I sincerely hope that I’m posting in the right place. I dread being told this belongs in Great Debates or IMHO. Kindly understand that I’m a guest and can not use the search function.
What were the arguments against Gingerich’s proposed definition of a planet which would’ve expanded the solar neighborhood to twelve planets?
What constitutes a body having cleared the neighborhood of its orbit? Are trojan asteroids, rings, satellites, near Earth objects, etc. considered in this definition? What if there’s a Mars-sized (imperial Mars not metric Mars for the pedants out there ) object discovered in the rubble-strewn system around Tau Ceti? Would this challenge the IAU’s definition?
Finally are there any prominent non-USian astronomers or scientists who are denouncing the new definition? Everything I’ve found has been about USians being upset but the rest of the world appears to be taking the change in stride.
I’m not an Astronomer of any description, but I’ve been following this Debate in the Newspapers, and I can answer your first question.
dry_oatmeal:What were the arguments against Gingerich’s proposed definition of a planet which would’ve expanded the solar neighborhood to twelve planets?
As our imaging gets better, we are very likely to discover many more objects meeting the that definition of planet in the Kuiper (Sp?) belt beyond Pluto. This seemed, apparently, just too inclusive and fluid. The number of planets could possibly be hundreds, according to the some top ‘asteroid hunter’. I’m afraid I don’t remember the scientist’s name, it was something I read in the newspaper many weeks ago, when the conference was happening.
I think the USian predjudice for keeping Pluto’s planethood stem from the fact it was discovered here, but none of the astronomer’s at the Lowell Observatory near Flagstaff seemed despondant or suicidal over the issue. Not even particularly inflamed. Our newspapermen went up and checked on on them.
My science through the humanities background is insisting that I mention nothing about Pluto changes, no matter what we call it.
I read that some 43 known objects would have been called planets under the proposal and who knows how many others. Neil de Grasse Tyson, director of the Rose Planetarium in NY (and the Natural History Museum’s chief astronomer) has been arguing against Pluto’s planethood for years.
By and large, scientists don’t much care. As that royal Sonoran guy points out, the objects themselves are the same as they’ve always been. Labels like “planet” are really just a matter of convenience (i.e., the word “planet” can be used as shorthand for a list of properties, rather than listing them all individually), but the label “Kuiper belt object” is even more convenient, for Pluto.
The thing is that under the new definition of planethood, Earth will eventually lose its status as a planet. While it won’t happen for a long time, Earth and Luna (like Pluto and Charon) will eventually both orbit around the sun rather than the smaller body around the larger. When that happens, Earth will no longer be considered to have cleared its orbit and will be demoted to a dwarf planet.
Wikipedia has, as one should expect, an article on what “clearing the neighborhood” means. Basically, it goes down to asking whether the object in question contains the bulk of mass in its orbital zone. I can’t find any specific criterion such as the 50 % condition mentioned by mittu, but I don’t think that sharp a line would be necessary at the moment, given that, according to the table in the Wiki article, Pluto only has 0.077 times the mass of all the other bodies in its orbital zone combined, whereas all the other bodies accepted as plents as of 2006 have at least 10,000 times the mass of their orbital neighbors.
That would all depend on how you define “has cleared it’s orbit”. I can’t find a cite for my 50% mass statement but i’m sure I read it somewhere and if that is true then the Earth would still be a planet as it is so much larger than the moon.
Without some regard for the comparative sizes of the objects Neptune would no longer be considered a planet because it shares it’s orbit with Pluto
I thought that wasn’t the case - that Pluto’s orbit doesn’t actually intersect Neptune’s, becuase of the tilt of Pluto’s orbi, and it only looks like it does so in two-dimensional maps?
Not an astronomer or anything, but I think the argument goes like this.
Right now, the centre of gravity of the Earth-Moon system is under the Earth’s surface, so the Moon is considered to be in orbit around the Earth.
However, the Moon is very gradually moving away from the Earth in its orbit.
