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What do we really know about the Kuiper belt? Fifth dispatch from the annual planets meeting

The views expressed are those of the author and are not necessarily those of Scientific American.


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Kuiper Belt object QuaoarFAJARDO, Puerto Rico—It smacked of a cunning plan. The organizers of last week’s planets conference put one of the best talks in the very last session of the very last day. Most scientists had either left for the airport or the beach. I almost didn’t make it myself—the room and time got switched at the last minute.

If the speaker, Wesley Fraser of the California Institute of Technology, is right, planetary scientists are going to have to rethink the Kuiper belt—the vast band of smallish planets that orbit beyond Neptune. Using the Hubble Space Telescope, Fraser and his colleague Mike Brown observed one of the largest of these objects, Quaoar, and its little moonlet, Weywoot, to refine estimates of its size and mass. They found that Quaoar is smaller than previously thought, only 900 kilometers in diameter. Consequently, it must be denser—about four grams per cubic centimeter. This makes it by far the densest Kuiper belt object (KBO). It outdoes even a fairly dense asteroid such as Vesta.

Planetary scientists typically explain dense KBOs by imagining that they started off larger and less dense; they then collided with one another, stripping off lighter material such as ice and leaving behind mostly rock. Yet by the new estimate Quaoar is denser even than rock, so even completely stripping its ice wouldn’t be enough. Moreover, astronomers detect ice on its surface. It makes no sense. "It’s confusing, to say the least," Fraser says.

That’s only half of it. Most KBO moonlets have circular orbits. That is thought to be a consequence of coalescing from collisional debris: debris rings naturally settle into a circular shape. Yet Weywoot’s orbit is distinctly oblong. "We have to completely rewrite the book," Fraser concludes.

One radical idea is that Quaoar is a refugee from the asteroid belt between Mars and Jupiter. Asteroids tend to be denser than KBOs, and Jupiter might conceivably flick one to the farthest reaches of the solar system. But the hapless exile would tend to wind up on a highly elliptical orbit around the sun, whereas Quaoar’s orbit is nearly circular.

Erik Asphaug of the University of California, Santa Cruz, offered another explanation based on work he presented earlier in the week. Perhaps Quaoar collided with a much bigger body—something approaching the proportions of Mars. Like a Mini blindsided by a Hummer, Quaoar would have gotten seriously banged up by such a collision, maybe enough to give it an anomalously high density. If so, Quaoar might be living proof that substantial planets used to orbit in the distant solar system and may still lurk out there.

Clearly this is one of those "more data are needed" situations, and Fraser has applied for additional telescope time. One thing is sure, though. People typically call KBOs icy bodies, like giant comets. But they can also be rocky, like small Earths.

Previously:
From carbon planets to the lakes of Titan: Dispatch from the annual planets meeting
What caused Saturn to lurch? Second dispatch from the annual planets meeting

Planetary bombardments, past and future: Third dispatch from the annual planets meeting

LCROSS strikes Earth’s moon as other moons continue to puzzle: Fourth dispatch from the annual planets meeting

Photo of Quaoar from the Hubble Space Telescope: NASA and M. Brown (Caltech)





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  1. 1. laurele 6:07 pm 10/13/2009

    The spherical bodies in the Kuiper Belt are planets. You should get someone representing this view to speak at these conferences. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASAs New Horizons mission to Pluto. One reason the IAU definition makes no sense is it says dwarf planets are not planets at all! That is like saying a grizzly bear is not a bear, and it is inconsistent with the use of the term dwarf in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Plutos orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless. Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity–a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned. I’m one of them and also a graduate school at Swinburne University planning to specialize in studying the Kuiper Belt.

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  2. 2. gigabetz 5:12 pm 10/14/2009

    For what it’s worth, my instincts tell me that the object was much larger and hit Mars when it had surface water. Something cataclysmic happened to Mars and the evidence is in the Kuiper Belt.

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  3. 3. Michael Hanlon 7:00 pm 10/14/2009

    May we discuss Apophis and Project Asimov here, since we lost the other site? Don’t want to intrude but I think we have a better chance of understanding the bodies of the Kuiper zone if we get "hands on" a possible progeny.

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  4. 4. Michael Hanlon 4:09 pm 10/15/2009

    Sorry to intrude again Kuiperians. I found that SciAm has a page behind this one on which the plane bombard discussion continues. Come on over and help out. I really think that if anything can be gleaned from 99942 Apophis’ existence which would add to the general knowledge of Solar System Origins, that may be it. Don’t be shy. sometimes we say things that are wrong and sometimes we say the wrong things. Bottom line we think this is do-able.

