I read an article in Natural History recently that indicated most astronomers and others interested in the workings of the solar system have blackballed Pluto. It has been kicked out of the official list of planets. Due to its excessive diminutivity, it has been determined to be just a rather large object in the Kuiper belt, the collection of rocks and general space debris that marks the boundary of the solar system.
This creates, among other things, the need for a new mnemonic device to remember the names of the planets in order. (The old one was “My Very Easy Mother Just Served Us Nasty Pizza” or something to that effect; I can’t come up with anything so catchy, so I’ll leave it to the Mnemonic Oversight and Review Organization interNational (MORON).)
Pluto was demoted, not because of it’s size, but because of it’s composition and origin. It’s basically cosmic garbage sharing the exact properties as thousands of other Kuiper Belt objects. I believe one of Neptune’s moons is also of the same origin. Perhaps a planet once?
I am a card-carrying MORON, so I came up with a catchy mnemonic for you, missdavis:
Much Violence Entered My Jungian Subconscious Until Neurosurgery
OK, OK, lemme see if I remember these right… Mercury, Venus, Earth, Luna, Mars, Ceres, Vesta, Pallas, Hera, Jupiter, Saturn, Uranus, Neptune, Pluto.
What’s that, you say? Luna, Ceres, Vesta, Pallas, and Hera aren’t planets? The problem is that we need a consistent, non-arbitrary way of defining “planet”. The best one that I’ve heard, is that a planet is a body that meets the following conditions:
[ul][li]Primary gravitational influence is a star or brown dwarf, not some other body (eliminates Ganymede, Titan, and all other satelites except Luna)[/li][li]Large enough to be rendered spherical by gravity (eliminates most asteroids)[/li][li]Not so large that a differential increase in mass would result in a decrease in size (eliminates brown dwarfs… Jupiter just barely squeaks by)[/ul][/li]Any definition that excludes the four largest asteroids is either arbitrary (a planet must mass at least X kilograms, with X some silly number), or it excludes Pluto and possibly Mercury, which most folks don’t want. Yes, Pluto is certainly a Kuiper belt object, but that doesn’t mean that it can’t be a planet, too.
Earth and the moon are sort “locked” into one side of the moon ever facing the earth.
I think I have read that the Pluto/Charon situation is even more “locked:” On Pluto (assuming there was enough light to view it), Charon would remain motionless in the sky, never moving. So, on one side of the planet, it would always be in the same spot in the sky. On the other, it would never be seen.
The reason: Charon’s revolution around Pluto is the same as Pluto’s rotation on its axis.
The earth/moon situation is (under our current models) the result of a catastrophic collision that split the planetisimal/planetoid/planet earth into two distinct bodies.
Might that also be the same with Pluto and Charon?
I bring this up solely on the one piece of inferred evidence about their revolution/rotation situation being similar to ours.
I read a piece in Scientific American last fall (September?) which suggested that actually, Pluto’s eccentric orbit wasn’t a sign that it was a captured Kuiper bely object, but rather indicated that Pluto once had a fairly regular orbit that had been perturbed by changes in
Neptune’s orbit.
For example, Pluto and Neptune ar in sync (3:2?), so that even when Pluro is crossing Neptube’s orbit, there is no cjance of a collision.
How about “Planets must be significantly larger than other objects whose orbits are in close proximity”? If one of the four largest asteroids were alone in it’s orbit, it may have been considered a planet all along. Isn’t tt the multitude of asteroids which gave them their title?
RM Mentock, is your link supposed to be to a book on “Improving Reading Comprehension” or did they change the URL on you? If it’s the right link, I don’t get it.
missdavis102 - Pluto was not downgraded. Here is a press release from the International Astronomical Union, the group that would make such determinations. IAU Press Release 01/99 http://www.iau.org/PlutoPR.html
The problem is as pointed out by Chronos - there is no consistent, non-arbitrary definition of a planet. Our names for heavenly bodies are heavily attributed to historical artifacts. Planet means “wanderer”, which just meant that while all the other stars (except those pesking “falling” ones) stayed put with respect to each other and turned around earth in a regular pattern, the “wanderers” (planets) moved around differently, being in different places on different nights. It was only centuries later that we started figuring out what they were - some are balls of rock, some are balls of gas. Now we can make distinctions between the inner rocky planets, the outer gaseous ones, and the icy debris around the outside. Pluto and Charon become the largest of the Kuiper belt objects. Several rocky moons are larger than our Moon, and larger than Pluto (and Mercury, I think). Thus there are moons that alone would be planets, planets that could easily be moons, and a planet that is really just a big pile of icy debris like a bunch of smaller piles of icy debris. Oh, and the asteroids, which are rocky type objects, most of which are too tiny to be considered, but a few (listed before) are large enough that their gravity pulls them to a spherical shape.
