How do we know if there isn't another earth on the other side of the sun?

The Earth's orbit is not a perfect circle - a planet on the other side of the sun would have to have exactly the same orbit we do.

But that orbit would not be stable - the sun and the other planets would cause some tiny perturbations in its orbit that would very quickly change its orbit.

Many space probes have done complete circuits of the solar system. If anything was there we would have seen it. Also, it would have effects upon asteroids and comets, none of which have ever been observed.

http://en.wikipedia.org/wiki/Cassini_mission#Traje...

Planets Neptune and Pluto were discovered due to minute effects upon other planets. The tiny discrepancy in the orbit of Mercury was accounted for by relativity. So another big planet in the solar system would produce effects that would be easy to see.

http://en.wikipedia.org/wiki/Discovery_of_neptune

BTW: Don't bother with "Journey to the Far side of the Sun". It is one of the worst sci-fi films I ever saw apart from LOTR and avatar (but since I only watched about half an hour of it, perhaps it improved after that)

Cheers!

The L1 point is gravitationally unstable, and quite a number of solar satellites have looked at that area and sent data almost live back to Earth about that area. There is no planet orbiting the Sun directly opposite, 180 degrees away, any planet or dwarf planet in the Solar System.

We'd know from the gravitational influence it had on the other planets and the asteroids and comets etc. Nice question though.

There was a sci-fi film about that in 1969 called Doppleganger or something like that, I think it is more commonly known by the title Journey to the Far Side of the Sun. It was a great film and well worth a watch if you haven't already seen it.

We would have none. Space agencies have sent a couple of satelites to orbit the sun and they could have spotted something on they way. Also we are always listening with satalites for radio waves coming from space. But imagine what would happen if the world knew there was another planet. The goverment would keep quiet.

That Lagrangian point is unstable, for starters.

Secondly, because of our elliptical orbit, that object would "peek out" from behind the Sun, it would be noticeable at perihelion and aphelion, because of Kepler's laws, it wouldn't be permanently behind the Sun because of the varying orbital velocities of each object.

Thirdly, the perturbations of Venus and Mars would have been detected.

And, finally, our satellites in solar orbit have examined this part of space, with no sighting of such an object.

I wonder the same thing all the time...