The Kuiper belt is an area of the
solar system extending from the
Neptune (at 30
AU) to 50 AU
from the Sun.
The objects within the Kuiper Belt, together with the members of the
scattered disk extending beyond, are collectively referred to as
trans-Neptunian, along with any hypothetical
The interaction with Neptune (2:1
orbital resonance) is thought to be responsible for the apparent edge at 48
AU (a sudden drop in number of objects, see Orbit distribution below) but
the current models have yet to explain this peculiar distribution in detail.
simulations show the Kuiper belt to have been strongly influenced by
Neptune. During the early period of the
System, Neptune's orbit is thought to have migrated outwards from the Sun
due to interactions with minor bodies. In the process, Neptune swept up, or
gravitationally ejected all the bodies closer to the Sun than about 40 AU (the
inner edge of the region occupied by
apart from those which fortuitously were in a 2:3
orbital resonance. These resonant bodies formed the
present Kuiper Belt members are thought to have largely formed in their present
position, although a significant fraction may have originated in the vicinity of
Jupiter, and been ejected by it to the far regions of the Solar system.
to suggest the existence of this belt were
Frederick C. Leonard in
Kenneth E. Edgeworth in
Gerard Kuiper suggested that the belt was the source of short period
orbital period of less than 200
detailed conjectures about objects in the belt were done by
Al G. W. Cameron in
Fred L. Whipple in
Julio Fernandez in
1980. The belt and the objects in it were named after Kuiper after the
(15760) 1992 QB1. No known object in the Kuiper belt is a
remotely possible candidate to become a comet.
An alternative name, Edgeworth-Kuiper belt is used to credit
Edgeworth. The term
trans-Neptunian object (TNO) is recommended for objects in the belt by
several scientific groups because the term is less controversial than all others
— it is not a
synonym though, as TNOs include all objects orbiting the Sun at the outer
edge of the solar system, not just those in the Kuiper belt.
Discoveries thus far
Over 800 Kuiper belt objects (KBOs) (a subset of
trans-Neptunian objects (TNOs)) have been discovered in the belt, almost all
of them since 1992.
This was primarily a result of advances in computer hardware/software and
allowing for cost effective automated KBO searching.
Among the largest are
Charon, but since the year
2000 other large
objects that approached and even exceeded their size were identified.
Quaoar, discovered in
2002, which is a
KBO, is half the size of Pluto and is larger than the largest
(136472) 2005 FY9 (nicknamed "Easterbunny") and
(136108) 2003 EL61 (nicknamed "Santa"), both announced on
29 July 2005, are larger
still. Other objects, such as
Ixion (discovered in 2001) and
Varuna (discovered in 2000) while smaller than Quaoar, are nonetheless quite
The exact classification of these objects is unclear, since they are probably
fairly different from the
asteroid belt. The largest recent discovery is
which is actually larger than Pluto. This led scientists to question the
definition of the term planet, as it is larger than
Pluto and was
often called a
planet by some sources.
Due to this discovery, on
IAU announced a
first-ever definition of 'planet', and these large Kuiper belt objects
accordingly became known officially as
planets. A number of astronomers around the world came out in public
disagreement with the definition in the days following it.
Triton is commonly thought to be a captured KBO.
Classification and Distribution
Resonant and classical objects
Orbital resonance with Neptune is the major factor of the current
classification of KBO, even if most of them (>600 objects as of October 2005)
resonant. These objects, called Classical Kuiper Belt objects or
are found between the 2:3 resonance (at ~39.4AU, populated by >150
the 1:2 resonance (at ~47.7AU, populated by a few
Minor resonances exist at 3:4, 3:5, 4:7 and 2:5 (this last, also fairly strongly
occupied). The 1:2 resonance appears to be an edge. It is not clear whether it
is actually the outer edge of the Classical Belt or just the beginning of a gap.
The next diagram shows the largest objects of the Kuiper belt:
Pluto with the
Ixion, a few big classical objects, and two
scattered objects (beyond the 1:2 resonance, in grey), notably
thought to be the biggest trans-Neptunian object known. The
eccentricity of the orbits is represented by the red segments (extending
inclination represented on Y axis. While eccentric orbits of many resonant
KBOs bring them inside Neptune's orbit periodically, classical KBOs are in more
circular orbits (short red segments,
Initially, the Kuiper belt was thought to be a flat belt (populated by
objects on moderately eccentric, low-inclination orbits), as opposed to high
inclination orbits of the "scattered" disk objects. With the discovery of the
large cubewanos, this belt became a thick disk or
torus. It now
appears that the distribution of orbit inclinations peaks around 4 and 30-40
degrees, giving rise to a division into two groups: cold and hot,
respectively. The cold group would have been born outside the Neptune's
orbit while the hot migrated outwards due to close interactions with
Neptune. The cold/hot terminology comes from analogy to particles in a gas,
temperature rises, so do the relative velocities between the
This grouping may yet be revised, however, as the current distribution of
known objects is likely to be strongly affected by observational bias. Most
observations have so far focused on near-ecliptic objects. Even objects with
high inclinations (e.g.
were actually found at near ecliptic positions. In addition, most of the known
KBOs are detected near their closest approaches to the Sun since they appear
dimmer at greater distances.
