# Earth 2 - how would we know? **URL:** \ **Category:** Factual Questions\ **Created:** [March 18, 2011, 1:26pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145 "2011-03-18T13:26:19Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Gyrate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gyrate/32/3588_2.png) [@Gyrate](https://boards.straightdope.com/u/Gyrate)\ **Post date:** [March 18, 2011, 1:26pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/1 "2011-03-18T13:26:19Z")
Some idle speculation for you all: Let us hypothesize that there exists a second planet in Earth’s orbit, which stays at exactly the opposite point in the orbit (i.e. half a year around the orbit). Let us further say that this planet matches exactly Earth’s size, mass and albedo, and - what the heck - let’s say it’s got a similar sized moon. What evidence would we have that it was there? Given our elliptical orbit would we be able to see it at cetain points of the year? Would it have an effect on Earth at all? Or other planets? (Do not Need Answer Fast) ---
**Author:** ![kferr](https://avatars.discourse-cdn.com/v4/letter/k/71e660/32.png) [@kferr](https://boards.straightdope.com/u/kferr)\ **Post date:** [March 18, 2011, 1:34pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/2 "2011-03-18T13:34:11Z")
It would perturb the orbits of the other planets, particularly Venus and Mars since they’re the closest. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 18, 2011, 1:40pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/3 "2011-03-18T13:40:36Z")
We could probably figure out it was there by observing its influence on other planets. In particular, the [perihelion precession of Mercury](http://en.wikipedia.org/wiki/Perihelion_precession_of_Mercury#Perihelion_precession_of_Mercury) depends in large part on the other planets in the solar system. Another Earth would cause Mercury’s orbit to (I think) precess faster than it already does. ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [March 18, 2011, 2:01pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/4 "2011-03-18T14:01:09Z")
The situation you describe would be unstable. Two planets exactly opposing each other will only remain stable so long as the positions of the planets are perfectly mirrored. If either gets even a tiny bit away from being perfectly opposite the other, the uneven gravitational forces will pull them further and further out of alignment, eventually resulting in the two worlds either hitting each other or passing close enough to throw both of them into very different orbits. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [March 18, 2011, 2:13pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/5 "2011-03-18T14:13:20Z")
Orbits sweep out equal areas in equal time. Since we have an elliptical orbit the swept out angles are different at different time. We’d see something though as has been mentioned it’s not a really stable configuration. ---
**Author:** ![Gyrate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gyrate/32/3588_2.png) [@Gyrate](https://boards.straightdope.com/u/Gyrate)\ **Post date:** [March 18, 2011, 2:28pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/6 "2011-03-18T14:28:01Z")
> [@AndrewL](#): > > The situation you describe would be unstable. Two planets exactly opposing each other will only remain stable so long as the positions of the planets are perfectly mirrored. If either gets even a tiny bit away from being perfectly opposite the other, the uneven gravitational forces will pull them further and further out of alignment, eventually resulting in the two worlds either hitting each other or passing close enough to throw both of them into very different orbits. I assumed this would be the case and that there’s no way to have multiple stable planets in the same orbit occur naturally. I was just wondering what it would look like if we did. And I figured during spring/autumn the planet opposite would be unobstructed by the sun but thought you good people might be able to confirm that through the mighty power of Math. ---
**Author:** ![sevenwood](https://avatars.discourse-cdn.com/v4/letter/s/6de8d8/32.png) [@sevenwood](https://boards.straightdope.com/u/sevenwood)\ **Post date:** [March 18, 2011, 2:30pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/7 "2011-03-18T14:30:49Z")
> [@Gyrate](#): > > Some idle speculation for you all: Hah! You can’t fool me! You’ve been reading [Alley Oop](http://comics.com/alley_oop/2011-02-20/)! ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [March 18, 2011, 2:36pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/8 "2011-03-18T14:36:55Z")
Well, as long as the super-science technology of the Priest-Kings is still working, the Counter-Earth can be kept in an artificially stable position directly opposite us. ---
**Author:** ![Gyrate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gyrate/32/3588_2.png) [@Gyrate](https://boards.straightdope.com/u/Gyrate)\ **Post date:** [March 18, 2011, 2:43pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/9 "2011-03-18T14:43:16Z")
> [@sevenwood](#): > > Hah! You can’t fool me! You’ve been reading [Alley Oop](http://comics.com/alley_oop/2011-02-20/)! Huh. I don’t think I’ve read Alley Oop in at least 25 years. Weird. I’d say “Great minds think alike” but they’ve drawn circular orbits so I’m denying any similarity of thought processes. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [March 18, 2011, 3:18pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/10 "2011-03-18T15:18:17Z")
> [@DrFidelius](#): > > Well, as long as the super-science technology of the Priest-Kings is still working, the Counter-Earth can be kept in an artificially stable position directly opposite us. Cybermen don’t have Priest-Kings. 😕 ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [March 18, 2011, 3:23pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/11 "2011-03-18T15:23:49Z")
