| I | INTRODUCTION |
Kuiper
Belt (pronounced KY-per), a collection of frozen objects made of ice,
dust, and rock that orbit the Sun in the outer solar system. The belt extends
from just beyond the orbit of the planet Neptune to well beyond the orbit of
Pluto. The objects in the Kuiper Belt are called Kuiper Belt Objects (KBOs) and
range in size from clumps of ice and dust up to planetary bodies larger than
Pluto. The orbits of these objects show that the belt is actually
disk-shaped.
Nearly 1,000 Kuiper Belt Objects have been
found. Astronomers estimate that more than 100,000 KBOs larger than 50 km (30
mi) in diameter may exist. The Kuiper Belt therefore is far more extensive and
contains far more large objects than the asteroid belt, a region of rocky debris
between the orbits of Mars and Jupiter. Including the billions of comets
believed to orbit in the Kuiper Belt, scientists estimate that the belt’s total
mass is about 1 to 3 percent the mass of Earth.
The small icy bodies in the Kuiper Belt
sometimes turn into visible comets when their orbits are disturbed to bring them
into the inner solar system. When the nucleus of a comet comes near enough to
the Sun, heat causes the object to give off gas and dust as a coma and a tail.
The Kuiper Belt is considered the likely source of short-period comets, which
orbit the Sun in the main plane of the solar system in periods shorter than 200
years. The largest KBOs have planetlike properties, including a rounded shape
from effects of their own gravity and an inner structure that likely has
separated into a rocky core surrounded by layers of ice. Dozens of such
planetlike objects may orbit in the Kuiper Belt.
The existence of the Kuiper Belt was first
predicted during the mid-20th century, most notably by Dutch American astronomer
Gerard Kuiper. Kuiper and other astronomers expected that a debris belt, similar
to the asteroid belt of rocky material that orbits the Sun between Mars and
Jupiter but composed of icy material, might lie beyond Neptune. The first
searches for the Kuiper Belt, however, were unsuccessful. Astronomers now know
the early searches failed because the photographic technology in use at the time
was not sensitive enough to find KBOs. By the late 1980s, astronomers had access
to a kind of electronic camera called a charge-coupled device (CCD). CCDs are
much more sensitive than traditional photography, allowing them to detect
fainter objects. In 1992 astronomers Jane Luu and David Jewitt found the first
Kuiper Belt Object. This KBO, designated 1992QB1, is more
than 1,000 times fainter than Pluto.
| II | CHARACTERISTICS |
Astronomers were studying the Kuiper Belt long
before the first KBO was detected. The orbits of objects within the belt fall
into two major ranges: the inner belt and the scattered belt. Most known KBOs,
called classical KBOs, orbit in the inner belt from 35 to 55 astronomical units
(AU) from the Sun. (An astronomical unit is the average distance from Earth to
the Sun, just under 150 million km [93 million mi].) Some KBOs have been
discovered in the scattered belt, which stretches out beyond 1,000 AU. Although
still largely unobserved, the scattered belt is thought to be as populous or
more so than the inner belt.
The term plutino is used for large
KBOs that are influenced by Neptune’s gravity and make two orbits around the Sun
for every three orbits Neptune makes (a 2:3 orbital resonance). This group
includes Pluto and smaller bodies such as Orcus, Ixion, Rhadamanthus, and Huya.
KBOs with orbits that are not directly linked to the orbital period of Neptune
are called classical Kuiper Belt Objects or “cubewanos” (after the temporary
name QB1 given the first object discovered). This group of KBOs
currently includes the named objects Eris, Quaoar, Varuna, Chaos, and Deucalion.
Also in this group are two objects that are about 75 percent the size of Pluto,
with the temporary designations 2003 EL61 and 2005 FY9. Another recently
discovered KBO, named Sedna, is more than half the diameter of Pluto and has an
orbit that extends out of the Kuiper Belt to about 130 billion km (about 84
billion mi) from the Sun at its farthest point. Sedna is currently at the
nearest point of its orbit, about 13 billion km (about 8 billion mi) from the
Sun.
The largest known KBO is Eris with a diameter
of about 2,400 km (1,490 mi), slightly larger than Pluto, which has a diameter
of about 2,360 km (1,475 mi). The discoverers of Eris—Michael Brown of the
California Institute of Technology, Chad Trujillo of Gemini Observatory, and
David Rabinowitz of Yale University—originally called the object the “tenth
planet” of the solar system because of its size. In 2006 the International
Astronomical Union (IAU) classified Eris as a dwarf planet, a new category that
included Ceres (formerly considered the largest asteroid) and Pluto, originally
counted as the ninth planet. Many scientists, however, have not accepted the
IAU’s new definition of a planet that changed the status of Pluto from a true
planet to a dwarf planet that the IAU now lists with a number in the catalog of
minor planets.