The last diagram shows the distribution of known Kuiper Belt objects. The
Neptune Trojans (1:1 resonance),
(1:2) and a handful of objects occupying other resonances are shown in red.
Confirmed plutinos are plotted in dark red. Beyond the 1:2 resonance,
scattered disk objects are plotted for reference.
Interestingly, low inclination regions which include the "cold" majority of
cubewanos appear devoid of the largest objects (see diagram). The (observed)
distribution has been a challenge to the theories of the formation of the Kuiper
belt as it is far too complex to be explained simply as being the remains of the
accretion disc dating back to the formation of the Solar System. Numerous
competing models are being developed, typically involving so called Neptune
migration. It was suggested in the 1980s that interaction between giant planets
and a massive disk of small particles would not only scatter the disk but also
cause (via momentum transfer) the giants to migrate to more distant orbits.
While all four giant planets would be affected, Neptune could have migrated as
far as 5AU outwards to reach its current position at around 30 AU. Such models
can explain for example, the ‘trapping’ of small bodies into the plutino 2:3
However, the present models still fail to account for many of the
characteristics of the distribution and, quoting one of the scientific articles,
the problems "continue to challenge analytical techniques and the fastest
numerical modeling hardware and software".
The belt should not be confused with the hypothesized
which is far more distant.
Bright objects are rare compared with the dominant dim population, as
expected from accretion models of origin, given that only some objects of a
given size would have grown further. This relationship N(D), the population
expressed as a function of the diameter, referred to as brightness slope, has
been confirmed by observations. The slope
is inversely proportional to some power of the diameter D.
where the current measures
 give q = 4 ±0.5.
The relationship is illustrated on the graph for q=4. Less formally, there is
for instance 8 (=23) times more objects in 100-200km range than
objects in 200-400km range. In other words, for a single object with the
diameter of 1000 km it should be there around 1000 (=103) objects
with diameter of 100km. Of course, only the magnitude is actually known, the
size is inferred assuming albedo (not a safe assumption for larger objects)
Missing mass dilemma
The total mass of Kuiper Belt objects can be inferred by models of the origin
of the Solar System from the known mass of the planets and known distribution of
mass closer to the Sun. While the estimates are model-dependent, the total mass
of around 30 MEarth is expected. Surprisingly, the actual
distribution appears to be well below that value, even accounting for the
observational bias. The observed density is at least 100 times smaller
the model calls for. This missing 99% of the mass can be hardly dismissed as it
is required for the accretion of bigger (>100km) objects ever taking place. At
the current low density these objects simply could not be created. Moreover, the
eccentricity and inclination of current orbits makes the encounters quite
"violent" resulting in destruction rather than accretion. It appears that either
the current residents of the Kuiper belt have been created closer to the Sun or
some mechanism dispersed the original mass. Neptune’s influence is too weak to
explain such a massive "vacuuming". While the question remains open, the
vary from a passing star scenario to grinding of smaller objects, via
collisions, into dust small enough to be affected by Solar radiation.
The "Kuiper cliff"
Earlier models of the Kuiper belt had suggested that the number of large
objects would increase by a factor of two beyond 50 AU;
however, observation has revealed that in fact, at 50 AU, the number of observed
objects in the Kuiper belt falls precipitously. This falloff is known as the
"Kuiper cliff," and its cause is unknown, though
Southwest Research Institute has claimed that a large planetary object might
Bernstein and Trilling et al. have found evidence that the observed rapid
decline in objects of 100 km or more in radius beyond 50 AU is a real decline in
the number of objects, and not just an observational effect.
The term "Kuiper belt object" (KBO)
Most models of solar system formation show icy planetoids first forming in
the Kuiper belt, while later gravitational interactions displace some of them
outwards into the so-named
scattered disc. Strictly speaking, a KBO is any object that orbits
exclusively within the defined Kuiper belt region regardless of origin or
composition. However, in some scientific circles the term has become synonymous
with any icy planetoid native to the outer solar system believed to have been
part of that initial class, even if its orbit during the bulk of solar system
history has been beyond the Kuiper belt (e.g. in the scattered disk region).
Michael E. Brown, for instance, has referred to
Eris as a KBO, despite it having a semi-major axis of 67 AU, well clear of
the Kuiper cliff. Other leading trans-Neptunian researchers have been more
cautious in applying the KBO label to objects clearly outside the belt in the
List of the brightest KBOs
The brightest known KBOs (with
absolute magnitudes < 4.0), are:
||1800 ± 200
C. Trujillo &
C. Trujillo &
C. Trujillo &
||1260 ± 190
C. Trujillo &
||400 – 550
||650 – 750
K. Lawrence &
M. Hicks /
A. Descour /
||450 – 750
K. Lawrence &