Time - Man of the Year/Time_Man_of_the_Year_Compact_Publishing_3YX-Disc-1_Compact_Publishing_1993.iso/moy/092892/0928320.000

Text File | 1993-04-08 | 6KB | 117 lines

                                                                                SPACE, Page 59

Beyond Pluto


Astronomers spy a faint icy body at the solar system's edge
-- evidence of the breeding grounds of comets

By MICHAEL D. LEMONICK


    Where does the solar system end? At Pluto, most folks
would reply. Or at Neptune, the cognoscenti might say, because
thanks to Pluto's odd, egg-shaped orbit, the eighth planet has
been outermost since 1979 and will be through 1998. But
astronomers suspect that the sun's family actually extends far
beyond either of these two planets. Out there in the frigid
darkness beyond any known planet, they believe, lies the Kuiper
belt, a ring of dusty ice chunks that surrounds the solar
system. Beyond that, astronomers say, is the similarly composed
Oort cloud, which forms a vast sphere around our planetary
system. The cloud stretches two light-years from the sun,
halfway to Alpha Centauri, the next nearest star. Occasionally
one of the icy lumps in these outer regions is nudged toward the
sun by a passing star or gas cloud. As it falls toward our
world, it flares into view as a streaking comet.

    Ever since these vast comet nurseries were first proposed
four decades ago, the only evidence for them has been indirect
and theoretical. At last there is something concrete. A tiny
reddish spot of light recorded on a sensitive electronic
detector in Hawaii last month appears to be the first component
of the Kuiper belt ever observed. The body, known for now as
1992 QB1, is about 200 km (120 miles) across, and a preliminary
calculation puts it at more than 5.1 billion km (3.2 billion
miles) away. That doesn't necessarily make it the most remote
object in the solar system, since Pluto retreats to more than
7 billion km from the sun. But it does imply that the Kuiper
belt and the Oort cloud really exist and that the solar system's
boundary may lie 10,000 times as far away as Pluto ever
ventures.

    The discovery was no accident. David Jewitt, a University
of Hawaii astronomer, and Harvard's Jane Luu, now a
postdoctoral fellow at the University of California, Berkeley,
had been searching for just such an object for five years. Says
Jewitt: "We were trying to understand why the outer solar system
is so empty." Is it because there is really nothing out there
or because things are just hard to see?

    There were already several reasons to think the latter is
true. For one thing, the existence of the belt and cloud are
natural consequences of established theories about the birth of
the solar system. According to such theories, the early sun,
formed from a cloud of gas and dust, was surrounded by a
disk-shaped nimbus made up of the leftovers. The newborn star's
heat drove smaller particles and gases, including water vapor,
out from the center. The heavier, metal-rich rock left behind
condensed into asteroids and the inner planets: Mercury, Venus,
Earth and Mars. Much of the gas and light dust farther out was
gathered up into the so-called gas giants: Jupiter, Saturn,
Uranus and Neptune. The rest was blown by solar heat and wind
to the outskirts, where it presumably congealed into chunks of
ice and dust. (Rocky Pluto is an anomaly, and many astronomers
believe it isn't a planet at all but a giant comet or asteroid
flung into its present position when it had a close
gravitational encounter with one of the outer planets.)

    The existence of comets provides further evidence for the
Oort cloud and the Kuiper belt. Comets are orbiting chunks of
dusty ice, whose surfaces evaporate in the warmth of the sun to
form halos and tails. In the early 1950s, Dutch astronomer Jan
Oort suggested that comets originate in a cloud surrounding the
solar system; he based his theory on their highly elongated
orbits, which reach into the inner solar system and out beyond
Jupiter. Shorter-period comets like Halley's, which returns
every 76 years, are believed to originate closer in, hurtling
out of the Kuiper belt, a region first proposed by Oort's
countryman and contemporary Gerard Kuiper. Because repeated
solar heating would boil a comet away after a few million years,
the fact that new ones keep appearing suggests that there is a
large supply.

    "What makes us happy," says Jewitt, "is not just that we
may have found the source of the short-period comets, but also
that these objects have stayed largely unchanged since the
solar system formed." QB1's color probably reflects what little
change has happened: carbon compounds on its surface have been
bombarded with cosmic rays for eons, turning it reddish.

    Jewitt and Luu caution that the object's identity will
have to be confirmed, a process that will take a month or two.
Agrees Brian Marsden, the Harvard-Smithsonian astronomer who
actually calculated its orbit: "All we can say for sure right
now is that it's far away, and that it is most likely one of the
larger members of the Kuiper belt. But it could be something
else." If it is part of the belt, a worldwide search will begin
for similar objects.

    Proof that the Kuiper belt exists would help demonstrate
that another long-sought object almost certainly does not. For
nearly a century, astronomers have been looking for a Planet X,
a world conjectured to lie far beyond Pluto. But the planet's
gravity would have scattered any belt of proto-comets far and
wide. Planet X was first dreamed up to explain apparent
irregularities in Neptune's orbit. Recent studies have shown
those irregularities to be an illusion -- and the sighting of
QB1 has probably dashed forever the hope of finding a 10th
planet.