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Earth's twin planet

  1. May 17, 2018 #1
    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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  3. May 17, 2018 #2

    Orodruin

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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.
     
  4. May 17, 2018 #3
    Thanks for the quick reply.
    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
     
  5. May 17, 2018 #4
    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.
     
  6. May 17, 2018 #5

    stefan r

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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.
     
  7. May 17, 2018 #6

    Janus

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    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.
     
  8. May 18, 2018 #7
    Not to labor the point but Hollywood produced some sci fi clunkers in the 50's excluding the classic Plan 9 From outer Space.
     
  9. May 18, 2018 #8

    stefan r

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    A trip to the moon dates to 1902.
     
  10. May 18, 2018 #9
    Very interesting, though somehow I got a psychedelic 60's impression of it.
     
    Last edited: May 18, 2018
  11. May 19, 2018 #10

    JMz

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    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.
     
  12. May 19, 2018 #11
    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.
    BTW, thanks for your reply.
     
  13. May 19, 2018 #12

    JMz

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    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.
     
  14. May 21, 2018 #13
    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?
     
  15. May 21, 2018 #14

    stefan r

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    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.
     
  16. May 21, 2018 #15
    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.
     
  17. May 21, 2018 #16
    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.
     
  18. May 21, 2018 #17

    JMz

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    This is Newtonian gravity: no need for Einstein (which was good for Lagrange, since he didn't know about GR ;-).
    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.
     
  19. May 22, 2018 #18
    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.
     
  20. May 23, 2018 #19
    Saw both of those movies, Another Earth I thought was pretty good as a low budget venture, interesting story. Melancholia was just plain strange..
     
  21. May 23, 2018 #20
    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.
     
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