Kuiper belt
From Wikipedia, the free encyclopedia
Known objects in the Kuiper belt, derived from data from the
Minor Planet Center.
Objects in the main belt are colored green, whereas scattered objects
are colored orange. The four outer planets are blue. Neptune's few known
trojans are yellow, whereas Jupiter's are pink. The scattered objects between
Jupiter's orbit and the Kuiper belt are known as
centaurs. The scale is in
astronomical units. The pronounced gap at the bottom is due to difficulties in detection against the background of the plane of the
Milky Way.
The
Kuiper belt //, sometimes called the
Edgeworth–Kuiper belt (after the astronomers
Kenneth Edgeworth and
Gerard Kuiper), is a region of the
Solar System beyond the
planets, extending from the
orbit of
Neptune (at 30
AU) to approximately 50
AU from the
Sun.
[1] It is similar to the
asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive.
[2][3] Like the asteroid belt, it consists mainly of
small bodies, or remnants from the Solar System's formation. Although most asteroids are composed primarily of
rock and metal, most Kuiper belt objects are composed largely of frozen
volatiles (termed "ices"), such as
methane,
ammonia and water. The Kuiper belt is home to at least three
dwarf planets:
Pluto,
Haumea, and
Makemake. Some of the Solar System's
moons, such as Neptune's
Triton and
Saturn's
Phoebe, are also believed to have originated in the region.
[4][5]
Since the belt was discovered in 1992,
[6]
the number of known Kuiper belt objects (KBOs) has increased to over a
thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are
believed to exist.
[7] The Kuiper belt was initially thought to be the main repository for
periodic comets,
those with orbits lasting less than 200 years. However, studies since
the mid-1990s have shown that the belt is dynamically stable, and that
comets' true place of origin is the
scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago;
[8] scattered disc objects such as
Eris have extremely
eccentric orbits that take them as far as 100 AU from the Sun.
[nb 1]
The Kuiper belt should not be confused with the
hypothesized Oort cloud,
which is a thousand times more distant. The objects within the Kuiper
belt, together with the members of the scattered disc and any potential
Hills cloud or Oort cloud objects, are collectively referred to as
trans-Neptunian objects (TNOs).
[11]
Pluto is the largest member of the Kuiper belt, and the second largest known TNO, the largest being
Eris in the scattered disc.
[nb 1] Originally considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a "
dwarf planet"
in 2006. It is compositionally similar to many other objects of the
Kuiper belt, and its orbital period is characteristic of a class of
KBOs, known as "
plutinos", that share the same 2:3
resonance with Neptune.
History
After the discovery of
Pluto,
many speculated that it might not be alone. The region now called the
Kuiper belt was hypothesized in various forms for decades. It was only
in 1992 that the first direct evidence for its existence was found. The
number and variety of prior speculations on the nature of the Kuiper
belt have led to continued uncertainty as to who deserves credit for
first proposing it.
Hypotheses
The first
astronomer to suggest the existence of a trans-Neptunian population was
Frederick C. Leonard. Soon after Pluto's discovery by
Clyde Tombaugh in 1930, Leonard pondered whether it was "not likely that in Pluto there has come to light the
first of a
series
of ultra-Neptunian bodies, the remaining members of which still await
discovery but which are destined eventually to be detected".
[12] That same year, astronomer
Armin O. Leuschner suggested that Pluto "may be one of many long-period planetary objects yet to be discovered."
[13]
In 1943, in the
Journal of the British Astronomical Association,
Kenneth Edgeworth hypothesized that, in the region beyond
Neptune, the material within the
primordial solar nebula
was too widely spaced to condense into planets, and so rather condensed
into a myriad of smaller bodies. From this he concluded that "the outer
region of the solar system, beyond the orbits of the planets, is
occupied by a very large number of comparatively small bodies"
[14]
and that, from time to time, one of their number "wanders from its own
sphere and appears as an occasional visitor to the inner solar system",
[15] becoming a
comet.
In 1951, in an article for the journal
Astrophysics,
Gerard Kuiper
speculated on a similar disc having formed early in the Solar System's
evolution; however, he did not believe that such a belt still existed
today. Kuiper was operating on the assumption common in his time that
Pluto
was the size of the Earth and had therefore scattered these bodies out
toward the Oort cloud or out of the Solar System. Were Kuiper's
hypothesis correct, there would not be a Kuiper belt today.
[16]
The hypothesis took many other forms in the following decades. In 1962, physicist
Al G.W. Cameron postulated the existence of "a tremendous mass of small material on the outskirts of the solar system".
[17] In 1964,
Fred Whipple, who popularised the famous "
dirty snowball"
hypothesis for cometary structure, thought that a "comet belt" might be
massive enough to cause the purported discrepancies in the orbit of
Uranus that had sparked the search for
Planet X, or, at the very least, massive enough to affect the orbits of known comets.