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  5. 5. tulcak 5:55 pm 10/15/2009

    why couldn’t it be an artifact? why do we try to explain anything outside of our thin atmosphere in terms of what we observe within our biosphere?

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  6. 6. Daniel35 5:22 pm 10/18/2009

    I believe the early definition of "planet" was "wanderer" an object that wandered among the background stars, therefore, today, objects that orbit the sun (or other star). We’ve since identified some smaller objects as moons, asteroids and such, but they still orbit the sun (perhaps indirectly), so it seems in some sense they should also be considered planets.

    In any case, I see I see this as nit-picking, and the great majority of astronomy, especially cosmology, to be recreational (even when I get into it) having little direct affect on life here and now. Of course all recreation, such as puzzles, has some value in stimulating the mind, but I think this could be done in less expensive ways. I exclude from this efforts like finding asteroids on Earth-collision courses and studying the sun to know how it will effect our future. Otherwise, we have lot’s of more important, and expensive, things to take care of here on Earth.

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  7. 7. rwilliston 5:14 pm 10/20/2009

    Daniel35, we have more important things to attend to, according to you. I might propose that learning about the universe we live in is the most important thing we are doing. Just the word "expensive" brings the element of money into it and money as an artificial construct created as a substitute for real goods or services shouldn’t decide what we as a species aspire to or are capable of. No, I don’t mean we should bankrupt the country in a space race, but if one-tenth the money was spent investigating our universe as was spent on trying to invent new enemies and then defeating them, I think the world would be a better place. And just because passively tracking rogue asteroids (that we can’t really do anything about) should help you sleep at night doesn’t mean that this study should be favoured over other intellectual pursuits.
    Personally I find tracking asteroids, searching for extrasolar planets, running around on Mars with remote controlled cameras and things like SETI to get far more glamour than they should. That’s because the average Joe can identify with these things and they perk up when they’re on the news. Passively watching for blips that we can’t influence is not as important as understanding the laws of the universe we live in. If we study the physics we will invent new technologies that might help us go (or not have to go) to these places and will have unimagined benefits for us right here at home as well. So rather than engineering NASA to a budget, let’s ask what theories we need to be testing and how do we do that and find a way to make it happen.

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  8. 8. Michael Hanlon 8:21 pm 10/20/2009

    Sorry to intrude again, but these issues seem to be closer to home than the Kuiper Zone. Everybody who can see a benefit if mankind had space faring capabilities raise your hand. Good, I’m glad we can agree that if we had them, spaceships wouldn’t be a bad thing. So, we all do want to do it on the cheap so buckeroos are around for other projects. Again, everyone agree? Good Then I’m going to outline a way to get to that space faring capability.
    Think of those tumbling bits of space jetsam, asteroids, as abandoned vessels, left by some previous sky crossing race. The power is gone, the life support is down and if there’s any communication ability, it won’t reveal itself. Applying law of the seas, any abandoned vessel need only be occupied to claim ownership.
    Now a parallel posit. To succeed in space and to go other places, not just low Earth orbit, we’d need a hundred short haul vessels to support ten long rangers. Well where do we stand on that tote board? We’re lucky if we can count ten short haul vessels in our navy of the vacuum. If I told you that some already built long haulers were out there and just needed fueling to make them ours, wouldn’t you agree that it would be well worth it, meet the previously stated requirements of procedure fiscally, and join the plan to increase the Earth’s Flotilla?
    Those vessels are the rocks that are currently being catalogued. If we put thrusters on them, swing them through gravity wells and other rocket science ploys, we could use their mass to carry us places we couldn’t get to otherwise. What makes them different from a ship? they have no quarters/habitats? strap them on! They need fuel? maybe the right ones supply their own fuel! Not enough fuel to get back? Find another rock and switch places and come home.
    With the long haulers built, or tumbling, we need and can concentrate on the short haulers. Like I said, we need 100 of them to be worthy of taking the name space farers.

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  9. 9. Daniel35 3:33 am 10/21/2009

    rwilliston, you saying " trying to invent new enemies" makes it sound like you thought I talking about having wars instead of going to space and understanding the universe. I meant that there are several different reasons our human community is about to collapse, and 2012 is pretty far down the list. My list would start with global warming, peak oil, government corruption, all caused by our excessive success as a species, leading to a runaway population. Space travel and advanced science is a fantasy of the past to those in the know. And still we wonder why we haven’t, provably, heard from ET. He probably went the way we’re headed. Maybe it’s an common scenario for ‘advanced’ cultures.

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