The Edgeworth-Kuiper Belt has been proposed by some astronomers
to be named after
Kenneth
Essex Edgeworth (1880-1972) and
Gerard
Peter Kuiper (1905-1973), but many astronomers refer to this
region of planetary objects more simply as the Kuiper Belt. Before
Jan
Hendrik Oort (1900-1992) raised his hypothesis of a cloud of
dormant comets surrounding the Solar System in
1950,
Edgeworth suggested in
1943
(with further elaboration in
1949)
that a reservoir of comets should exist beyond the planets as a
very large number of small bodies, or clusters of bodies, which
occasionally enter the inner solar system as comets. He noted
that, at such great distances from Sol, collisions between
planetesimals in the protoplanetary disk
were so infrequent that only small bodies (or clusters) could form.
In 1951, Kuiper forcefully gave his view (in a written summary of
developments in planetary science up to the middle of the 20th
Century at the 50th anniversary symposium of Yerkes Observatory)
that some comets may originate in a region closer to the Solar
System than 100,000 AU, where the Oort Cloud
is. In other words, the region of space beyond Neptune outward to
120 AU could be populated with enormous numbers of icy bodies that
are essentially dormant comets. (Continuing discussion over an
appropriate name for these objects -- e.g., Trans-Neptunian
Objects -- and their "belt" can be found at the
Kuiper
Belt Page and in
David
C. Jewitt, 1999.)
During the 1980s, a belt of Edgeworth-Kuiper "proto-comets" was
proposed by
Julio
A. Fernandez (1980) and by M. Duncan, T. Quinn, and
S. Tremaine as the origin of short-period comets. These comets
orbit the Sun in the same direction as the planets, only slightly
inclined from the
plane of
the ecliptic (near which all of the planets orbit). Since
short-period comets could not have originated from the more distant
spherical Oort Cloud as originally believed, they come from a
second, more flattened reservoir of "dormant" comets beyond the
orbit of Neptune.
NASA/NOAO/Deep
Ecliptic Survey
(Buie,
Millis, Wasserman, Chiang,
Elliot, Kern, Trilling, and Wagner)
Concentrations of EKOs found
reflect astronomers' search fields
up to June 2000, with orbits of
outer planets
(Jupiter
to Pluto)
and ecliptic longitude. (See a
plot
of projected orbits from
the
Kuiper
Belt Page.)
A real-time
Plot
of the Outer Solar System is available from the
Minor Planet
Center.
Although the first Edgeworth-Kuiper object (EKO) was not discovered
until 1992, a multitude has been discovered since -- over 1,100
by September 2006. As predicted by planetary theorists (e.g.,
Duncan
et al, 1994; and
Levison
and Duncan, 1993), the vast majority (over 90 percent) of these objects
travel in highly circular orbits beyond Neptune where they appeared
to have formed (also called
Trans-Neptunian
Objects or TNOs -- more definitions at the
Kuiper
Belt Page). About two dozen EKOs that may have formed closer
to the Sun but subsequently were scattered by
Uranus and Neptune
(called
"Centaurs
and scattered disk objects") have also been discovered in highly
inclined, elliptical orbits extending as far out as 200 AU.
Courtesy of
Johns Hopkins University
Larger image
Most Edgeworth-Kuiper Belt objects have
been found in orbits within 55 AUs from
the Sun, but some orbit much farther.
The outer edge of the Edgeworth-Kuiper (E-K) Belt is currently
uncertain, however. Until the
announced discovery
of 2004 XR 190 in December 2005, no large EKOs beyond 50 AUs
had been found as yet in circular orbits that were unperturbed by
the giant planets (Neptune, Uranus, Saturn, and Jupiter).
It is possible that the outer E-K Belt has been "dynamically
cold" and undisturbed by Neptune's formation and hypothesized
early migration outwards by as much as eight AUs. One hypothesis
is that these outer EKOs may be relatively smaller (less than or
equal to 400 km or 250 miles in diameter) and inhabit a thin plane
that is inclined by at least 0.5° from the plane of the ecliptic.
Thus far, these distant EKOs have escaped detection by deep ecliptic
surveys, leaving astronomers to consider whether the outer E-K Belt
was perturbed by something else other than the giant planets or that
the primordial Solar nebula was smaller than currently hypothesized
(Joseph
M. Hahn, 2000; and explanation of
Lynne Allen's
summary of research links on the
Kuiper
Belt).
