Asperion Posted June 18, 2015 Report Share Posted June 18, 2015 I got thinking about the current defination of "planet". According to the IAU, the defination is: “is in orbit around the Sun…” “has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape…” “has cleared the neighborhood around its orbit.” Applying that to Earth: 1 - Earth is clearly orbiting the Sun - check 2 - After centuries of exploration the Earth has been well mapped - check 3 - Pressence of the Moon orbing with Earth is taking the same space - negative Does this mean that Earth fails the planetary test? Putting the same test on all the gas giants will get the same results - should they be removed from the planetary lists. If so, then the only actual planets on this system would be Mercury and Venus since they are the only ones that truly meet all three criteria. Is there some flaw in my reasoning? Quote Link to comment Share on other sites More sharing options...
L. Marcus Posted June 18, 2015 Report Share Posted June 18, 2015 Yes -- an overly picky definition of "clearing the orbit." It means "clearing the orbit of items on the same size scale as the main object." Quote Link to comment Share on other sites More sharing options...
Christopher Posted June 18, 2015 Report Share Posted June 18, 2015 The moon is explicitly a sattelite of the earth. Those are not considered for the whole "cleared neighborhood" requirement: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood "In the end stages of planet formation, a planet will have "cleared the neighbourhood" of its own orbital zone (see below), meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence." If moons would be considered, the only planets in our whole solar system would be Venus and Merkur (and we are not certain they might not have a moon we just can't see aganst the sun). And that would be stupid. The rule is more about differentaiting dwarf planets from real planets, then the other way around. It just happens that pluto does not fit the new definition anymore, wich is a minor hickup. It is still a dwarf planet. Quote Link to comment Share on other sites More sharing options...
薔薇語 Posted June 19, 2015 Report Share Posted June 19, 2015 The Earth and the Moon don't have the same orbital path. The moon orbits around the Earth and incidentally revolves around the sun. The Earth orbits around the Sun and leaves no substantive bodies in its wake. La Rose. Quote Link to comment Share on other sites More sharing options...
Old Man Posted June 20, 2015 Report Share Posted June 20, 2015 Clearing the Neighborhood is a defined term that specifically excludes bodies that are orbiting the one in question. It occurs to me that it might exclude binary pairs, however. If there were a pair of spherical bodies orbiting a star in an arrangement like Janus - Epimetheus, then I suppose they would not be planets but a pair of moons. Quote Link to comment Share on other sites More sharing options...
procyon Posted June 20, 2015 Report Share Posted June 20, 2015 Clearing the Neighborhood is a defined term that specifically excludes bodies that are orbiting the one in question. It occurs to me that it might exclude binary pairs, however. If there were a pair of spherical bodies orbiting a star in an arrangement like Janus - Epimetheus, then I suppose they would not be planets but a pair of moons. If we were to find a pair of bodies orbitting a star that were in an arrangement like Janus-Epimetheus around Saturn, they wouldn't be moons. Moons are a satellite of a primary body to include major/minor/dwarf planets. If they were small, they would likely be classified as asteroids/comets depending on composition. If they were big, it could get a little sticky. Two objects the size of Mars following the same orbit and passing one another every few years and on the opposite sides of the star at other times...isn't something that is addressed particularly well by current definitions. Quote Link to comment Share on other sites More sharing options...
Cancer Posted June 21, 2015 Report Share Posted June 21, 2015 I'll look around Monday when I can use the university library site licenses. I have this dim memory of an exoplanet system with coorbital planets but I am not finding it now, unless it's Kepler-223. We'll see if this sort of thing is addressed at the IAU General Assembly in Hawaii later this summer. Quote Link to comment Share on other sites More sharing options...
Christopher Posted June 21, 2015 Report Share Posted June 21, 2015 I'll look around Monday when I can use the university library site licenses. I have this dim memory of an exoplanet system with coorbital planets but I am not finding it now, unless it's Kepler-223. We'll see if this sort of thing is addressed at the IAU General Assembly in Hawaii later this summer. That is called a Barycentric Orbit (two objects orbiting a "middle point" that is outside each planets body). https://en.wikipedia.org/wiki/Orbit https://en.wikipedia.org/wiki/Barycentric_coordinates_%28astronomy%29 Actually Pluto and Charon are prime examples of that. But at the very least the centers of all orbits are slightly shifted towards the orbiting body - even the moon does not orbit the excact center of mass of the earth. Quote Link to comment Share on other sites More sharing options...
Zeropoint Posted June 22, 2015 Report Share Posted June 22, 2015 Two objects the size of Mars following the same orbit and passing one another every few years and on the opposite sides of the star at other times...isn't something that is addressed particularly well by current definitions. As I understand it, that isn't possible with Mars-sized bodies. Being big enough to clear up nearby orbiting bits is part of what makes a planet a planet. In any case, "following the same orbit" and "passing one another every few years" are contradictory concepts. Quote Link to comment Share on other sites More sharing options...
Cancer Posted June 23, 2015 Report Share Posted June 23, 2015 OK, Kepler-223 has four planets in very close to mean motion resonances, but not co-orbital. Mean motion resonances ARE allowed within the present definition of planets. Quote Link to comment Share on other sites More sharing options...
Old Man Posted June 24, 2015 Report Share Posted June 24, 2015 As I understand it, that isn't possible with Mars-sized bodies. Being big enough to clear up nearby orbiting bits is part of what makes a planet a planet. It ought to be possible; the ratio of Janus/Epimetheus' masses to Saturn's are about the same as the ratio of Mars' to that of the Sun. I can't think of any other reason the phenomenon couldn't scale up. In any case, "following the same orbit" and "passing one another every few years" are contradictory concepts. Technically what they're doing is exchanging each other's very, very close orbits on each pass. Quote Link to comment Share on other sites More sharing options...
procyon Posted June 24, 2015 Report Share Posted June 24, 2015 That is called a Barycentric Orbit (two objects orbiting a "middle point" that is outside each planets body). https://en.wikipedia.org/wiki/Orbit https://en.wikipedia.org/wiki/Barycentric_coordinates_%28astronomy%29 Actually Pluto and Charon are prime examples of that. But at the very least the centers of all orbits are slightly shifted towards the orbiting body - even the moon does not orbit the excact center of mass of the earth. The problem would be that Janus & Epimetheus do not actually fulfull a barycentric orbit. A barycentric orbit is centered on the two bodies center of mass, where Janus & Epimetheus orbit Saturn (and not their common center of mass at any given time) and trade momentum as they pass each other with the inner moon aquiring energy from the outer moon as it catches up on its slightly shorter orbit. As the inner moon aquires this energy, it speeds up and this additional energy pushes it's orbit farther out. At the same time as the outer moon loses its momentum/energy to the inner moon, it slows and its orbit moves slightly closer to Saturn. So as they pass, the moons trade orbital positions. But the difference between their average orbital radii is actually less then either moon's radius. It is just when passing that their respective orbits differ enough for them to not intersect. And there is no reason larger bodies could not share the same dynamic while orbitting a star. It is just (IMHO) very unlikely. Quote Link to comment Share on other sites More sharing options...
procyon Posted June 24, 2015 Report Share Posted June 24, 2015 In a barycentric pair, one of the pair will be more massive and deemed the primary. If the pair fulfill the definition for a planet, the primary will be a planet and the less massive body will be the moon. Just as Pluto is the dwarf planet and Charon the moon. Quote Link to comment Share on other sites More sharing options...
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