   #copyright

Kuiper belt

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Artist's rendering of the Kuiper Belt and hypothetical more distant
   Oort cloud.
   Enlarge
   Artist's rendering of the Kuiper Belt and hypothetical more distant
   Oort cloud.

   The Kuiper belt ( pronounced /ˈkaɪpɚ/, to rhyme with 'viper') is an
   area of the solar system extending from the orbit of 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 Hills cloud and Oort cloud
   objects.

   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.

Origins

   These debris disks around the two remote stars seem equivalent of our
   own Solar system's Kuiper Belt. The left image is the "top view", and
   the right image "edge view". The black central circle is produced by
   the camera's coronagraph which hides the central star to allow the much
   fainter disks to be seen. Observed with Hubble Space Telescope
   Enlarge
   These debris disks around the two remote stars seem equivalent of our
   own Solar system's Kuiper Belt. The left image is the "top view", and
   the right image "edge view". The black central circle is produced by
   the camera's coronagraph which hides the central star to allow the much
   fainter disks to be seen. Observed with Hubble Space Telescope

   Modern computer simulations show the Kuiper belt to have been strongly
   influenced by Jupiter and Neptune. During the early period of the Solar
   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 cubewanos),
   apart from those which fortuitously were in a 2:3 orbital resonance.
   These resonant bodies formed the plutinos. 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.

Hypothesis

   The first astronomers to suggest the existence of this belt were
   Frederick C. Leonard in 1930 and Kenneth E. Edgeworth in 1943. In 1951
   Gerard Kuiper suggested that the belt was the source of short period
   comets (those having an orbital period of less than 200 years). More
   detailed conjectures about objects in the belt were done by Al G. W.
   Cameron in 1962, Fred L. Whipple in 1964, and Julio Fernandez in 1980.
   The belt and the objects in it were named after Kuiper after the
   discovery of (15760) 1992 QB[1]. No known object in the Kuiper belt is
   a remotely possible candidate to become a comet.

Name

   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

                                        TNOs and similar bodies
                                 * cis-Neptunian objects
                                      + centaurs
                                      + Neptune Trojan
                                 * trans-Neptunian objects (TNOs)
                                      + Kuiper belt objects (KBOs)
                                           o classical KBOs (cubewanos)
                                           o resonant KBOs
                                                # plutinos (2:3 resonance)
                                      + scattered disc objects (SDOs)
                                      + Oort cloud objects (OCOs)

   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 CCD-enabled telescopes allowing for cost
   effective automated KBO searching.

   Among the largest are Pluto and Charon, but since the year 2000 other
   large objects that approached and even exceeded their size were
   identified. 50000 Quaoar, discovered in 2002, which is a KBO, is half
   the size of Pluto and is larger than the largest asteroid, 1 Ceres.
   (136472) 2005 FY[9] (nicknamed "Easterbunny") and (136108) 2003 EL[61]
   (nicknamed "Santa"), both announced on 29 July 2005, are larger still.
   Other objects, such as 28978 Ixion (discovered in 2001) and 20000
   Varuna (discovered in 2000) while smaller than Quaoar, are nonetheless
   quite sizable. Sedna, a red planetoid with a diameter roughly half-way
   between Pluto and Quaoar, was first sighted on November 14, 2003.

   The exact classification of these objects is unclear, since they are
   probably fairly different from the asteroids of the asteroid belt. The
   largest recent discovery is Eris, 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 tenth planet by some
   sources. Due to this discovery, on August 24, 2006, the IAU announced a
   first-ever definition of 'planet', and these large Kuiper belt objects
   accordingly became known officially as dwarf planets. A number of
   astronomers around the world came out in public disagreement with the
   definition in the days following it.

   Neptune's moon Triton is commonly thought to be a captured KBO.

Classification and Distribution

Resonant and classical objects

   Orbit classification (schematic of semi-major axes).
   Enlarge
   Orbit classification (schematic of semi-major axes).

