Two Planets Found Sharing One Orbit 175
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."
Re:And bolster my theory (Score:4, Informative)
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?
That would be STEREO [nasa.gov].
Gravity of an Earth-size body at L3 (Score:5, Informative)
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 [wikipedia.org], Lagrangian point L3 on Wikipedia [wikipedia.org], and Counter-Earth on TV Tropes [tvtropes.org]. 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.
First? (Score:5, Informative)
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.
Re:First? (Score:5, Informative)
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.
I googled "scottish reel orbit" and of course the first result was your own post. However, I did come across this, for those who are interested: http://faculty.ifmo.ru/butikov/Projects/Collection3.html [faculty.ifmo.ru]
Re:Time for another IAU meeting (Score:5, Informative)
The definition that makes Pluto a dwarf planet specifically apples only to our solar system, and the part that calls for clearing the orbit was inserted in case a Kuiper belt object actually bigger than Mercury was found later, so the IAU would not have to debate the subject again, not as a straight-forward rule based on any physical fact. Incidentally, the belt is named after Kuiper because he was a. the third major working astronomer to propose such as zone, and b. the first to be fundamentally wrong about its nature, as he claimed such a belt could not still exist.
All the debate about how to define a an extra-solar planet will be driven by the very people who have totally screwed up any rational, scientific definitions when it comes to our own solar system. Expect a rule about how planets in the 'northern' part of the galaxy must have an eccentricity of less than 5.2%, and planets in the direction of Virgo are allowed 7.1%, but only if they move in square orbits on alternate St. Swithen'sdays.