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Traab
2013-10-03, 05:59 AM
Ok, lets say we have the technology to create a planet. As one random example, we can smash enough asteroids together into a single mass to form a planet sized body. We then place it into orbit around the sun at the exact opposite point as the earth, carefully designed so they stay that distance apart. Does this have any effect on earth itself? Does having an extra planet across the way from us do anything to our orbital path or the paths of the other planets in our system?

Spiryt
2013-10-03, 06:13 AM
Depends on it's size... 'Planet sized' is pretty broad.

Manga Shoggoth
2013-10-03, 06:27 AM
Assuming that the planet has the same mass as Earth, and is inserted with the correct orbital velocity:

Initially, not a lot. Your counter-earth would take up its orbit and happly fling itself round the sun. You would have difficulty keeping it "exactly opposite" as planets do not move at a constant speed - instead, they sweep out the same area in their orbit in a given time (google Kepler's Laws for the details), but it should stay more-or-less on the opposite side of the orbit.

Gravitational effects between planetry bodies are very small, and generally swamped by the Sun. The only reason the Moon has an effect on us is because it is 1/6 our size and very close. Over a very long period of time you would get some pertubations.

Over extremely long periods of time I suspect that the system is unstable, and one or both planets would end up ejected from orbit.

The most immedate effect, however, is that Earth would no longer be a planet according to the IAU definition, as it would no longer have has cleared the neighbourhood around its orbit. As a result we would all come from Dwarf Planet Earth.

Traab
2013-10-03, 06:51 AM
Depends on it's size... 'Planet sized' is pretty broad.

Yeah sorry, I meant earth sized. It was something that popped into my head durin a discussion on alien invasions, the reasons for them, if there is a way we could actually fight back against them, and what they might do instead. I found myself thinking, if they can mine asteroids and do terraforming, why couldnt they basically make their own earth and put it into orbit far enough away from us to not cause trouble? Then I wondered if that would even work.


Manga, Yeah, I wasnt sure if they could naturally stay opposite of each other or not. Of course, then my mental train went off the rails and considered the possibility of planet moving sized orbital thrusters to help correct any drift involved. Hey, clearly we had the means to move asteroids and slam them together into a single planetary body, upgrading an extra couple of sizes should be possible. Especially since its less "slam together" and more "nudge gently back into position." Of course, that all depends on what "an extremely long time" might be. Are we talking personal extremely long time? Or astrological extremely long time? Cause one could mean second earth would be secure till the sun is about ready to go red giant anyways. The other could mean a few generations of stable orbit then things start getting wonky.

factotum
2013-10-03, 07:47 AM
It wouldn't be stable. There is a Lagrange point (L3) which is on the opposite side of the Sun from us (albeit slightly closer to the Sun than we are)--a body in that location would orbit in the same period as Earth due to the combination of the Sun and Earth's gravity. However, it's not a stable Lagrange point, so any planet there would rapidly deviate from its opposite position due to gravitational influences from other planets.

If you wanted to put a planet in the same orbit as Earth that would actually stay where you put it, you'd need to be looking at the L4 and L5 points which are sixty degrees ahead of and behind the Earth in its orbit--those points are stable. In the case of Jupiter both its L4 and L5 points (also called Trojan points) are home to large collections of asteroids that have settled there over the years.

Manga Shoggoth
2013-10-03, 08:33 AM
In fact, building on Factotum's comments: Wikipedia gives a brief description of Counter-Earth (http://en.wikipedia.org/wiki/Counter-Earth) with a pretty picture.

The system would be stable(ish) for a few thousand years, but would still fall apart fairly quickly (in geological and astronomical terms, at any rate).

Jormengand
2013-10-03, 08:53 AM
Given that g=GM/R^2, G is always 6.67*10^-11, M is 5.972*10^24 kg, and R is 149,600,000*2 km, g would be 6.67*5.972*10^13/8952.064*10^13= 0.00445 m/s^2, which is not a lot, assuming I did the maths right (big assumption there). Long story short, you're probably more likely to be thrown off course by a meteor strike than Earth's evil twin slowly dragging you away. Of course, the other problem is that it might do things to the other planets, causing a sort of butterfly effect on a planetary scale as Nega-Earth drags mars out of orbit, slightly, causing the other planets to go out of orbit slightly, and so on. We're not sure.