At some point, the centre of gravity of the system may no longer be within the Earth, and the two bodies will then be revolving around each other, much like Charon and Pluto.
when that happens, the Earth may no longer be considered to have cleared its orbit.
therefore Earth would not be a planet any longer.
However, the rate of the expansion of the Moon’s orbit is really small, so I don’t think it will happen for a very long time. It will likely be the Alpha Centuri Astronomers’ Union that will be having to decide the issue.
Okay, so it’s people using terminological whinging to whinge about terminology.
That is, arbitrarily deciding a dividing line between “A orbits B” and “A and B orbit each other”, and using that to complain about another arbitrary line between “planet” and “not-planet”.
Look, the “cleared its orbit” thing is a bit screwey, I’ll give you. Still, the Earth is so much bigger than the moon that it still will have “cleared”.
I think in the future we’ll find some property of solar system formation that explains why the eight planets’ orbits are flatter than Kansas (which is, in turn, flatter than a pancake). Seriously, the inclinations are so tiny that I’m sure there’s some mechanism in how planets form that pushes them pretty near a single plane. If the protoplanet is big enough or close enough in or whatever to have this mechanism act significantly on it, it forms a planet. If it’s too small or too far out (evidently KBOs don’t stay as close to the plane) then it’s not a planet. At least “it’s big enough to pull itself into a spheroid” says something about why the planet is what it is, rather than effectively describing an accident of history.
That said, I’d rather kick Pluto to the Kuiper Belt (where I think it belongs because of its inclination) rather than let in “Planet Ceres”, “Planet Charon”, “Planet Sedna”, and “Planet Limbaugh”.
Why? Is this characteristic common to all moons in our Solar System? If so, then most planets (but not Venus or Mercury) are doomed to be demoted as well.
The tidal bulge of the earth exerts a force that slightly speeds up the moon in its orbital path. The effect of the speeding up is to send the moon into a slightly higher orbit. Since the pull that results in the speeding up is continual the moon is always moving away from the earth and the earth’s rotation is gradually slowing. Eventually, if the earth-moon system lasts long enough, the earth rotation will slow to the point where the same face of the earth always poiints at the moon and the receding will stop.
The effect would be there in all cases since there is always a tidal bulge in the planet. However the moons of Mars, for example, are so small that Mars’ tidal bulge would be so small as to make the speeding up of the moons insignificant.
Tidal forces exerted by the Moon are slowing the Earth’s rotation. This causes the Moon to move further out to conserve the angular momentum of the Earth-Moon system.
Actually, I believe that when the moon’s orbit makes it such that the center of gravity is outside the Earth, they will be reclassified as a double-planet system, and the Moon will be added as a planet.
When that happens, the Earth might still be considered to have “cleared its orbit”, but the Moon sure as heck wouldn’t. So under the current definition, the Moon still wouldn’t be a planet.
But then, I’m quite willing to argue that the Moon should be considered a planet, right now. You can make a stronger case for it orbiting the Sun than for it orbiting the Earth (the center of mass of the Moon-Sun system is inside the Sun, if that’s the definition you want to use), it’s certainly big enough, and its orbital plane is closer to the ecliptic than it is to Earth’s equator.
Actually, the Moon would escape from the Earth before this happens—but this would only occur well after Sun evolves into a red giant.
I’m afraid this is entirely untrue. The orbits of Phobos and Deimos certainly are evolving tidally. Their small mass means that they raise a small tidal bulge, yes, but their smaller inertia means they are, in turn, more easy to move. Because Phobos is inside the synchronous orbit (the distance from Mars where a satellite’s orbital period equals Mars’ rotational period), tidal evolution actually decreases the size of Phobos’ orbit, and because Phobos is so close, this process is quite rapid, a couple of meters per century. Phobos will not survive long. In about 50 million years, it will be so close to Mars that it will be pulled apart by tides.
) object discovered in the rubble-strewn system around Tau Ceti? Would this challenge the IAU’s definition?