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I am drafting a short story about a small object orbiting the Sun at the precise 'speed' as Earth's opposite the Sun, undiscovered by us due to its position. The problems I see with this include the following.

  1. We would have detected it by now, because of the gravity that impacts other objects in the solar system. How small would it have to be (just an order of magnitude is fine) to not be detectable?
  2. Would our spacecraft beyond Earth have 'accidentally' snapped a picture of it?
  3. Must it reside at Lagrange Point 3 ($L_3$) in order to have our orbital period?

Side note: I was re-phrasing the question to go from "small planet" to "dwarf planet" and accidentally deleted the wrong word. Before posting I looked up to see I had written, "What are the possibilities of a dwarf orbiting opposite Earth's orbit?" and truly laughed out loud. That would have gotten some weird looks haha.

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    $\begingroup$ I would like to clarify that I am not trying to make this a habitable planet, but it will have objects left by intelligent visitors. $\endgroup$ – Mikey Jan 13 '15 at 14:14
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    $\begingroup$ At the L3 point it will have the tendency to move away unless it actively restores its position $\endgroup$ – ratchet freak Jan 13 '15 at 14:19
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    $\begingroup$ For a quick tutorial on the Lagrange points, listen to this Cheap Astronomy podcast portaltotheuniverse.org/podcasts/eps/view/344367 $\endgroup$ – user3106 Jan 13 '15 at 14:44
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    $\begingroup$ The STEREO spacecraft were in L4/L5 in 2009, the Spitzer space telescope is in that area now (en.wikipedia.org/wiki/List_of_objects_at_Lagrangian_points#L4). Spitzer, being an IR observer, is certainly able to pick up small objects, but I'm not sure it would've been pointed in the right direction (you don't want it looking into the sun). $\endgroup$ – user3106 Jan 13 '15 at 14:50
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    $\begingroup$ As a point of interest, this is where Doctor Who's cybermen originally came from in 1966's 'The Tenth Planet', Earth's twin which they called Mondas. Sadly we've looked there and even placed a satellite there, so no. No Mondas. $\endgroup$ – IchabodE Jan 13 '15 at 19:14
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The point directly opposite Earth on the other side of the sun is called the L3 Lagrange point. It's not quite at the same distance from the sun as the Earth is, assuming that the body residing there is smaller than the Earth, since both the gravity of the Earth and the sun pull on it.

However, the L3 point is a saddle point in terms of gravitational potential, meaning that it is an unstable equilibrium point. Anything in the L3 point will eventually drift away, given sufficient time. Eventually, it would probably drop into an orbit around either the L4 or the L5 points, which would be visible from Earth.

We also probably would have seen the planet by now, given that there were quite a few people who theorized exactly such a planet, starting with Aristotle and continuing into some works of modern science fiction.

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    $\begingroup$ This. The L3 points of the planets (and other unstable points) are the least likely places in the solar system to find anything orbiting the Sun :-) $\endgroup$ – Steve Jessop Jan 13 '15 at 16:21
  • $\begingroup$ The Big TV Blackout by Jan K. Möller is another science-fiction work which features such a planet. It is not discussed very deeply, however. $\endgroup$ – a CVn Jan 14 '15 at 12:12
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Under 100km, and probably not even then.

http://en.wikipedia.org/wiki/Counter-Earth#Scientific_analysis

Probes sent to Venus and Mars wouldn't have arrived if there was anything bigger there, as they used gravitational slingshot effects.

Venus would pull it out of orbit. Smaller bodies would be more affected by Venus's pull.

STEREO probe launched in 2006 would've detected it. It's in the Sun's orbit, not Earth's.

If there are EM emissions, the Sun's wobbling motion (rotation) around its barycenter would reveal them for part of the year.

Earth doesn't orbit in a circle, but an ellipse - and thus is moving faster during parts of its orbit, and thus can see things during parts of the year that it couldn't see during other parts of the year.

You need to make your counter-Earth recent or specially hidden (high-tech).

Also, what's the story purpose? If you want something hidden, there are other places to hide it. If you want something that's easy to get to, that's not particularly easy to get to.

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As others have stated, the L3 point is unstable. As well as the already mentioned L4/L5 that are well observed.

An interesting alternative orbit might be a Horseshoe Orbit. Combined with really-good-radiation-absorption, and with sufficient smallness, such a body would be tricky to spot.

