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Two Planets Found Sharing One Orbit
dweezil-n0xad writes "Buried in the flood of data from the Kepler telescope is a planetary system unlike any seen before. Two of its apparent planets share the same orbit around their star. If the discovery is confirmed, it would bolster a theory that Earth once shared its orbit with a Mars-sized body that later crashed into it, resulting in the moon's formation."
Possibly. As neither has "cleared its neighbourhood" of other masses in their neighborhood, they might be back to being called planetoids like Pluto. Both are to be considered "dwarf planets" until they collide and one becomes obviously dominant. There's already bits that cover things like this, but people are already arguing about the exampled in our own solar system. I be something like this would cause even more hub bub and another conference to further define the meaning of planet yet again.
That there's a duplicate Earth on the exact opposite side of the Sun!
OK, just for the fun of it: what would be the most efficient method to check this hypothesis?
By checking how its gravity would effect other planets in the same star system. For background: Counter-Earth on Wikipedia, Lagrangian point L3 on Wikipedia, and Counter-Earth on TV Tropes. Executive summary: We don't have one, and we know this because if we did, we'd be able to detect its pull. Furthermore, such an orbit would be unstable.
It's not clear that this is anything new. A number of astronomers have suggested that we should treat the Earth/Luna and Pluto/Charon pairs as "double planets" sharing an orbit. And there's a pair of Saturn's moons that share an orbit. Of course, whether these are counterexamples depends on the picky, legalistic details of how you define the term "planet", which we've discussed to death here on /. already. Fun as such pseudo-arguments may be, the fact is that they're not terribly significant.
Thus, for the Pluto/Charon pair, reclassifying Pluto as a "dwarf planet" make it especially an edge case, since it still includes the term "planet" in its classification. But they're both large, spherical bodies in a single orbit around the sun, while also orbiting each other.
The Earth/Luna pair is a bit of a mathematical curiosity. One of the arguments supporting calling our moon a "planet" orbiting the sun is that its orbit is everywhere convex with respect to the sun. You'd expect a "moon" to have a much more wiggly orbit, parts of which are curved away from the sun, and this is true of the other objects in the solar system that we call moons. OTOH, the barycenter of the Earth/Luna pair is (slightly) inside the Earth, which can be used with some definitions to say that it's really a satellite of the Earth.
And, of course, Saturn's two moons in a single orbit can be disqualified because they're obviously not "planets". They're not even big enough to be spheroidal, which is required by most definitions of a planet.
But the fact remains that our solar system contains at least three example of paired bodies sharing an orbit about their primary, and periodically exchanging the lead position. The mechanics of such orbits have been long understood, and astrophysicists can tell you when such orbits are stable. So while this may be "news" in the sense that it's about such orbits around another star, it's hardly news in the astrophysics sense.
What'll be interesting news is the discovery of three astronomical bodies in a "Scottish reel" orbit, which was proved possible several years ago, but to my knowledge hasn't actually been observed yet. Possible places to find them are in the asteroid belt, in Jupiter's "Trojan" asteroid clumps, and in the Kuiper Belt.
Those who do study history are doomed to stand helplessly by while everyone else repeats it.