Possibility of undiscovered planet hiding behind the sun?

Factual Questions
1

How do we know that there isn’t a planet similar to Earth in the same orbit as Earth on precisely the other side of the sun?[sup]1[/sup] We wouldn’t be able to see it because the sun would be in the way.

It would have to have similar mass, and be almost exactly the same distance away from the sun, and in order for us never to have ever observed it, its orbital period would have to be precisely the same length as ours.[sup]2[/sup]

Besides the fact that the existence of this planet is ludicrously improbable, is there any way to disprove it? For example, I know the gravity of the sun bends light. Would light from our hypothetical planet reach us by bending around the sun, or would it be too close? Do we have pictures of that presumably empty spot in space taken from Voyager probes? Would any other orbits in our system be affected in any detectable way by this planet?

[sup]1[/sup] Is this the basis of a S.F. story I’ve read? Have I heard this idea somewhere?

[sup]2[/sup] Well, actually, it would only have to be fairly close: if our mystery planet were only hiding behind the sun for the past few decades, it would elude radio telescopy. But if the orbital period were dissonant enough with Earth that it would not have been behind the sun in days of yore, our ancestors STILL wouldn’t have detected it because it would be visible only during the day, and thus obscured by the atmosphere.

2

We could see the gravitational affects on the other planets. It’s not there.

3

We’d have to notice its gravitational effects on the rest of the solar system, wouldn’t we?

4

[QUOTE=Randy Seltzer]
Is this the basis of a S.F. story I’ve read? Have I heard this idea somewhere?
[/QUOTE]

It’s the basis for a lot of SF and comics stories, often under the name Counter-Earth.

5

[QUOTE=Randy Seltzer]
How do we know that there isn’t a planet similar to Earth in the same orbit as Earth on precisely the other side of the sun?[sup]1[/sup] We wouldn’t be able to see it because the sun would be in the way.

It would have to have similar mass, and be almost exactly the same distance away from the sun, and in order for us never to have ever observed it, its orbital period would have to be precisely the same length as ours.[sup]2[/sup]

Besides the fact that the existence of this planet is ludicrously improbable, is there any way to disprove it? For example, I know the gravity of the sun bends light. Would light from our hypothetical planet reach us by bending around the sun, or would it be too close? Do we have pictures of that presumably empty spot in space taken from Voyager probes? Would any other orbits in our system be affected in any detectable way by this planet?

[sup]1[/sup] Is this the basis of a S.F. story I’ve read? Have I heard this idea somewhere?

[sup]2[/sup] Well, actually, it would only have to be fairly close: if our mystery planet were only hiding behind the sun for the past few decades, it would elude radio telescopy. But if the orbital period were dissonant enough with Earth that it would not have been behind the sun in days of yore, our ancestors STILL wouldn’t have detected it because it would be visible only during the day, and thus obscured by the atmosphere.
[/QUOTE]

Please tell me this wasn’t really intended as a serious question? :eek:

The story line was used by the infamous Tarl Cabot of Gor novels, written by John Norman. Initially started with the sort of fantasy premise utilized by Edgar Rice Burroughs, they veered quickly into a sexual fantasyland that had to be read to be experienced. But Counter-Earth (Gor) was diametrically opposite the Sun from Earth, and the Priest-Kings kept us from noticing with some super-science.

We have sent probes around the sun. We’ve sent probes to planets which were on the opposite side of the sun from us. We’ve sent probes to the solar pole where they have looked down at our system. We’d notice the perturbation of the orbits of Mercury, Venus and Mars for sure (hell, we have those down so fine we use the Mercury orbit to help callibrate the effects of Relativity). In short, there simply is no such planet.

6

I recall that this was the premise for a movie too, and I’m not talking about a “Gor” movie. I even remember a scene from the movie, where a satellite suddenly rotated upwards to show a planet with land and seas.

Anybody remember the name of that movie? If not, I may have to query in CS!

As regards the OP, yeah what everyone else said about gravity…
ETA: Never mind, I found it via Otto’s link. It’s Journey to the Far Side of the Sun

7

Lagrange Points
Such an object would be at what’s called the L3 position, but:
“[The] Sun–Earth L3 is highly unstable, because the gravitational forces of the other planets outweigh that of the Earth (Venus, for example, comes within 0.3 AU of L3 every 20 months).”

