# Earth's twin planet

Johnnyallen
I was watching a really bad Japanese sci-fi movie in which the writers employed a frequent plot device. The Earth had a twin planet opposite the sun which made it undetectable to earthlings.
This, of course, would be possible if the Earth's orbit was a perfect circle, which it isn't. It is, of course, elliptical.
Question: Given that the Earth's speed in it's orbit varies according to its distance from the sun, would a planet opposite the Earth always maintain its position behind the sun?

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In the ideal case that the Sun's gravitation was completely dominant and the eccentricities and radii were the same and their perihelion positions were on opposite sides of the Sun, yes. Now, any perturbations to this (such as the influence from other bodies in the solar system) would accumulate over time and break this situation, not to mention that you could discover the opposite planet using spacecraft.

PeroK
Johnnyallen
I'm new to this forum and am looking forward to future discussions, etc.
I didn't mention the obvious about discovering the planet with satellites or spacecraft because this was a very bad Japanese movie from the 60's

rootone
a very bad Japanese movie from the 60's
Well it might not have been as bad as you think considering that western sci fi movies at the time were concentrating on super heroes with magical powers.

russ_watters
...
This, of course, would be possible if the Earth's orbit was a perfect circle, which it isn't. It is, of course, elliptical...

I do not think circular orbits matter. Could be an extremely elliptical orbit, like a comet. Just has to be perfectly opposite.

You can hang out at 5 Lagrange points. Only L3 is on the far side of the sun.

There is also an asteroid called 3753 cruithne which has a 1 year orbit around the sun.

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I do not think circular orbits matter. Could be an extremely elliptical orbit, like a comet. Just has to be perfectly opposite.

You can hang out at 5 Lagrange points. Only L3 is on the far side of the sun.

There is also an asteroid called 3753 cruithne which has a 1 year orbit around the sun.
The problem with the L3 point is that it is unstable. The slightest drift from that perfect balance point and the combined gravity of the sun and Earth will tend pull it even further out of position. This is unlike the L5 and L4 points, where a slightly dislodged object will tend drift back into place.

Johnnyallen
Johnnyallen
Well it might not have been as bad as you think considering that western sci fi movies at the time were concentrating on super heroes with magical powers.
Not to labor the point but Hollywood produced some sci fi clunkers in the 50's excluding the classic Plan 9 From outer Space.

Not to labor the point but Hollywood produced some sci fi clunkers in the 50's excluding the classic Plan 9 From outer Space.
A trip to the moon dates to 1902.

rootone
A trip to the moon dates to 1902.
Very interesting, though somehow I got a psychedelic 60's impression of it.

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JMz
Back on the OP: The Earth's Twin's gravity will also perturb the orbits of the inner planets. My guess is that, even as early as the 19th Century, people would have easily been able to deduce its presence from them.

This is the same story as the discovery of non-Newtonian effects on Mercury's orbit and of Neptune itself, both of which were indeed detected in the 19th C (though people didn't know what to make of Mercury's orbital anomalies at the time). However, I don't have the numbers for an Earth's Twin.

unusually_wrong and Johnnyallen
Johnnyallen
The problem with the L3 point is that it is unstable. The slightest drift from that perfect balance point and the combined gravity of the sun and Earth will tend pull it even further out of position. This is unlike the L5 and L4 points, where a slightly dislodged object will tend drift back into place.
Briefly, I don't have a degree in Physics. I credit Carl Sagan's Cosmos for igniting my interest in it and Cosmology. Your reference to Lagrange points got me to look it up. I had heard something about them, but that's about it.
The material I read mentions the GAIA satellite at L2. I had heard of GAIA but not exactly what it does.
I subscribe to a YouTube channel Space Time. Those guys are awesome. Their short lectures are captivating although a little hard to follow sometimes. And sometimes the material is a little over my head.
Gabe's series on General Relativity is, as promised, mind blowing.
I recently watched one on GAIA'S contribution to Astrometry. Wow is about all I can say.

JMz
JMz
Briefly, I don't have a degree in Physics. I credit Carl Sagan's Cosmos for igniting my interest in it and Cosmology. Your reference to Lagrange points got me to look it up. I had heard something about them, but that's about it.
The material I read mentions the GAIA satellite at L2. I had heard of GAIA but not exactly what it does.
I subscribe to a YouTube channel Space Time. Those guys are awesome. Their short lectures are captivating although a little hard to follow sometimes. And sometimes the material is a little over my head.
Gabe's series on General Relativity is, as promised, mind blowing.
I recently watched one on GAIA'S contribution to Astrometry. Wow is about all I can say.
Gaia is in a (gravitationally) unstable Lagrange point. It is actively stabilized by non-gravitational means. Like being on the top of a hill, gravity works against staying there, but it's a lot easier to keep yourself in place there, with small forces, than to keep yourself in place on the side of hill, where you need much larger ones.

Johnnyallen
Gaia is in a (gravitationally) unstable Lagrange point. It is actively stabilized by non-gravitational means. Like being on the top of a hill, gravity works against staying there, but it's a lot easier to keep yourself in place there, with small forces, than to keep yourself in place on the side of hill, where you need much larger ones.
I had to do some reading up on Lagrange points. So GAIA is at L2 and is in a, for lack of a better phrase, lock step orbit with the Earth opposite the sun. Was it Newton that attributed the inverse square law to Keppler's planetary motions, so wouldn't GAIA's orbital speed be slower than the Earth's or is the Earth's mass (General Relativity) dragging the satellite along with it?

