[meteorite-list] Pluto's Exotic Playmates

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Sep 13 12:55:11 2006
Message-ID: <200609131632.JAA00381_at_zagami.jpl.nasa.gov>

http://www.nytimes.com/2006/09/12/science/space/12belt.html

Pluto's Exotic Playmates
By KENNETH CHANG
New York Times
September 12, 2006

With a quick vote last month, the International Astronomical Union
decreed that Pluto was no longer the ninth planet, but just a dwarf
planet - and not even the largest dwarf - orbiting in a distant
ring of icy debris.

But perhaps that should not be seen as a slight to Pluto.

For many astronomers, that ring of icy debris, known as the Kuiper Belt,
has become an exciting spot for innovative research and has changed how
they view the solar system.

"It's a lot bigger now," said Marc W. Buie, an astronomer at the Lowell
Observatory in Flagstaff, Ariz. "For me, it's like somebody invented a
new field of science."

Harold F. Levison of the space studies department in the Southwest
Research Institute in Boulder, Colo., said, "The more we learn, the
weirder it looks."

More than 1,100 Kuiper Belt objects have been found so far. Astronomers
estimate that half a million bodies larger than 20 miles wide are
floating out there. At least one appears to be mostly rock with a
coating of ice. Some are mostly ice. Some are less dense than ice,
indicating a Swiss-cheese-like structure. A surprising number of them
have moons.

Some move in clockwork with Neptune; Pluto, for example, is in what is
called a 3:2 resonance, taking 1.5 times as long as Neptune to loop the
Sun. Many Kuiper Belt objects have been flung into orbits crazily tilted
to the rest of the solar system.

"This is really a very exotic zoo out there," said S. Alan Stern,
executive director of the space science and engineering division at the
Southwest Research Institute and principal investigator of NASA's
New Horizon spacecraft, which is currently heading to Pluto.

The distribution of Kuiper Belt objects has already provided decisive
evidence that Neptune was once perhaps nearly a billion miles closer to
the Sun and was then gravitationally nudged outward. Astronomers also
hope that the Kuiper Belt preserves a frozen record of the earliest
building materials of the solar system.

"It's kind of like the solar system's attic," Dr. Stern said. "It's like
an archaeological dig into the history of our solar system."

Scientists had initially expected a simple structure for the belt: a
thin disk of objects traveling in circular orbits in the plane of the
solar system. Some Kuiper Belt objects do fit that profile, and those
are called the classical Kuiper Belt objects. (One mystery is why there
appears to be a sharp edge at about 4.5 billion miles, with no classical
Kuiper Belt objects beyond that distance. Some think a passing star did
that.)

Other Kuiper Belt objects share orbits similar to Pluto's, in resonance
with Neptune. Those in the same 3:2 resonance as Pluto have been called
the Plutinos.

Still others are called the scattered-disk Kuiper Belt objects. These
appear to have been tossed into highly elliptical orbits, often at a
sharp angle to the rest of the solar system. Surprisingly, these include
some of the larger Kuiper Belt objects, including 2003 UB313, nicknamed
Xena, which is larger than Pluto.

An estimated 15 percent of Kuiper Belt objects are binaries - pairs of
bodies of similar size and mass. Among some classical Kuiper Belt
objects, that fraction may be as high as 30 percent - possibly higher,
because even the Hubble Space Telescope cannot distinguish two separate
objects if they are too close to each other.

Theorists puzzled about how such small bodies, with weak gravitational
pull, could have paired up so often. The answer, it turns out, is that
as two objects flew past each other, the gravitational drag generated by
many other much smaller Kuiper Belt objects slowed them enough to
capture each other.

That mechanism requires a fairly dense Kuiper Belt with a total mass of
at least 10 Earths. But while Kuiper Belt objects are many, they do not
amount to much today. Adding the masses of Pluto, Xena and the half
million other objects, even those not yet seen, gives an estimate of
just one-tenth the mass of Earth.

"The mass we measure is pathetic," said David C. Jewitt, a professor of
astronomy at the University of Hawaii.

That, in turn, produces a quandary. Where has 99 percent of the Kuiper
Belt gone? This is, as the planetary scientists quaintly put it, the
cleanup problem. (One popular idea: repeated collisions smashed most of
them to dusty bits, and the bits were blown away by solar radiation.)

Just 15 years ago, the Kuiper Belt was not on the map at all. The known
solar system essentially ended at Neptune, except for the occasional
comet interloper from far away. (Fifteen years ago, Pluto was traveling
along the inner part of its eccentric orbit, closer to the Sun than
Neptune.)

Gerard Kuiper, a prominent astronomer for whom the belt is named,
speculated about the possibility of it in 1951. Kuiper was prescient,
but wrong on a major point. He thought that the belt had existed early
in the solar system's history but that Pluto, then thought to be a more
massive planet, had scattered it away.