[18] Observation, however, ruled out this hypothesis.
[17]
In 1977,
Charles Kowal discovered
2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. He used a
blink comparator, the same device that had allowed
Clyde Tombaugh to discover
Pluto nearly 50 years before.
[19] In 1992, another object,
5145 Pholus, was discovered in a similar orbit.
[20] Today, an entire population of comet-like bodies, called the
centaurs,
is known to exist in the region between Jupiter and Neptune. The
centaurs' orbits are unstable and have dynamical lifetimes of a few
million years.
[21]
From the time of Chiron's discovery in 1977, astronomers have
speculated that the centaurs therefore must be frequently replenished by
some outer reservoir.
[22]
Further evidence for the existence of the Kuiper belt later emerged
from the study of comets. That comets have finite lifespans has been
known for some time. As they approach the Sun, its heat causes their
volatile
surfaces to sublimate into space, gradually evaporating them. In order
for comets to continue to be visible over the age of the Solar System,
they must be replenished frequently.
[23] One such area of replenishment is the
Oort cloud, a spherical swarm of comets extending beyond 50,000
AU from the Sun first hypothesised by astronomer
Jan Oort in 1950.
[24] The Oort cloud is believed to be the point of origin of
long-period comets, which are those, like
Hale–Bopp, with orbits lasting thousands of years.
There is, however, another comet population, known as
short-period or
periodic comets, consisting of those comets that, like
Halley's Comet, have
orbital periods
of less than 200 years. By the 1970s, the rate at which short-period
comets were being discovered was becoming increasingly inconsistent with
their having emerged solely from the
Oort cloud.
[25]
For an Oort cloud object to become a short-period comet, it would first
have to be captured by the giant planets. In 1980, in the
Monthly Notices of the Royal Astronomical Society, Uruguayan astronomer
Julio Fernández
stated that for every short-period comet to be sent into the inner
Solar System from the Oort cloud, 600 would have to be ejected into
interstellar space. He speculated that a comet belt from between 35 and
50
AU would be required to account for the observed number of comets.
[26] Following up on Fernández's work, in 1988 the Canadian team of Martin Duncan, Tom Quinn and
Scott Tremaine
ran a number of computer simulations to determine if all observed
comets could have arrived from the Oort cloud. They found that the Oort
cloud could not account for all short-period comets, particularly as
short-period comets are clustered near the plane of the Solar System,
whereas Oort-cloud comets tend to arrive from any point in the sky. With
a “belt”, as Fernández described it, added to the formulations, the
simulations matched observations.
[27]
Reportedly because the words "Kuiper" and "comet belt" appeared in the
opening sentence of Fernández's paper, Tremaine named this hypothetical
region the "Kuiper belt".
[28]
Discovery
The array of telescopes atop
Mauna Kea, with which the Kuiper belt was discovered
In 1987, astronomer
David Jewitt, then at
MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System".
[6] He encouraged then-graduate student
Jane Luu to aid him in his endeavour to locate another object beyond
Pluto's orbit, because, as he told her, "If we don't, nobody will."
[29] Using telescopes at the
Kitt Peak National Observatory in
Arizona and the
Cerro Tololo Inter-American Observatory in Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a
blink comparator.
[29] Initially, examination of each pair of plates took about eight hours,
[30] but the process was sped up with the arrival of electronic
charge-coupled devices
or CCDs, which, though their field of view was narrower, were not only
more efficient at collecting light (they retained 90% of the light that
hit them, rather than the 10% achieved by photographs) but allowed the
blinking process to be done virtually, on a computer screen. Today, CCDs
form the basis for most astronomical detectors.
[31] In 1988, Jewitt moved to the Institute of Astronomy at the
University of Hawaii. Luu later joined him to work at the University of Hawaii's 2.24 m telescope at Mauna Kea.
[32]
Eventually, the field of view for CCDs had increased to 1024 by 1024
pixels, which allowed searches to be conducted far more rapidly.
[33]
Finally, after five years of searching, on August 30, 1992, Jewitt and
Luu announced the "Discovery of the candidate Kuiper belt object"
(15760) 1992 QB1.
[6] Six months later, they discovered a second object in the region,
(181708) 1993 FW.
[34]
Studies conducted since the trans-Neptunian region was first charted
have shown that the region now called the Kuiper belt is not the point
of origin of short-period comets, but that they instead derive from a
linked population called the
scattered disc. The scattered disc was created when Neptune
migrated outward
into the proto-Kuiper belt, which at the time was much closer to the
Sun, and left in its wake a population of dynamically stable objects
that could never be affected by its orbit (the Kuiper belt proper), and a
population whose
perihelia
are close enough that Neptune can still disturb them as it travels
around the Sun (the scattered disc). Because the scattered disc is
dynamically active and the Kuiper belt relatively dynamically stable,
the scattered disc is now seen as the most likely point of origin for
periodic comets.
