Movie inspired question (Another Earth)

[Eats Crayons]

Basic premise of the movie: a duplicate planet Earth appears in our sky, including duplicates of us all.

Okay, that's fine, but wouldn't the gravitational forces of another Earth THAT CLOSE to us completely and utterly make a righteous mess of our world? I'm thinking tsunamis and earthquakes and other catastrophes. Yes?... No?.. Bueller? Bueller?

[Pere]

Yes. If the whole idea wasn't, like, a metaphor.

[Kat]

Movie physics work different.

[Flying Saucerian]

You know, you can just go ahead and hate the movie if you want. You don't have to try to justify it...

[Random Precision]

This sounds like a good question for the XKCD guy. In this weekly series, he gives very serious and thoughtful answers to somewhat inane questions. This week, he answers "What if a glass were truly half empty?" (as in a vacuum). I especially liked the one about a .99 light speed fastball and a mole (Avogadro's number) of moles.

[Eats Crayons]

Originally Posted by Peremensoe

Yes. If the whole idea wasn't, like, a metaphor.

I get that, and in any case, I just waved my arms in the air and said: "It's magic!" It didn't affect anything really in the story. I'm not interested in discussing the movie, or I would have gone to Arts and Entertainment.

I would like to know what would be the aftermath if a planet that size turned up in our orbit one day.

[Wolf Larsen]

Bad.

The earth-moon system orbits something called the barycenter, which is not at the center of the earth, but is inside it. Adding a second earth sized object makes a chaotic system where the moon probably gets ejected and the two planets end up orbiting a spot not inside either of them. This makes the tides worse, may change the length of the day and depending on the plane the other earth is moving in, may change the Earth's axis relative to the sun, which changes the seasons. Seismic activity probably increases.

If the other Earth is too close, it gets worse. There is a distance called the Roche limit. Closer than that, and one or both of the planets break up to form a ring of debris. If this Earth breaks up, game over. If the other one, we get pelted by zillions of meteors and, game over.

Plus, in gravity calculations, you can't just have something appear, it comes in from a long distance and starts perturbing orbits from the beginning. If the other body is big enough, the earth may even get pitched out of the solar system. The exact answer depends on how massive the other body is and what path it takes getting here and how close it gets.

[Jaglavak]

Adding any Earth-size mass to the solar system would be an unstable situation. It would eventually result in a planetary collision even if it started out on the other side of the sun from us.

Somehow putting one near the real Earth, say at the distance of the moon, would get us 40 foot tides, quakes across every continent, and tidal waves in every ocean. Those would probably be the only immediate effects. But it wouldn't be long before one of the planets ate the moon, and then it's all over. All the inner planets would be bombarded with debris for thousands of years.

[Mustard Stain]

I bet it would it look like the opening credits to Thundar the Barbarian. Y'know, the whole "runaway planet" part that makes the moon break into and creates creatures like Ooklah the Mok.

[Ganryu Kojiro]

Fritz Leiber wrote a novel about this back in the sixties called The Wanderer that goes into this idea (well, and Leiber's cat fetish goes in a wholly weird direction, but that's neither here nor there). It won a Hugo.

Plot follows a lot of different people around the globe and you see a lot of the nasty stuff a body like this so close to Earth would do.

[mothedrine]

Originally Posted by Jaglavak

Adding any Earth-size mass to the solar system would be an unstable situation. It would eventually result in a planetary collision even if it started out on the other side of the sun from us.

Somehow putting one near the real Earth, say at the distance of the moon, would get us 40 foot tides, quakes across every continent, and tidal waves in every ocean. Those would probably be the only immediate effects. But it wouldn't be long before one of the planets ate the moon, and then it's all over. All the inner planets would be bombarded with debris for thousands of years.

Dr Ray Stantz: What he means is Old Testament, Mr. Mayor, real wrath of God type stuff.
Dr. Peter Venkman: Exactly.
Dr Ray Stantz: Fire and brimstone coming down from the skies! Rivers and seas boiling!
Dr. Egon Spengler: Forty years of darkness! Earthquakes, volcanoes...
Winston Zeddemore: The dead rising from the grave!
Dr. Peter Venkman: Human sacrifice, dogs and cats living together... mass hysteria!