Earth’s solar L-3 point must be more crowded than a number 6 train at rush hour. ---
**Author:** ![The\_Man\_In\_Black](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@The\_Man\_In\_Black](https://boards.straightdope.com/u/The_Man_In_Black)\ **Post date:** [March 18, 2011, 3:40pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/12 "2011-03-18T15:40:12Z")
Wasn’t this the idea behind a movie? An astronaut leaves earth on a trip around the sun, and somehow ends up back at earth in half the time. Turns out he is on the mirror earth. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [March 18, 2011, 3:51pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/13 "2011-03-18T15:51:27Z")
> [@sevenwood](#): > > Hah! You can’t fool me! You’ve been reading [Alley Oop](http://comics.com/alley_oop/2011-02-20/)! Or [Philolaus](http://en.wikipedia.org/wiki/Counter-Earth). ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [March 18, 2011, 3:56pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/14 "2011-03-18T15:56:05Z")
Since no one has yet mentioned it in this thread, the [LaGrangian Points](http://en.wikipedia.org/wiki/Lagrange_point) would be useful for the OP to look at. And even though LP #3 looks like it would do the trick, this works only for circular orbits and only for 3 bodies. More than that, or for eliptical orbits, anything at #3 would wobble enough that it would “peek out” from behind the Sun enough times that someone would notice from Earth (“Hey, look over there!”). Not to mention it would be discovered from photography on board the various Solar System devices man has launched into space. ---
**Author:** ![Gyrate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gyrate/32/3588_2.png) [@Gyrate](https://boards.straightdope.com/u/Gyrate)\ **Post date:** [March 18, 2011, 3:56pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/15 "2011-03-18T15:56:54Z")
> [@The\_Man\_In\_Black](#): > > Wasn’t this the idea behind a movie? An astronaut leaves earth on a trip around the sun, and somehow ends up back at earth in half the time. Turns out he is on the mirror earth. I think it gets used a lot as a sci-fi idea, to be honest. I’ve read the Wiki pages on Counter-Earth and the L-3 point since posting the OP and they pretty much cover my questions, but feel free to continue to discuss. ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [March 18, 2011, 5:53pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/16 "2011-03-18T17:53:41Z")
> [@The\_Man\_In\_Black](#): > > Wasn’t this the idea behind a movie? An astronaut leaves earth on a trip around the sun, and somehow ends up back at earth in half the time. Turns out he is on the mirror earth. [Journey to the Far Side of the Sun (also known as Doppelgänger)](http://www.imdb.com/title/tt0064519/) ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 18, 2011, 6:49pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/17 "2011-03-18T18:49:08Z")
> [@Gyrate](#): > > And I figured during spring/autumn the planet opposite would be unobstructed by the sun but thought you good people might be able to confirm that through the mighty power of Math. Oh, right, that. The answer there is that Earth-2 would be completely hidden from Earth-1 to a first approximation. If you took Earth’s orbit and rotated the whole thing around the Sun by 180 degrees (so that Earth-2’s perihelion was the 180 degrees opposite Earth-1’s, and same for Earth-2’s aphelion), then Earth-1 and Earth-2 would always be directly opposite the Sun from each other. However, this is only the case so long as you can legitimately ignore the gravitational pull of Earth-1 on Earth-2 and vice versa. In reality, they’ll start to pull on each other (as well as having the sun pull on them, and these perturbations will pull their orbits out of perfect anti-alignment. ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [March 18, 2011, 7:00pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/18 "2011-03-18T19:00:04Z")
Two commemts: Not to mention we’ve sent rockets to Venus, Mars, asteroids, etc. Didn’t any of these have cameras that filmed the skies? The wonderful Al McWilliams comic strip “Twin Earths” from the 50s and early 60s had this as its premise. ---
**Author:** ![Telperion](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telperion/32/509_2.png) [@Telperion](https://boards.straightdope.com/u/Telperion)\ **Post date:** [March 18, 2011, 8:46pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/19 "2011-03-18T20:46:02Z")
If there was anything at all on the other side of the sun, the STEREO satellites would have seen it. [http://www.nasa.gov/mission\_pages/stereo/main/index.html](http://www.nasa.gov/mission_pages/stereo/main/index.html) ---
**Author:** ![Todderbob](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@Todderbob](https://boards.straightdope.com/u/Todderbob)\ **Post date:** [March 18, 2011, 8:49pm UTC](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145/20 "2011-03-18T20:49:39Z")
> [@Gyrate](#): > > I assumed this would be the case and that there’s no way to have multiple stable planets in the same orbit occur naturally. I was just wondering what it would look like if we did. And I figured during spring/autumn the planet opposite would be unobstructed by the sun but thought you good people might be able to confirm that through the mighty power of Math. > **[Two planets seen sharing the same orbit - UPI.com](https://www.upi.com/Science_News/2011/02/24/Two-planets-seen-sharing-the-same-orbit/UPI-81051298596586/)** > > U.S. astronomers say they've discovered a distant planetary system unlike anything seen before, with two of its planets apparently sharing the exact same orbit. Not according to some astronomers. But we’d have seen one if it was at any one of the L points. [Next page](https://boards.straightdope.com/t/earth-2-how-would-we-know/575145.md?page=2)