Youtube vid of Horseshoe Orbit

An in-universe explanation for the lack of detection could be a covering of near-perfect solar panels (ie absorbs all light/radar for internal power)

It would have to be spotted by a transit in front of other stars/bodies, and this difficulty would be compounded by the changing relative positions between the body and the Earth.

Edit: Added link to video

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    $\begingroup$ I like this option. Not only is it vaguely plausible its also sufficiently interesting to add a little fillip to the story. $\endgroup$ – Gordon Coale Jan 14 '15 at 8:42
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Well the first problem is that if there was Earth 'technically' isn't a planet! ;) Hasn't cleared it path...

But I don't think we would have gotten a picture of it until the 70's or 80's but there is a reasonable chance that it would have been caught on film somewhere.

Also the planetoid/dwarf planet,asteroid would have to have the exact same path as the earth or the two would have collided long ago. I tried looking up info on orbital decay, but didn't find what I was looking for. I do suspect that the Earth and a smaller body, even if starting on the exact same orbit would decay at different rates and thus eventually collide.

You could always use any other dwarf planet/large asteroid we have identified so far (or a new one!) to host alien artifacts. Next best would be some kind of stabilizing tech on the dwarf planet to keep it in place exactly opposite the earth. Just have someone 'discover' it in the old photos from one of the missions. Any one of the mars missions might have spotted it, since mars takes twice as long as earth to go around the sun.

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    $\begingroup$ Could it have a different orbit (closer/further from the sun), orbit at the same speed, and always be opposite the sun where we would not have seen it, albeit highly unlikely? I like your suggestion of just putting it on something we do know exists and just be done with it, though :). $\endgroup$ – Mikey Jan 13 '15 at 14:43
  • $\begingroup$ I could be wrong but my understanding of orbital speed is a directly related to the distance from the sun. thus closer would be faster, farther would be slower... $\endgroup$ – bowlturner Jan 13 '15 at 14:47
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    $\begingroup$ Clearing the neighborhood is one of the most misunderstood parts of the IAU definition of a planet. It doesn't mean there's nothing else in the orbit, it means the body gravitationally dominates the orbit. Everything else in the orbit is either a satellite (the Moon), in resonance (Earth Trojans), or on its way to being kicked out (Near Earth Asteroids). $\endgroup$ – Schwern Jan 14 '15 at 0:33
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    $\begingroup$ @Schwern reminds me of the movie the man who went up a hill and came down a mountain. Similar fuzzy boundry for what is a river as opposed to a creek or drainage channel. $\endgroup$ – JDługosz Jan 14 '15 at 6:09
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    $\begingroup$ The IAU definition of a planet is just an excuse for anti-Plutonian bigotry. $\endgroup$ – Oldcat Jan 28 '15 at 0:32
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If your goal is to have a body near Earth which is in a stable orbit for aliens to have stashed stuff, go for an Earth Trojan. The L4 and L5 points are stable, meaning the aliens can rely on the rock being there for a while. Trojans potentially take less delta-V to reach than the Moon making an immediate manned or robotic recovery mission plausible.

The first, and only, Earth Trojan yet discovered was 2010 TK7 in 2010. It's small, 300 meters, so it has no gravitational impact. It has the albedo of asphalt or a forest, so it's hard to see. You don't have to be on the opposite side of the Sun to have not been spotted yet, it's totally plausible there's more and we just haven't seen them.

A scenario could be this. A new space telescope is launched, or an old one is repurposed to look for Earth Trojans. Your target is spotted. NASA decides this would be a great candidate for their Asteroid Redirect Mission. A probe is sent to pluck a "boulder" off the surface and it turns out to be not a boulder. A manned mission is sent.

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As others have said, the L3 point is unstable, and any mass of significant size would have been detected already through perturbations of Venus's and Mercury's orbits.

However, space is vast, and small things are really small in space. A more likely location for a hidden object like this would be in either the asteroid belt, or as a Jupiter Trojan (in the L4 or L5 points). The object would also need a very low albedo (reflectivity) to escape detection.

If you wanted to push it, you could conceivably make your object an Earth Trojan (only one Earth Trojan has yet been discovered). Deimos would be a good reference. It is only 24 nanoEarths in mass, with an escape velocity just a little faster than you can jump. If it had a very, very low albedo, something that small could conceivably have gone undetected (but we could have taken a picture of it).

