread before I post: Does anyone? Neil
 
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crazyeddie

Guest
Re: Earthlike Double Planet Possible?

Fallingstar1971":3nez9rvx said:
I meant exactly what I said. Any object that wants to share Earth(A)s orbit and not be disturbed by Earth(B) would have to reside at one of Earth(A)s Lagrange points. At the same time, Earth(A) would have to reside in one of Earth(B)s Lagrange points.

Just used more words..............

Im not factoring in the rest of the solar system here, one star, two planets on opposite sides of the star. Of course it would not work in our solar system. But thats a lot different than saying that it cant exist in ANY solar system.

Star
I understand now....you wish to place your hypothetical planet at the L-3 point. Even if there were no other objects in this imaginary star system, it still wouldn't work. No planet orbit's it's star in a perfect circle, which would be a requirement for your "counter-Earth" to remain hidden. All planetary orbits follow elliptical paths, and because they speed up and slow down as they approach aphelion and perihelion, any object at L-3 would be visible regularly to it's opposite world.
 
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5hot6un

Guest
Preface - The following question is just for the sake of conversation.

If there were another planet in our solar system that had the same orbit length as our Earth and was on the opposite side of the sun from us, how could we detect it? Would it have to be the same size as Earth and be the same distance from the sun as Earth to match the speed at which we orbit?

Wouldn't this be a cool premise for a sci-fi story? I think so!
 
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MeteorWayne

Guest
Yes. If I might make a suggestion for new users. PLEASE take a little time to look around to see if there are other discussions already underway on the subject you are starting.

This will be merged with the latest one, which I think resides in the Unexplained or Sci Fi.
 
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Shpaget

Guest
As it was already stated in earlier posts, that arrangement of planets is very unstable and impossible in a system with anything else orbiting around that star. Just a slight perturbation of one of the "sister planets" (such as from a pass of a third planet, or an asteroid) would cause it to either slow down or speed up bringing it to a position that is no longer stable. Once those planets are no longer in perfect balance, they would start getting closer and closer and faster and faster.
 
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