   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) are not resonant. These objects, called Classical Kuiper Belt
   objects or cubewanos, are found between the 2:3 resonance (at ~39.4AU,
   populated by >150 plutinos) and the 1:2 resonance (at ~47.7AU,
   populated by a few twotinos). 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.
   Large cubewanos, plutinos and near scattered objects.
   Enlarge
   Large cubewanos, plutinos and near scattered objects.

   The next diagram shows the largest objects of the Kuiper belt: Pluto
   with the largest plutinos: 90482 Orcus and 28978 Ixion, a few big
   classical objects, and two scattered objects (beyond the 1:2 resonance,
   in grey), notably Eris thought to be the biggest trans-Neptunian object
   known. The eccentricity of the orbits is represented by the red
   segments (extending from perihelion to aphelion) with 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, Quaoar).

   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, where, as temperature rises, so do the relative
   velocities between the particles.

   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. 2004 XR[190]) 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.

Orbit distribution

   Distribution of cubewanos, plutinos and near scattered objects.
   Enlarge
   Distribution of cubewanos, plutinos and near scattered objects.

   The last diagram shows the distribution of known Kuiper Belt objects.
   The resonant objects: Neptune Trojans (1:1 resonance), plutinos (2:3),
   twotinos (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 original 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
   resonances.

   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 Oort cloud, which
   is far more distant.

Size distribution

   Illustration of the power law.
   Enlarge
   Illustration of the power law.

   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.

          \frac{d N}{d D} \sim D^{-q} 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 (=2^3) 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 (=10^3)
   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 M[Earth] 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 than 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 conjectures 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 Alan Stern of
   the Southwest Research Institute has claimed that a large planetary
   object might be responsible. 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). Discoverer Michael E. Brown, for instance, has
   referred to Eris as a KBO, despite it having a mean orbital radius 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 current epoch.

List of the brightest KBOs

   The brightest known KBOs (with absolute magnitudes < 4.0), are:
   Permanent
   Designation Provisional
   Designation Absolute magnitude Albedo Equatorial diameter
   (km) Semimajor axis
   (AU) Date found Discoverer Diameter method
   Pluto −1.0 0.6 2320 39.4 1930 C. Tombaugh occultation
   136472 2005 FY[9] −0.3 0.8 ± 0.2 1800 ± 200 45.7 2005 M. Brown, C.
   Trujillo & D. Rabinowitz assumed albedo
   136108 2003 EL[61] 0.1 0.6 (assumed) ~1500 ^(1 43.3 2005 M. Brown, C.
   Trujillo & D. Rabinowitz assumed albedo
   Charon S/1978 P 1 1 0.4 1205 39.4 1978 J. Christy occultation
   Orcus 2004 DW 2.3 0.1 (assumed) ~1500 39.4 2004 M. Brown, C. Trujillo &
   D. Rabinowitz assumed albedo
   Quaoar 2002 LM[60] 2.6 0.10 ± 0.03 1260 ± 190 43.5 2002 C. Trujillo &
   M. Brown disk resolved
   Ixion 2001 KX[76] 3.2 0.25 – 0.50 400 – 550 39.6 2001 DES thermal
   55636 2002 TX[300] 3.3 > 0.19 < 709 43.1 2002 NEAT thermal
   55565 2002 AW[197] 3.3 0.14 – 0.20 650 – 750 47.4 2002 C. Trujillo, M.
   Brown, E. Helin, S. Pravdo,
   K. Lawrence & M. Hicks / Palomar Observatory thermal
   55637 2002 UX[25] 3.6 0.08? ~910 42.5 2002 A. Descour / Spacewatch
   assumed albedo
   Varuna 2000 WR[106] 3.7 0.12 – 0.30 450 – 750 43.0 2000 R. McMillan
   thermal
   2002 MS[4] 3.8 0.1 (assumed) 730? 41.8 2002 C. Trujillo, M. Brown
   assumed albedo
   2003 AZ[84] 3.9 0.1 (assumed) 700? 39.6 2003 C. Trujillo, M. Brown, E.
   Helin, S. Pravdo,
   K. Lawrence & M. Hicks assumed albedo

   Retrieved from " http://en.wikipedia.org/wiki/Kuiper_belt"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