Pluto and Charon are
suspected as being among
the larger examples of these icy worlds of the outer Solar System,
where there may be as many as 100,000 EKOs with diameters greater than
100 km (62 miles) -- as well as a billion or more kilometer-sized
comets -- orbiting within 50 AUs of the Sun. In general, however,
EKOs are generally very small at 10 to 50 km (six to 31 miles) across
and not very bright. They take hundreds of years to complete an orbit
around the sun, which makes their detection difficult. On August
9, 2006, astronomers announced that their use of archived observations
from NASA's Rossi
X-ray Timing Explorer to detect 58 dips in X-ray radiation from
the background neutron star,
Scorpius X-1,
and its eclipsing stellar companion (which are caused by the
occultations of nearer, small extremely dim objects) leads them to
estimate that the number of even smaller EKO's of around 100 meters
(330 feet) in size may number around a quadrillion (more from the
Observatoire
de Paris-Meudon and
New
Scientist).
Ann Feild, STScI,
ESA,
NASA
Larger illustration.
Including Eris
("Xena" or 2003
UB313), the largest
dwarf planet
candidates include
many recently
discovered EKOs
that orbit the Sun
beyond the orbit of
Neptune
(more).
On July 29, 2005, a team of astronomers (including
Mike Brown,
Chad Trujillo,
and David Rabinowitz) announced the discovery of a planetary
body (Eris or 2003 UB313) that is
now estimated to be around three percent larger than
Pluto (NASA
press
release;
latest news;
and Brown et al, 2005, in
pdf).
As a result, there is renewed debate among the astronomical
community over the definition of planet and how large planetary
bodies roughtly close to the size of Pluto that are being found
in the outer Solar System beyond Neptune's orbit should be classified.
Gerhard Hahn/DLR,
Astrovirtel,
ESO,
ESA,
Institute
for Astronomy -- larger image
The largest known EKOs include Eris
(2003 UB313), Pluto,
Quaoar, Orcus,
and Pluto's moon, Charon. The next
largest EKOs are Ixion
(KX76),
Varuna,
and
2002
AW197. KX76 was
originally
estimated to be larger than Charon -- as well as Ceres, the largest
main belt asteroid -- just beyond the orbits of Neptune and Pluto. An
icy, reddish object that is now thought to be as small as 760 km (470
miles) in diameter, KX76 was found with the
Deep
Ecliptic Survey (a NASA-funded search for EKOs), in images of the
southern sky taken with the 4-metre Blanco Telescope at the
Cerro Tololo Inter-American
Observatory in Chile. (See image of KX76
in motion as colored dots.)
Most EKOs orbit the Sun in a disk-shaped region lying mostly within
the plane of the planets beyond the orbit of Neptune, or more than
30 AUs outwards from the Sun. Unlike active comets, these icy
planetary bodies are thought to be primordial planetesimals that
were left relatively untouched (other than from mutual collisions)
in the plane of the dust disk from which all planets formed during
the first 100 million years or so of the Solar System's birth.
Unlike the rocky asteroids, EKOs and comets lie so far out
from the Sun that they still contain a lot of water and gas ices
(like methane) on their surface.
Greg Bacon,
STScI, NASA
(Artwork derived from Hubble Space Telescope
images)
8405
Asbolus is a chunk of ice and debris called a Centaur object which
orbits the sun between Saturn and Uranus. It has a surprisingly bright
impact crater that may be less than 10 million years old.
Centaurs,
which orbit the Sun between Jupiter and Neptune, are thought to be icy
bodies that were perturbed from their original orbits in the
Edgeworth-Kuiper Belt, which extends to at least 10 times the distance
of Pluto from the Sun.
Donald
W. McCarthy, an University of Arizona researcher, speculates that
the collision creating the impact crater dislodged Asbolus and sent it
on its way to its closer orbit around the Sun. (See also
Kern
et al, 2000.)
The EKOs discovered during the 1990s, such as 1992 QB1
and 1993 SC, appear to be icy planetary bodies similar to Pluto and
Triton, if smaller. Since 1992, over 300 of these "Plutinos"
("trans-Neptunian" objects orbiting in 3:2 resonance with Neptune like
Pluto) have been identified. Color measurements of some of the
brightest of these objects indicate that they are unusually red.
Even if there were only somewhat more than 35,000 EKOs greater than
100 km (62 miles) in diameter, their total mass would total several
hundred times that of similar sized objects in the main asteroid belt,
given the much larger volume of space in the belt. Moreover,
astronomers using the Hubble Space Telescope have detected much
fainter EKOs that are much smaller, measuring perhaps only 20 km
(12 miles) or so across. There may be as many as 100 million of
these smaller objects in low-inclination orbits.
Yale
University and CIDA Observatory
-- field image
A very large EKO was discovered on March 15, 2000, between the orbits
of Neptune and Pluto.