Lord Raziere
2013-10-03, 08:59 AM
Heh, now I have idea for a very advanced civilization where a stupid rich guy makes a counter-earth because he can and when people protest that it makes the gravitational orbit stuff unstable, he replies "yeah but won't happen for thousands years! we got plenty of time until we need to worry about that, whose up for party-earth? lets make this planet much better than old earth!"

shawnhcorey
2013-10-03, 09:04 AM
It wouldn't be stable. There is a Lagrange point (L3) which is on the opposite side of the Sun from us (albeit slightly closer to the Sun than we are)--a body in that location would orbit in the same period as Earth due to the combination of the Sun and Earth's gravity. However, it's not a stable Lagrange point, so any planet there would rapidly deviate from its opposite position due to gravitational influences from other planets.

If you wanted to put a planet in the same orbit as Earth that would actually stay where you put it, you'd need to be looking at the L4 and L5 points which are sixty degrees ahead of and behind the Earth in its orbit--those points are stable. In the case of Jupiter both its L4 and L5 points (also called Trojan points) are home to large collections of asteroids that have settled there over the years.

Those points would be unstable too because the planet is the same size as Earth. L4 and L5 would only work for small masses.

This is what would happen (http://en.wikipedia.org/wiki/Giant_impact_hypothesis) in a few million years.

Jormengand
2013-10-03, 09:06 AM
Heh, now I have idea for a very advanced civilization where a stupid rich guy makes a counter-earth because he can and when people protest that it makes the gravitational orbit stuff unstable, he replies "yeah but won't happen for thousands years! we got plenty of time until we need to worry about that, whose up for party-earth? lets make this planet much better than old earth!"

The problem is probably not that it would make Earth's orbit unstable so much as Mars'. Because it's essentially going to be in places it shouldn't at times it shouldn't, it will have a funny effect on Mars, especially because it's closer to Mars at some points than it will ever be to Earth. Essentially, Earth's pull on Mars is on average being equaled by Nega-Earth, meaning that Mars is being pulled by the combo at double speed. That's going to knock Mars out of orbit, which will knock other things out of orbit...

Essentially, bad idea guys. The specifics would depend on which exact cubic centimetre you stuck it in, so it would be unpredictable at best.

pendell
2013-10-03, 09:06 AM
Hmmm .... let's assume the aliens need an earthlike planet. Instead of creating a new earth from scratch, could they instead use the mass to bulk up Mars? My understanding is that Mars is uninhabitable due in part to its small size . Give it more bulk to maintain a proper magnetic field so the atmosphere doesn't escape, then modify the atmosphere such that it is much more efficient at trapping heat. While I am unhappy with modern global warming theories (another topic for another day), might it be possible to take the same principles that might melt the icecaps on earth to give Mars a breathable atmosphere? To take modern climate change theory as sort of a precursor to terraforming other planets by deliberately engineering the changes there we may accidentally be causing here?

So if we have the technology to create a 'counter earth', would terraforming Mars be a simpler problem that would be in the grasp of the same technology, with fewer problems related to orbital stability?

Respectfully,

Brian P.

Jormengand
2013-10-03, 09:08 AM
fewer problems related to orbital stability?

By changing the mass of Mars, you're essentially inflicting this problem on a smaller scale. However, stuff in that order of magnitude that happens every few hundred thousand years anyway, so we'd probably be fine.

thubby
2013-10-03, 09:19 AM
it would be fine on any useful scale. it would eventually disrupt our orbit but I'm pretty sure the sun would explode first.

Traab
2013-10-03, 11:12 AM
It wouldn't be stable. There is a Lagrange point (L3) which is on the opposite side of the Sun from us (albeit slightly closer to the Sun than we are)--a body in that location would orbit in the same period as Earth due to the combination of the Sun and Earth's gravity. However, it's not a stable Lagrange point, so any planet there would rapidly deviate from its opposite position due to gravitational influences from other planets.