Some people have suggested that the belt should instead be named after
Kenneth Edgeworth, an Irish astronomer who vaguely hypothesized an icy
disk a few years before Kuiper. But that was just a guess.

"The outer solar system was just this sort of empty space," said Michael
E. Brown, a professor of planetary astronomy at the California Institute
of Technology.

Astronomers did have a few clues that something was out there. One was
Pluto, an oddball. Unlike the four rocky inner planets or the four outer
gas giants, Pluto is half ice, and its orbit is quite elliptical and
tilted 17 degrees to the solar system's ecliptic plane. It did not fit in.

A second clue showed up in 1977 with the discovery of Chiron, an icy
body between 90 and 130 miles wide that loops around on an elliptical
path taking it as close to the Sun as Saturn and as far out as Uranus.
Astronomers argued over whether it should be called an asteroid or a
comet. In the end, they decided both, and created a new category,
Centaurs, to describe small bodies orbiting among the giant planets.

More interestingly, Chiron's orbit is unstable, meaning that within a
million years or so, it will probably swing too close to Saturn and be
tossed out of the solar system or onto a cometlike trajectory passing
closer to the Sun. That also means that Chiron entered its current orbit
in astronomically recent times. Like a lamb wandering the streets of
Manhattan, it had to have come from somewhere else. No one knew where.

A third, crucial clue came from comets. Some comets, known as
long-period comets, visit the inner solar system once in thousands or
millions of years, or simply once. Others like Halley's Comet are
short-period comets that swing by more frequently, every few decades or
centuries or so.

For a long time, most astronomers theorized that short-period comets
were long-period comets that had been deflected by the gravity of a planet.

In 1988, computer simulations by three Canadian astrophysicists - Martin
J. Duncan, Thomas R. Quinn and Scott Tremaine - showed that the orbits
of deflected long-period comets would not match those of observed
short-period comets and that their more likely origin was a ring of
debris by Neptune. Those computer simulations spurred several teams to
start searching more actively the outskirts of Neptune.

Dr. Jewitt, then at the Massachusetts Institute of Technology,
and his graduate student Jane X. Luu had already started looking.

"Our search was very simply motivated by the surprising emptiness at the
edge of the solar system," Dr. Jewitt said. "We would have been happy
with either answer: empty because it was empty, or empty because no one
had looked."

For years, they found nothing. To find moving objects requires taking
repeated photographs of a region and looking for the points of light
that moved between the images. The limits of digital technology stymied
the early searches.

Finally, in 1992, they found 1992 QB1, probably about 100 miles wide,
the first Kuiper Belt object.

Dr. Brown had a reaction similar to many astronomers. "It's like, 'Oh my
god, Pluto finally makes sense,'" he said. "It's no longer an oddball
at the edge of the solar system."

Pluto was instead the harbinger of many properties now seen in Kuiper
Belt objects: the resonant orbits, the moons, the icy ingredients.

About that time, Renu Malhotra, a professor of astronomy at the
University of Arizona, was calculating the effects of Neptune?s
migrating outward, as some had hypothesized, early in the history of
the solar system. Her calculations indicated that Neptune would
effectively snowplow smaller objects into resonant orbits.

The first Kuiper Belt objects in resonant orbits were discovered in
1993. The distribution of resonant Kuiper Belt objects fit with what she
predicted, and now planetary scientists uniformly agree that the giant
planets were not born in their current orbits but migrated there.

Scientists are looking for more clues about the early history of the
solar system. Eugene Chiang, a professor of astronomy at the University
of California, Berkeley, theorizes that the solar system used to
contain several more gas giant planets that were subsequently ejected,
but that their gravitational effects remain imprinted in the Kuiper Belt.

Dr. Levison of the Southwest Research Institute, however, offers a
different account. He and his collaborators have created a model where
the solar system was initially much more compact, with all the giant
planets forming well within the current orbit of Uranus.

That hypothesis sidesteps the cleanup problem, because shrinking the
Kuiper Belt increases its density. His model predicts that gravitational
wobbling between Jupiter and Saturn created wild oscillations in the
orbits of Neptune and Uranus, with the two swapping places repeatedly.

Dr. Levison said he could not prove that his model was correct, just
that it reproduces what is seen today in the solar system. And the
Kuiper Belt was a key component in creating his model.

"Sometimes how the blood is splattered on the wall tells you more about
what happened than the body," he said.

The Kuiper Belt, Dr. Levison said, is "the blood splattering on the
wall," adding, "If we're going to understand what happened, it's going
to be by studying the Kuiper Belt."
Received on Wed 13 Sep 2006 12:32:25 PM PDT