<p>It would not be a stable configuration, but it wouldn't be too far from the stability.</p>

<p>The are 5 points where a third body could have a semi-stable orbit in a 2-body system:</p>

<p><a href="https://i.sstatic.net/Yt1N0.gif" rel="noreferrer"><img src="https://i.sstatic.net/Yt1N0.gif" alt="Lagrange points"></a></p>

<p>The Earth is the blue point. The "anti-Earth" on the other side would be in the third Lagrange point (<span class="math-container">$\rm{L}_3$</span>).</p>

<p>However, this is not a stable orbit. Any small deviation, including the effect of possible fourth bodies, would change the system so, that it results yet more deviation. Only <span class="math-container">$\rm{L}_4$</span> and <span class="math-container">$\rm{L}_5$</span> are stable, and even this has some requirements (the third body can be at most <span class="math-container">$\approx$</span> 3.5% of the second).</p>

<p>Corresponding this, the space on the other side of our Sun was examined by some space probes already, and there is nothing.</p>

CC BY-SA 4.0
2019-09-12T18:02:11.360Z
Sorry this was a duplicate question, thank you for answering though.
throwaway
CC BY-SA 4.0
2019-09-12T20:51:30.597Z
@throwaway No, it is not a really dupe - only one of the questions cited by uhoh are on this site, this: https://astronomy.stackexchange.com/q/16239/1891. It could be an original candidate. There are no cross-site dupes in the StackExchange model. If you are not sure that it would be a dupe, a possibility is that you initiate its closure as dupe. But, only posting a dupe is not a big problem (if the post is voted up). Don't remove the post, just close as dupe. I rollbacked your edition.
peterh
CC BY-SA 4.0
2019-09-18T10:36:43.730Z
Perhaps worth noting we also examined L4 and L5 and DID find something in L4, 2010 TK7.
Maury Markowitz
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2019-09-12T16:31:17.933Z
2019-09-12T16:31:17.933Z
peterh
<p>Would two Earth-like planets on opposite sides of a Sol-like star at about 1 AU be a relatively stable configuration? I understand that there are tadpole-like orbits and the two planets may not stay exactly opposite each other, but oscillate around those position.</p>

<p>But given perturbations from other planets, say a Jupiter-like one at say 5 AU, might they eventually crash into each other, or would one end up moving to a higher orbit and the other to a lower orbit?</p>

CC BY-SA 4.0
2019-09-12T15:34:43.913Z
@MikeG rather than have the question closed, I've modified it to stay on-topic.
uhoh
CC BY-SA 4.0
2019-09-12T16:04:40.780Z
Note, some probes already checked, what is on the other side of the Sun. The answer is: nothing. Just empty space. Interplanetar gas. Nearly empty vacuum.
peterh
CC BY-SA 4.0
2019-09-12T16:23:29.397Z
@peterh that reminds me! [Why would Hollywood's Planet X (at earth's L3) be unstable?](https://space.stackexchange.com/q/13947/12102). Also see [Have any co-orbital exoplanet pairs been discovered (and not subsequently retracted)?](https://astronomy.stackexchange.com/q/16239/7982) as well as [In “The Man from Planet X” was Planet X ever directly behind the sun w/ same orbit as earth?](https://scifi.stackexchange.com/q/117611/51174) and while we're at it, Wikipedia's [Co-orbital configuration](https://en.wikipedia.org/wiki/Co-orbital_configuration)
uhoh
CC BY-SA 4.0
2019-09-12T16:38:40.097Z
@uhoh Well, then it might be a dupe.
peterh
CC BY-SA 4.0
2019-09-12T17:56:59.993Z
@peterh Sorry, I didn't realize it was a dupe, I searched and didn't find anything. Sorry this was outside my area and didn't know, I will delete

Nevermind can't delete, sorry
throwaway
CC BY-SA 4.0
2019-09-12T18:13:04.550Z
I don't think it's a duplicate. A modified version of your original question would be OK.
Mike G
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2019-09-12T18:16:51.527Z
Still was wrong place for question as was hypothetical, I should have done more research before posting, my apologies
throwaway
CC BY-SA 4.0
2019-09-12T20:48:33.220Z
Possible duplicate of [Have any co-orbital exoplanet pairs been discovered (and not subsequently retracted)?](https://astronomy.stackexchange.com/questions/16239/have-any-co-orbital-exoplanet-pairs-been-discovered-and-not-subsequently-retrac)
peterh
CC BY-SA 4.0
2019-09-12T22:35:19.110Z
@throwaway don't worry too much. Some more info: 1) to be considered a duplicate your question would have to be *answered* there, 2) it takes five votes to close (vtc) to put the question on hold, 3) there's nothing wrong with an occasional question being closed. In this case it would have been hard for you to have found a question about co-orbital planets not knowing the specific term ahead of time. 4) Don't be hasty to delete things unless they start collecting down votes. When an answer is posted, it becomes nearly impossible to delete a question. *Welcome to Stack Exchange!*
uhoh
CC BY-SA 4.0
2019-09-12T23:11:21.903Z
This is a useful question with a clear, useful answer, it makes a worthwhile addition to our site library and it’s certainly *not* a duplicate of the other Astronomy.SE question. It should neither be closed nor deleted! :-)
Chappo Hasn't Forgotten
<orbit><exoplanet><orbital-mechanics>
Would two Earth-like planets on opposite sides of a Sol-like star at about 1 AU be a stable configuration?
throwaway
# Would two Earth-like planets on opposite sides of a Sol-like star at about 1 AU be a stable configuration?

