h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I disagree with the statement that a "hidden planet" in the L3 point would be unstable and could not exist. Not that I'm saying that there is one--I doubt there is--but if there was, it would not be as unstable as claimed.

An asteroid, yes. An asteoid has little or no effective gravity of its own, and would be a "passive" body in the system. A whole planet, however, is a much different scenario--in fact, it doesn't even apply to Langrange points any more, because instead of a "huge-big-tiny" scenario you would have a "huge-big-big" scenario.

Imagine that there was a second planet earth in the L3 point. It seems logical that this planet' orbit would be exactly as stable as our own planet's orbit--it has the same mass, is moving in the same orbit with the same velocity, and is in a position where it would not be adversly affected by the "real earth"'s gravity. I don't claim to be an astrophycisist, but such an opposing system seems perfectly stable to me, since neither of the two "earths" is wholly dependent on the other for its stability.

On another topic, this article reminds me of another gravitational henomenon, but I can't remember what it's called. A Kepler Rosette, I think, but I'm not sure. Anyway, take three or more planet-size bodies, and place them at equidistant points from each other in a single orbital path around a sun. In theory, the planets will stay in their orbits, stable relative to each other and the star system. In fact, once in place, the star can actually be removed, and the planets will continue orbiting stably around their own mutual center of gravity.

No. The hidden planet could never be exactly opposite the earth, because the earth travels in an ellipse at slightly different speeds at different points in the orbit. Once the earth/sun/hidden planet are not quite in line, the Earth's gravity will have an effect on the other planet and the two planets will be tugged out of their orbits, ending up in a completely different configuration. This is what is meant by unstable.

Well said. I commend Gnomon's statement to the House.

Ah yes. That guy Tycho, wasn't it?