8

Try this and this for lots of thorough discussion in prior threads.

9

[QUOTE=CookingWithGas]
Try this and this for lots of thorough discussion in prior threads.
[/QUOTE]
Gah! Did searches for “planet” and “orbit.” Didn’t think to search for “Earth”!
ETA: And I don’t know how I missed it in “orbit.”

10

I have nothing valuable to add to this thread. I just had to come in and mention that I just said EXACTLY THIS to my husband while we were watching some special on Global Warming the other day.

EXACTLY THIS. How weird.

11

[QUOTE=malkavia]
I have nothing valuable to add to this thread. I just had to come in and mention that I just said EXACTLY THIS to my husband while we were watching some special on Global Warming the other day.

EXACTLY THIS. How weird.
[/QUOTE]

You said “EXACTLY THIS” to your husband?

12

[QUOTE=Qadgop the Mercotan]
I recall that this was the premise for a movie too, and I’m not talking about a “Gor” movie. I even remember a scene from the movie, where a satellite suddenly rotated upwards to show a planet with land and seas.

Anybody remember the name of that movie? If not, I may have to query in CS!

As regards the OP, yeah what everyone else said about gravity…
ETA: Never mind, I found it via Otto’s link. It’s Journey to the Far Side of the Sun
[/QUOTE]

Well, that’s not quite the movie I was thinking of. I was thinking of Gamera, the Protector of Earth and Friend to All Children. Any Counter-Earth movie would be better with Gamera.

13

[QUOTE=Qadgop the Mercotan]
You said “EXACTLY THIS” to your husband?
[/QUOTE]

Well, no. :cool: I said, “Do you think there’s a planet on the other side of the sun in the same orbit that we’re in? With maybe the same climate? You know, like a new planet that’s move-in ready? I don’t wanna mess with a fixer-upper like Mars if we can avoid it.”

But the premise was exactly the same. :wink:

14

[QUOTE=asterion]
Any Counter-Earth movie would be better with Gamera.
[/QUOTE]

Any movie period would be better with Gamera.

15

[QUOTE=garygnu]
Lagrange Points
Such an object would be at what’s called the L3 position, but:
“[The] Sun–Earth L3 is highly unstable, because the gravitational forces of the other planets outweigh that of the Earth (Venus, for example, comes within 0.3 AU of L3 every 20 months).”
[/QUOTE]

I came in here to say just this. If there were a counter-Earth, it wouldn’t stay there for long, because the L3 Lagrange point is unstable. All of the Lagrange points on the line between the Earth and the Sun are unstable, which means the forces are in balance there but if you get slightly away from them, the gravitational forces don’t draw you back toward that point (even if you ignore the forces from other planets). That means that things at those points don’t stay there for long. We’d notice a counter-Earth when it came out of its orbit and started careening around the solar system.

Even if a counter-Earth were stable, we would have seen it by now from one of our space probes, like Mercury Messenger that flew by Mercury recently. In fact, we probably would have had at least one space probe go way off course because of its gravity, which we wouldn’t have included in our calculations of how the probe would move.

16

[QUOTE=DSYoungEsq]
Please tell me this wasn’t really intended as a serious question? :eek:
[/QUOTE]
I don’t get it: what’s wrong with the question besides betraying ignorance of planetary physics and sci fi pop culture? It’s not like I actually believe such a planet to be there; I was just wondering why we know it’s not.

Of course, now that I’ve been pointed towards previous threads on the subject (and a fascinating article about lagrange points), I can educate myself.

17

[QUOTE=Otto]
Any movie period would be better with Gamera.
[/QUOTE]

Citizen Kane would be better with Gamera?

18

Damn right it would. “Gamera was a sled” has a hell of a ring to it.

19

How do we know the sun is not hollow and there’s a planet inside? And the people there (who have tiny eyes, because the sky is so bright) wonder if there’s a planet on the other side of the “sky” (which they know is a burning star - they’re not stupid), but they have no way to detect it because nothing can penetrate the wall of energy (can it?).

20

I thought Lagrange points were only for a third object that had negligible mass compared to the other two masses?