Eris completes one orbit in 560 years and is
currently the most distant known body in the solar system at 14.5 billion km (9
billion mi) from the Sun. At the near point of its orbit, Eris comes to within
38 AU (5.7 billion km/3.5 billion mi)—inside the orbit of Pluto. Eris’s orbit is
tilted 44° to the main plane of the solar system, much more inclined than
Pluto’s orbit, which is tilted 17.2°. Because of its eccentric and tilted orbit,
Eris is considered a scattered disk object that ranges outside the inner Kuiper
Belt.
Small KBOs greatly outnumber large ones.
Computer simulations indicate that the Kuiper Belt, like the asteroid belt,
formed early in the history of the solar system. One theory holds that the
Kuiper Belt was coalescing into one or more large planets when the growth
process was interrupted. The formation of Neptune may have disturbed the region
gravitationally and interrupted this growth.
The Kuiper Belt differs from the asteroid belt
in two significant ways. First, KBOs formed more than ten times as far from the
Sun as most asteroids. Second, they contain far more ice than asteroids, which
are primarily rocky objects.
Based on their knowledge of the composition
of Pluto and its moon, Charon, astronomers expect that KBOs consist of water-ice
and rock, with some organic and other complex compounds as well. KBOs have a
wide range of surface colors, varying from almost gray to very red. Their
surfaces are usually quite dark, only reflecting from 3 percent to perhaps 25
percent of the light that falls on them. As a result, if one were to stand on a
KBO, the surface would appear blacker than dirt. Based on the distance of KBOs
from the Sun and their surface reflectivity, astronomers estimate that KBO
surface temperatures are typically about -220°C (-360°F), not much warmer than
absolute zero.
Collisions in the Kuiper Belt create craters
on the surfaces of KBOs and reduce their mass. Small KBOs shrink over time
because their gravity is so weak that material thrown up when impacts occur
never falls back to their surfaces. Large KBOs, however, are not much affected
by this process. Astronomers studying results from collision models believe that
most KBOs smaller than about 50 km (30 mi) in diameter cannot have survived the
collisional bombardment over the lifetime of the solar system. KBOs smaller than
about 50 km in diameter, therefore, must be either remnants of larger KBOs or
fragments created by collisions. It is now widely accepted by researchers that
almost all Jupiter family comets, which originate in the Kuiper Belt, are bits
of KBOs chipped off by collisions within the belt.
In 2001 astronomers learned that some KBOs
have moons. Seven KBOs with moons were discovered in 2001 and 2002. As of 2005
about 20 KBOs are known to have satellites, suggesting that 10 to 20 percent of
all KBOs have moons. In September 2005 astronomers at the Keck Observatory in
Mauna Kea discovered that Eris (then called 2003 UB313) has a moon, which was
later given the name Dysnomia. Surprisingly, the KBO satellites are much larger,
relative to their parent KBOs, than planetary moons are relative to their
planets. Whereas most moons have diameters just a few percent as large as that
of the planet they circle, the KBO moons so far discovered are typically half as
large as their parent KBO. One KBO has a companion that is apparently as large
as the KBO itself. Such a pair is known as a binary KBO.
| III | RESEARCH |
The Kuiper Belt is an exciting area of
research in astronomy. The discovery and astronomical exploration of the Kuiper
Belt over the past decade have fueled a revolution in scientists’ views of the
solar system. Today astronomers recognize the Kuiper Belt as the third major
region of the solar system (the other two regions are the inner solar system,
with its small rocky planets, and the outer solar system, with its gas-giant
planets). They believe that the belt helps explain Pluto’s small size, eccentric
orbit, and icy composition—characteristics so different from those of other
planets but so similar to KBOs. Astronomers also see the belt as the site of the
initial stages of planet-building in this part of the solar system long ago.
This recognition, combined with the intense scientific interest in Pluto and its
moon, Charon, prompted astronomers to request the first mission to explore
Pluto, Charon, and the Kuiper Belt Objects.
The National Aeronautics and Space
Administration (NASA) launched the New Horizons Pluto-Kuiper Belt mission in
January 2006. After swinging past Jupiter for a gravity-assist boost in 2007,
New Horizons should reach Pluto in 2015. The New Horizons spacecraft will
explore the Pluto-Charon system with cameras, spectrometers, and other
instruments, and then fly on to visit a number of KBOs in the following few
years.
In the meantime, astronomers will continue
to use telescopes on the ground and the Hubble Space Telescope in Earth orbit to
discover new KBOs and to learn more about this population of ancient relics left
over from the birth of the solar system. NASA’s Spitzer Space Telescope,
launched in 2003, is an extremely useful tool for observing KBOs. Spitzer can
determine the temperatures, reflectivities, and sizes of KBOs directly by
detecting their infrared emissions.
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