[RealityChuck]

Read The Wanderer by Fritz Leiber.

[SmartAleq]

And on the other side of the equation, the planet that came wandering in would be motherfucking cold and frozen and nobody'd be alive on that at all. It would probably make one hell of a comet tail coming in though.

[Harry]

That's just one of the many criticisms of the movie Independance Day. Spacecraft the size of those mother ships, parked in near Earth orbit, would do catastrophic damage all by themselves, without even considering an Earth clone in an orbit similiar to the moon's.

[Eats Crayons]

Originally Posted by Wolf Larsen

If the other Earth is too close, it gets worse. There is a distance called the Roche limit. Closer than that, and one or both of the planets break up to form a ring of debris. If this Earth breaks up, game over. If the other one, we get pelted by zillions of meteors and, game over.

That's fascinating thanks. Why would one or both break up as opposed to, for example, just smashing together like too magnets on a table that get too close to each other?

Plus, in gravity calculations, you can't just have something appear, it comes in from a long distance and starts perturbing orbits from the beginning.

But if it did just *poof* appear suddenly in the sky, would things get a bit more explosive? Or is it simply not something than can even be considered if you're actually considering reality?

[Eats Crayons]

Originally Posted by Harry

That's just one of the many criticisms of the movie Independance Day. Spacecraft the size of those mother ships, parked in near Earth orbit, would do catastrophic damage all by themselves, without even considering an Earth clone in an orbit similiar to the moon's.

How big or massive would something have to be before it started affecting Earth <edit:> every noticeably</edit>? I've blocked most of that silly movie out of my head, but I didn't think they were that massive, but maybe there were very large numbers of them.

[Wolf Larsen]

Originally Posted by Eats Crayons

That's fascinating thanks. Why would one or both break up as opposed to, for example, just smashing together like too magnets on a table that get too close to each other?

In stuff moving in space, there is no friction and momentum is strictly conserved. Unless the planets are aimed exactly at each other, they will do a dosi-do maneuver at closest approach. For the Earth, the Roche limit appears to be about 3 Earth radii. Closer than that and tidal forces are going to start tearing things apart. Here's a pretty good explanation. http://en.wikipedia.org/wiki/Roche_limit

Originally Posted by Eats Crayons

But if it did just *poof* appear suddenly in the sky, would things get a bit more explosive? Or is it simply not something than can even be considered if you're actually considering reality?

Can't do that. Massive objects bend space-time and the bending propagates at the speed of light. So unless you can move something in faster than light, the solar system has been reacting to it as it came in from far away.

[Wolf Larsen]

Originally Posted by Eats Crayons

How big or massive would something have to be before it started affecting Earth <edit:> every noticeably</edit>? I've blocked most of that silly movie out of my head, but I didn't think they were that massive, but maybe there were very large numbers of them.

It's a size and distance problem. Something a tenth the mass of the Earth is going to cause noticeable effects if it gets as close as the moon.

Here's a sim that you can probably get to do the interaction you want.

http://phet.colorado.edu/sims/my-sol...system_en.html

[Jaglavak]

Keep in mind that beneath a very thin rigid crust the earth is still mostly liquid. If you give it a gravitational shove, it's going to slosh. Sloshing is bad.

[Fish]

Originally Posted by Eats Crayons

But if it did just *poof* appear suddenly in the sky...

If Earth₂ suddenly appeared motionless in the sky, our Earth (E_P) would continue to move relative to that stationary position at something close to



or at about 66,660 miles per hour, plus or minus whatever gravitional effects of E₂. Of course, the solar system itself is also moving ... and the galaxy is moving ... and the universe expanding ... so "appearing at a single place" almost certainly means that we, on Earth, will zip away from it in jig time.

[Flying Saucerian]

But what if Superman or The Spectre or someone is pushing them apart?

[Mustard Stain]

Originally Posted by Flying Saucerian

But what if Superman or The Spectre or someone is pushing them apart?

Depends on if it's the usual Superman or the John Byrne's Superman who has a telekinetic force field so he can move things like whole buildings by one little corner without the building crumbling from its own weight.

[Jaglavak]

Not to worry. It would be only minutes before the IRS colonized the place and started sucking them dry too. Everything's under control.