(Asked by: username_0 on 2019-09-12)

Would two Earth-like planets on opposite sides of a Sol-like star at about 1 AU be a relatively stable configuration? I understand that there are tadpole-like orbits and the two planets may not stay exactly opposite each other, but oscillate around those position.

But given perturbations from other planets, say a Jupiter-like one at say 5 AU, might they eventually crash into each other, or would one end up moving to a higher orbit and the other to a lower orbit?

**Tags:** orbit, exoplanet, orbital-mechanics
> **Comments:**
> **username_1**: @MikeG rather than have the question closed, I've modified it to stay on-topic.  
> **username_2**: Note, some probes already checked, what is on the other side of the Sun. The answer is: nothing. Just empty space. Interplanetar gas. Nearly empty vacuum.  
> **username_1**: @peterh that reminds me! [Why would Hollywood's Planet X (at earth's L3) be unstable?](https://space.stackexchange.com/q/13947/12102). Also see [Have any co-orbital exoplanet pairs been discovered (and not subsequently retracted)?](https://astronomy.stackexchange.com/q/16239/7982) as well as [In “The Man from Planet X” was Planet X ever directly behind the sun w/ same orbit as earth?](https://scifi.stackexchange.com/q/117611/51174) and while we're at it, Wikipedia's [Co-orbital configuration](https://en.wikipedia.org/wiki/Co-orbital_configuration)  
> **username_2**: @uhoh Well, then it might be a dupe.  
> **username_0**: @peterh Sorry, I didn't realize it was a dupe, I searched and didn't find anything. Sorry this was outside my area and didn't know, I will delete  Nevermind can't delete, sorry  
> **username_3**: I don't think it's a duplicate. A modified version of your original question would be OK.  
> **username_0**: Still was wrong place for question as was hypothetical, I should have done more research before posting, my apologies  
> **username_2**: Possible duplicate of [Have any co-orbital exoplanet pairs been discovered (and not subsequently retracted)?](https://astronomy.stackexchange.com/questions/16239/have-any-co-orbital-exoplanet-pairs-been-discovered-and-not-subsequently-retrac)  
> **username_1**: @throwaway don't worry too much. Some more info: 1) to be considered a duplicate your question would have to be *answered* there, 2) it takes five votes to close (vtc) to put the question on hold, 3) there's nothing wrong with an occasional question being closed. In this case it would have been hard for you to have found a question about co-orbital planets not knowing the specific term ahead of time. 4) Don't be hasty to delete things unless they start collecting down votes. When an answer is posted, it becomes nearly impossible to delete a question. *Welcome to Stack Exchange!*  
> **username_4**: This is a useful question with a clear, useful answer, it makes a worthwhile addition to our site library and it’s certainly *not* a duplicate of the other Astronomy.SE question. It should neither be closed nor deleted! :-)  

---

## 1 answers

### Answer by username_2 on 2019-09-12. Score: 5

It would not be a stable configuration, but it wouldn't be too far from the stability.

The are 5 points where a third body could have a semi-stable orbit in a 2-body system:

[![Lagrange points](https://i.sstatic.net/Yt1N0.gif)](https://i.sstatic.net/Yt1N0.gif)

The Earth is the blue point. The "anti-Earth" on the other side would be in the third Lagrange point ($\rm{L}\_3$).

However, this is not a stable orbit. Any small deviation, including the effect of possible fourth bodies, would change the system so, that it results yet more deviation. Only $\rm{L}\_4$ and $\rm{L}\_5$ are stable, and even this has some requirements (the third body can be at most $\approx$ 3.5% of the second).

Corresponding this, the space on the other side of our Sun was examined by some space probes already, and there is nothing.

> **Comments:**
> **username_0**: Sorry this was a duplicate question, thank you for answering though.  
> **username_2**: @throwaway No, it is not a really dupe - only one of the questions cited by uhoh are on this site, this: https://astronomy.stackexchange.com/q/16239/1891. It could be an original candidate. There are no cross-site dupes in the StackExchange model. If you are not sure that it would be a dupe, a possibility is that you initiate its closure as dupe. But, only posting a dupe is not a big problem (if the post is voted up). Don't remove the post, just close as dupe. I rollbacked your edition.  
> **username_5**: Perhaps worth noting we also examined L4 and L5 and DID find something in L4, 2010 TK7.  

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