If you wanted to put a planet in the same orbit as Earth that would actually stay where you put it, you'd need to be looking at the L4 and L5 points which are sixty degrees ahead of and behind the Earth in its orbit--those points are stable. In the case of Jupiter both its L4 and L5 points (also called Trojan points) are home to large collections of asteroids that have settled there over the years.

Too be honest, I mainly said opposite earth to try and make it as far away from earth as possible while still being in the same general area for an earth-like set of conditions as possible. Gravity, energy from the sun, etc etc etc. I figured that would cause the fewest potential problems. If putting this second earth in those other spots would be stable and not cause excessive problems then awesome. The whole thing about effecting mars though, yeesh, thats a stickier problem since, unlike an unstable orbit that could possibly be adjusted by alien tech capable of creating worlds, thats a gradual effect on everything in the solar system with no way too be sure of the eventual result.

shawnhcorey
2013-10-03, 12:20 PM
But we are sure of the eventual result: it will be flung out to the outer solar system or crash with one of the inner planets. Anything the size of another Earth inside Jupiter's orbit will do this.

pendell
2013-10-03, 01:31 PM
Maybe so, but on what time scale? The solar system is only projected to last 5 billion years. If we introduce a problem that only becomes serious in multiple billion years, it's not as much of an issue as, say, trying to share the earth with aliens, which would be a problem in the years to decades .

Respectfully,

Brian P.

Manga Shoggoth
2013-10-03, 02:21 PM
The Wikipedia article linked above suggests a lifetime in the order of a few thousand years.

I'm searching for something a little more solid, but the echo chamber mostly points back to the Wikipedia article...

shawnhcorey
2013-10-03, 02:48 PM
I was thinking 1 to2 million years.

Traab
2013-10-03, 03:11 PM
Can you define "flung?" Are we talking one day thousands of year, (if not longer) from when the new planet is placed our orbit extends so far we drift off into space and the next 5 years get considerably less hospitable on earth as we gradually drift away? Or are we talking some horrible sling shot effect where within a year we are zooming out past jupiter but we dont realize it because the forces involved have killed us all already?

shawnhcorey
2013-10-03, 03:21 PM
Can you define "flung?" Are we talking one day thousands of year, (if not longer) from when the new planet is placed our orbit extends so far we drift off into space and the next 5 years get considerably less hospitable on earth as we gradually drift away? Or are we talking some horrible sling shot effect where within a year we are zooming out past jupiter but we dont realize it because the forces involved have killed us all already?

The last one; a close encounter with another big object.

factotum
2013-10-03, 05:03 PM
Give it more bulk to maintain a proper magnetic field so the atmosphere doesn't escape

Couple of misconceptions there: firstly, adding bulk to Mars would not affect its magnetic field, because that's generated by the core, and the core of Mars appears to be pretty much dead. However, it's mainly gravity that prevents the atmosphere disappearing off into deep space--yes, magnetic fields help to deflect the solar wind, which will help somewhat, but you only have to look at Venus (which I believe has little or no detectable magnetic field, yet an atmosphere 90 times denser than Earth's) to see that magnetic field isn't the whole story by any means.

Salbazier
2013-10-04, 10:47 AM
Couple of misconceptions there: firstly, adding bulk to Mars would not affect its magnetic field, because that's generated by the core, and the core of Mars appears to be pretty much dead. However, it's mainly gravity that prevents the atmosphere disappearing off into deep space--yes, magnetic fields help to deflect the solar wind, which will help somewhat, but you only have to look at Venus (which I believe has little or no detectable magnetic field, yet an atmosphere 90 times denser than Earth's) to see that magnetic field isn't the whole story by any means.

Venus just have too much atmosphere that it still would have maintain a lot despite erosion by solar wind EDIT: from short check to Wikipedia, apparently thicker atmosphere also make it harder for solar wind to erode it. Those gases from came its ocean boiling over. Obviously, from how Venus look, keeping atmosphere that way isn't the way to make a planet habitable.

I don't have any number for how fast the rate atmospheric loss by solar wind erosion and by escaping gas, but if we can move something around Earth mass easily enough, perhaps we could do a constant atmosphere replenishment for Mars? Send a lot of gas there for a start and maintain a constant supply afterward. No need to do mass increase nor magnetic field reactivation