So while I really like Chronos’ choice, there is a lot of historical cultural pressure to retain the current listing of nine, and thus the large asteroids are too small (arbitrarily so), and Pluto is big enough to be distinct from the other Kuiper Belt objects.
RM - your link didn’t seem to be correct.
Mjollnir, you are correct that both Pluto and Charon are tidally locked to each other. It is possible that Charon is a piece off of Pluto, but also possible it is captured.
jti - you are right about the respective orbits of Neptune and Pluto precluding a collision. I have not heard about the disturbed orbit hypothesis vs. the captured hypothesis, but suppose it is possible.
ZenBeam, your definition does not adequately distinquish the asteroids. They are, after all, very far away from each other. You probably would not be able to see one to the next with the naked eye. Also, define “close proximity” of orbits. Neptune and Pluto’s orbits cross, doesn’t that make them in close proximity? Here is a good link on asteroids. http://www.seds.org/nineplanets/nineplanets/asteroids.html
The reason asteroids were seen as distinct from planets is size - they are lots smaller than even moons. But the distinction is still arbitrary.
it’s been a while since I read the article, but I think the bit about Pluto was sort of a by-product of the author’s main thesis.
She started out wondering how the four gas giants could form so far out from the sun, when most computer simulations suggested that the thickest part of the primordial planetary cloud would have been closer to the sun.
Her conclusion, backed up by math that I frankly could not follow, was that all four of the gas giants formed much closer in than their current orbits. However, that arrangement was unstable, and they all migrated outwards, to varying degrees, creating the stable orbits we currently observe.
By this thesis, Pluto originally had a reasonably normal orbit, about where you would expect a minor planet to form - right on the edge of the planetary cloud. When Neptune migrated outwards, its much larger gravitational pull perturbed Pluto into its current, highly eccentric orbit - or so the theory goes.
(The theory may also account for Neptune’s unusual axis - if it was moving outwards, it could have collided with debris as it went, sufficient to tip the planet.)
I did some looking around, and those asteroids don’t seem to satisfy your second condition: 1 Ceres seems distinctly football shaped, 4 Vesta has a noticeable non-sphericity, and although I didn’t find a photo of 2 Pallas, its dimensions are listed as 570 x 525 x 482km. 103 Hera is quite small, but you probably meant 10 Hygiea, right? I didn’t find axial data for Hygiea, but we probably wouldn’t call it a planet if the other three weren’t.
That first condition would almost eliminate Pluto! But it doesn’t. Charon’s gravitation attraction is only 7/8 that of the sun, by the nine planets site data.
Apparently, you’ve hit upon a “non-arbitrary” way to define the nine planets! Excepting the moon, and we know why they don’t call it a planet: it’s the original moon, with a small m even.
From your site, the two largest asteroids differ in mean orbital diameter by 0.2 percent. The third largest asteroid has a 15 percent smaller mean, but there are other asteroids between them, only a few percent different in orbital diameter.
Contrast this with Neptune vs. Pluto: Pluto’s orbit is 30 percent larger in mean distance, and there aren’t any sizeable bodies between them. Earth’s orbit is only 38 percent larger in mean distance than Venus, so the Pluto/Neptune ratio isn’t out of line with others.
I intentionally didn’t define an arbitrary value for “close proximity”, but clearly the asteroids in the main belt are significantly closer to each other than even Pluto and Neptune.
Okay, I realize this doesn’t have much to do with the OP, but I have a question. Is the asteroid belt between Mars and Jupiter a former planet, or is it a failed planet, or is is just left over debris from the formation of the solar system?
adam: Nobody seriously believes anymore that the Belt is the remnants of a planet… I don’t think that anyone other than sci-fi authors (not that there’s anything wrong with sci-fi) and philosophers ever took that idea seriously. I’m not sure what the distinction is between “failed planet” and “debris”, I’m afraid, but either could reasonably be used to describe the situation.
A few posts up, someone mentioned the tidal lock of Pluto and Charon to each other, and that of Luna to Earth, and considered that it might be a clue as to formation. Alas, it’s not: Any two bodies of sufficient mass which are sufficiently close for a sufficient time wil become tidally locked. Distance is the most significan factor, and the composition of the bodies can make a difference, too. Most satellites are locked to their primaries, Venus is in the process of becomming locked to the Sun, and Mercury is in a “harmonic lock”: its rotational period is exactly 2/3 of its orbital period. Interesting factoid about tidal lock, BTW: the Moon (say) would only actually keep the same face to Earth if the orbit were perfectly circular, wheras in actuality, it actually ends up keeping the same face towards the empty focus of the ellipse. Turns out that point is good for something, after all!
RM, I thought that Hera was in the top 4… What’s number 3? And I’ll take your word on those measurements; all of my astronomy books are up at the office. I still maintain that Luna is a planet in its own right, however.