I had to do some reading up on Lagrange points. So GAIA is at L2 and is in a, for lack of a better phrase, lock step orbit with the Earth opposite the sun. Was it Newton that attributed the inverse square law to Keppler's planetary motions, so wouldn't GAIA's orbital speed be slower than the Earth's or is the Earth's mass (General Relativity) dragging the satellite along with it?

GAIA or any satelite place at L2 is orbiting both the Sun and the Earth. It has a 365 day orbit. Both 365 days around Earth and 365 days around the sun.

For L2 the orbital speed is higher than Earth's orbital speed. For L1 the orbital speed around the sun is lower. Compare to the moon which is sometimes orbiting the sun faster (full moon), sometimes orbiting slower (new moon) and sometimes about the same (quarter). You are also orbiting the sun faster at midnight than you are during lunch. It is not breaking Kepler's laws.

Yes Newton did the inverse square.

Johnnyallen
GAIA or any satelite place at L2 is orbiting both the Sun and the Earth. It has a 365 day orbit. Both 365 days around Earth and 365 days around the sun.

For L2 the orbital speed is higher than Earth's orbital speed. For L1 the orbital speed around the sun is lower. Compare to the moon which is sometimes orbiting the sun faster (full moon), sometimes orbiting slower (new moon) and sometimes about the same (quarter). You are also orbiting the sun faster at midnight than you are during lunch. It is not breaking Kepler's laws.

Yes Newton did the inverse square.
Thanks for describing GAIA's orbit. I was confused. The material that I read about Lagrange points had a graphic of the points with a little picture of GAIA at L2. But after double checking the material, it shows an orbital path for GAIA around L2 which I really kinda overlooked.
With what you describe and what the graphic shows, it is making sense now.

Johnnyallen
GAIA or any satelite place at L2 is orbiting both the Sun and the Earth. It has a 365 day orbit. Both 365 days around Earth and 365 days around the sun.

For L2 the orbital speed is higher than Earth's orbital speed. For L1 the orbital speed around the sun is lower. Compare to the moon which is sometimes orbiting the sun faster (full moon), sometimes orbiting slower (new moon) and sometimes about the same (quarter). You are also orbiting the sun faster at midnight than you are during lunch. It is not breaking Kepler's laws.

Yes Newton did the inverse square.
Thanks for describing GAIA's orbit. I was confused. The material that I read about Lagrange points had a graphic of the points with a little picture of GAIA at L2. But after double checking the material, it shows an orbital path for GAIA around L2 which I really kinda overlooked.
With what you describe and what the graphic shows, it is making sense now.

JMz
is the Earth's mass (General Relativity) dragging the satellite along with it?
This is Newtonian gravity: no need for Einstein (which was good for Lagrange, since he didn't know about GR ;-).
orbital path for GAIA around L2
Yes, this is the "balancing on top of the hill" part: The satellite would spiral away from L2, slowly at first, were it not for active stabilization. Slowly is the key for stabilization, and it's generally available only at the 5 Lagrangian points.

Johnnyallen
Mr Wolf
I was watching a really bad Japanese sci-fi movie in which the writers employed a frequent plot device. The Earth had a twin planet opposite the sun which made it undetectable to earthlings.
This, of course, would be possible if the Earth's orbit was a perfect circle, which it isn't. It is, of course, elliptical.
Question: Given that the Earth's speed in it's orbit varies according to its distance from the sun, would a planet opposite the Earth always maintain its position behind the sun?

What movie was that? Googling it, I couldn't find it. :/
Some times ago, I watched a movie called "Another Earth", in which a "copy" of the Earth was discovered.
Then, there is "Melancholia" (by von Trier - one the worst directors ever ), in which a planet from nowhere is going to collide with Earth.

Glenstr
What movie was that? Googling it, I couldn't find it. :/
Some times ago, I watched a movie called "Another Earth", in which a "copy" of the Earth was discovered.
Then, there is "Melancholia" (by von Trier - one the worst directors ever ), in which a planet from nowhere is going to collide with Earth.

Saw both of those movies, Another Earth I thought was pretty good as a low budget venture, interesting story. Melancholia was just plain strange..

Mr Wolf
Johnnyallen
What movie was that? Googling it, I couldn't find it. :/
Some times ago, I watched a movie called "Another Earth", in which a "copy" of the Earth was discovered.
Then, there is "Melancholia" (by von Trier - one the worst directors ever ), in which a planet from nowhere is going to collide with Earth.
The movie was called Gorath. The gist was that a red hot planet was heading to collide with the Earth. All the plot descriptions that I found don't allude to it as being a twin planet.
One of those extra off-air networks called Comet TV showed it. They rerun a lot of stuff, so according to their website, it will be shown at 12 PM ET on May 29. I'll be sure to watch it again.

Mr Wolf
Johnnyallen
Saw both of those movies, Another Earth I thought was pretty good as a low budget venture, interesting story. Melancholia was just plain strange..
The movie was called Gorath. The gist was that a red hot planet was heading to collide with the Earth. All the plot descriptions that I found don't allude to it as being a twin planet.
One of those extra off-air networks called Comet TV showed it. They rerun a lot of stuff, so according to their website, it will be shown at 12 PM ET on May 29. I'll be sure to watch it again.

BTW I'm new to this forum. I copied and pasted this from a reply to another person because I don't know if you'll get an alert.
Also, what does quote do?

Johnnyallen
Saw both of those movies, Another Earth I thought was pretty good as a low budget venture, interesting story. Melancholia was just plain strange..
I stand corrected, The movie was not Gorath but Warning From Space.

rootone
Warning From Space.
"Spacemen Appear in Tokyo!" seems to be close to the correct translation

Mentor
This thread has become a movie review, not Science. Move to general discussion SF section.