In fact, the only way we'd find something like that is if we watched it occult one or more stars/planets. However, since we know this is a stable point, we would be looking for things here.

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    $\begingroup$ very dark is very possible for an artificial object. We've made incredibly black substances in the news the last few years. So cover it with ultrablack or geometric light traps (or both) $\endgroup$ – JDługosz Jan 14 '15 at 6:06
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    $\begingroup$ Note that reflectivity and emissivity go one against the other - so while your object wouldn't reflect a lot of light, it would still emit the same power in infrared - it would be very easy to see against the background in IR. Or maybe even visible light, given that it's completely black and closer to the Sun than we are :D $\endgroup$ – Luaan Jan 14 '15 at 9:55
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    $\begingroup$ @Luaan to emit visible light, it would need to be quite warm, still a lot warmer than a black body heated just from the sun at that distance would be. If it were an artificial object, it could also have active cooling and emit its heat to the other direction (where no one is looking). $\endgroup$ – Paŭlo Ebermann Jan 25 '15 at 19:40
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    $\begingroup$ @PaŭloEbermann Yeah, that's a good point. It would have the same amount of incident energy, but it would radiate in twice the area, so Sun wouldn't be quite enough for visible light I guess. Active cooling doesn't sound likely - unless it would have a large radiator hidden in its own shadow. And even then, it'd be partially illuminated most of the time, so it would really need a large radiating surface - while still hiding the radiator from our observation. $\endgroup$ – Luaan Jan 26 '15 at 11:51
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I'd like to point out that the L3 spot, although not parked, is passed through by some solar observatories that co-orbit with the Earth like STEREO. While passing through, they searched L4 and L5 for small bodies.

Their current position is rather interesting; here is a chart and a static snapshot, rather close to crossing at the point you are interested in.:

snap

So, here is an idea to use in your story: this is how the hard-to-see object is discovered by humans.

The spot is like parking at the top of a hill wit no parking brake; you don't expect anything to stay there. So it is a surprise to find something that looks like a tamed comet! It is covered with a regolith to keep the ices shielded in side and has an odd shape that causes it to tumble rather than rotate on a normal axis. It sometimes allows puffs of gas to escape in just the right direction to keep it parked. In fact, the jet as tenuous as it is was how it was spotted. The two crafts send a radio signal and mutual optical inspections as they passed. The jet let them back to the object.

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1 Maybe. It depends on how small it is. Small bodies do not cause so big perturbations on bigger bodies. Something like Earth itself would be detectable by perturbations on Mars and Venus, but Ceres most probably not. Anyway, such a planet is most probably nonexistent since it would interfere with Cruithne, and it does not.

2 No. It is exactly at the other side of the Sun. It is impossible to detect it from any spacecraft near Earth-Moon. It may have been photographed by missions to other planets, like the Voyager ones, but their cameras hardly point towards Sun.

3 No. Lagrange points are for smaller bodies, and L3 is not a stable point. The location would be at the same distance from the Sun than the Earth is (1AU), since orbit is independent from mass.

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  • $\begingroup$ My fault. I pasted the link I use for my reading. Fixed. $\endgroup$ – Envite Jan 14 '15 at 16:34
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I believe that others are mistaken about the L3 point. Rather than the L3 point being on the other side of the Sun from the earth, it is on the other side of the Earth from the sun. This is why the point is an unstable "orbit" - because it is not going around the sun in an ellipse, but rather in an orbit that is one of 5 solutions to the 3 body problem. The actual point on the other side of the sun from the earth would have a true elliptical orbit and would be stable, although probably not for billions of years just because it would have a slightly different rate of orbit decay than the earth due to its differing surface area/mass ratio.

Edit. After the comment below was posted I did some research and it turns out my memory was wrong.

The L3 point is indeed an elliptical orbit, but around the Earth-Sun balance point rather than around the sun's center of mass. The reason it is unstable is not because it lacks the stability of an elliptical orbit, but rather because the stability requirements of this particular elliptical are much more stringent than stability requirements of other planetary orbits, in that if the other planets of a solar system pull a planet out of its ellipse, it just goes into another, very similar ellipse. In this case, they would interfere with the relationship between the hidden planet and Earth, which would provide consistent gravitational bias.

Sorry about the confusion.

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    $\begingroup$ No, the point you're thinking of is $L_2$; both are unstable. $\endgroup$ – HDE 226868 Jan 14 '15 at 21:58

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