Designated EB 173, this reddish, icy object is estimated to be
about 370 miles (600 km) in
diameter assuming an albedo of four percent, which makes it about a fourth
of Pluto's diameter
and next in size to Ceres, the largest asteroid. It is classified as a
"Plutino" ("trans-Neptunian"
object in a 3:2 orbital resonance with Neptune) that swings between
28.5 to almost 50 AUs
from the Sun every 244 years
(Baltay
and Bruzual, ApJ Letters,
2000-2001; and Yale press
release of
October
25, 2000).
On the other hand, the Edgeworth-Kuiper Belt was once immensely more
massive than it is today. Computer simulations suggest that virtually
all (99 percent) of the mass in the original disk as far out as 70 AUs,
was lost as the development of Neptune fostered collisions that
grounded many EKOs into fine dust, which was eventually blown out into
interstellar space by the Solar wind over the eons
(S. Alan Stern,
Astronomy,
September 2000). Beyond 70 AUs, however, massive remnants of
that primordial Belt that are possibly larger than Pluto may yet lurk
undetected today.
Spacewatch
Survey -- false-color field image
On November 28, 2000, an even larger EKO was discovered
beyond the orbit of Neptune. Designated WR 106 but now named
Varuna,
this icy object is about 550 miles (900 km) in diameter
with a very dark albedo of seven percent -- about a third to a half
the size of Pluto and around the size of Asteroid Ceres. Varuna
is located at about 43 AUs from the Sun in a near circular orbit
(e=0.06). The slow-moving object was found in a crowded corner
of the night sky through manual observation by
Robert S.
McMillan
and confirmed with the help of new observations by
Jeffrey Larsen
(See Spacewatch
Discovery
News). Although first sighted in 1953,
its nature was unrecognized at the time.
The orbit of a EKO can become so disturbed by the gravitational
attraction of the giant planets that it moves to cross the orbit of
Neptune. If the object has a close encounter with Neptune or Uranus,
then the planet's gravitational force can sling it into the Oort Cloud
and perhaps onward into interstellar space, into the inner Solar
System as a comet, or into an unstable orbit crossing those of the
other giant planets.
Canada-France-Hawaii
Telescope -- artist's conception
and orbit
First discovered in 1998, WW 31 was the second EKO to be found with
a satellite (after Pluto and Charon). At 75-93 miles (120-150 km) and
62-75 miles (100-120 km) in diameter, the pair orbit each other around
a common center of gravity every 570 days (more from
STScI
and CFHT).
The Edgeworth-Kuiper Belt is the source of the short-period comets.
For example, there are at least nine known objects orbiting between
Jupiter and Neptune, including 2060
Chiron
(aka 95 P/Chiron) and 5145 Pholus, that have been designated as
"Centaurs,"
whose orbits are not stable. Some of these objects show cometary
activity as fuzziness in their image, indicating the presence of a
diffuse coma. The largest known Centaur is Chiron which is about 170 km
(106 miles) in diameter or 20 times larger than Halley's Comet. If
Chiron is ever is perturbed into an orbit that approaches the Sun, it
will be become a truly spectacular comet.
D. Cruikshank and J. Stansberry,
SIRTF/Spitzer,
CalTech,
JPL,
NASA
Larger and
jumbo infrared images.
EKOs between the orbits of Jupiter
and Neptune (known as
"Centaurs")
are heated sufficiently by the Sun
to have gas and dust erupt through
their crust periodically like Comet
Schwassmann-Wachmann I
(more).
By January 2006, of the one thousand some EKOs discovered, only a few
were found to be currently orbiting farther from the Sun than 55 AUs,
despite recent surveys that were capable of detecting objects out to
65 AUs out. Lynne Allen and Gary Bernstein of the University of
Michigan and Renu Malhotra, of the University of Arizona and the
Lunar Planetary Laboratory suggest that some event may have stripped
away most of the planet-building material beyond 50 AUs from the Sun
(more
information). Lists of known EKOs are available for
Trans-Neptunian
Objects and for
Centaurs
and Scattered Disk Objects.
David C. Hewitt,
co-discoverer of the first EKO after Pluto and Charon, has a
plot
of the projected orbits of the various types of EKOs. Real-time
orbit animations of
known EKOs (and other comets, asteroids, and planets) may be available
from the Orbit Viewer, originally written by
Osamu Ajiki of AstroArts and
modified by Ron Baalke of NASA's
Jet Propulsion Laboratory.
D. Cruikshank and J. Stansberry, 
NASA, JPL, University of Florida 
DIRBE
Team,
COBE,
NASA --
larger infrared mage. 
© Astronomical Journal /
Liou
and Zook, 1999 