[8]
Name
Astronomers sometimes use the alternative name Edgeworth–Kuiper belt
to credit Edgeworth, and KBOs are occasionally referred to as EKOs.
However,
Brian G. Marsden
claims that neither deserves true credit: "Neither Edgeworth nor Kuiper
wrote about anything remotely like what we are now seeing, but
Fred Whipple did".
[35] David Jewitt comments: "If anything ... Fernández most nearly deserves the credit for predicting the Kuiper Belt."
[16]
KBOs are sometimes called
kuiperoids, a name suggested by
Clyde Tombaugh.
[36] 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
an exact
synonym though, as TNOs include all objects orbiting the Sun past the orbit of
Neptune, not just those in the Kuiper belt.
Origins
Simulation showing outer planets and Kuiper belt: a) before
Jupiter/Saturn 2:1 resonance, b) scattering of Kuiper belt objects into
the Solar System after the orbital shift of Neptune, c) after ejection
of Kuiper belt bodies by Jupiter
The precise origins of the Kuiper belt and its complex structure are
still unclear, and astronomers are awaiting the completion of several
wide-field survey telescopes such as
Pan-STARRS and the future
LSST,
which should reveal many currently unknown KBOs. These surveys will
provide data that will help determine answers to these questions.
[2]
The Kuiper belt is believed to consist of
planetesimals, fragments from the original
protoplanetary disc around the
Sun
that failed to fully coalesce into planets and instead formed into
smaller bodies, the largest less than 3,000 kilometres (1,900 mi) in
diameter.
Modern
computer simulations show the Kuiper belt to have been strongly influenced by
Jupiter and
Neptune, and also suggest that neither
Uranus
nor Neptune could have formed in their present positions, as too little
primordial matter existed at that range to produce objects of such high
mass. Instead, these planets are believed to have formed closer to
Jupiter. Scattering of planetesimals early in the Solar System's history
would have led to
migration of the orbits of the giant planets:
Saturn,
Uranus and Neptune drifted outwards while Jupiter drifted inwards.
Eventually, the orbits shifted to the point where Jupiter and Saturn
reached an exact 2:1 resonance; Jupiter orbited the Sun twice for every
one Saturn orbit. The gravitational repercussions of such a resonance
ultimately disrupted the orbits of Uranus and Neptune, causing Neptune's
orbit to become more eccentric and move outward into the primordial
planetesimal disc, which sent the disc into temporary chaos.
[37][38][39] As Neptune's orbit expanded, it excited and scattered many TNO planetesimals into higher and more eccentric orbits.
[40]
Many more were scattered inward, often to be scattered again and in
some cases ejected by Jupiter. The process is thought to have reduced
the primordial Kuiper belt population by 99% or more, and to have
shifted the distribution of the surviving members outward.
[39]
However, this currently most popular model, the "
Nice model", still fails to account for some of the characteristics of the distribution and, quoting one of the scientific articles,
[41]
the problems "continue to challenge analytical techniques and the
fastest numerical modeling hardware and software". The model predicts a
higher average eccentricity in classical KBO orbits than is observed
(0.10–0.13 versus 0.07).
[39] The frequency of paired objects, many of which are far apart and loosely bound, also poses a problem for the model.
[42]
Structure
At its fullest extent, including its outlying regions, the Kuiper
belt stretches from roughly 30 to 55 AU. However, the main body of the
belt is generally accepted to extend from the 2:3 resonance (
see below) at 39.5 AU to the 1:2 resonance at roughly 48 AU.
[43] The Kuiper belt is quite thick, with the main concentration extending as much as ten degrees outside the
ecliptic plane and a more diffuse distribution of objects extending several times farther. Overall it more resembles a
torus or doughnut than a belt.
[44] Its mean position is inclined to the ecliptic by 1.86 degrees.
[45]
The presence of
Neptune has a profound effect on the Kuiper belt's structure due to
orbital resonances.
Over a timescale comparable to the age of the Solar System, Neptune's
gravity destabilises the orbits of any objects that happen to lie in
certain regions, and either sends them into the inner Solar System or
out into the
scattered disc or interstellar space. This causes the Kuiper belt to possess pronounced gaps in its current layout, similar to the
Kirkwood gaps in the
asteroid belt.
In the region between 40 and 42 AU, for instance, no objects can retain
a stable orbit over such times, and any observed in that region must
have migrated there relatively recently.
[46]
Classical belt
Between the 2:3 and 1:2 resonances with Neptune, at approximately
42–48 AU, the gravitational influence of Neptune is negligible, and
objects can exist with their orbits essentially unmolested. This region
is known as the
classical Kuiper belt, and its members comprise roughly two thirds of KBOs observed to date.
[47][48] Because the first modern KBO discovered,
(15760) 1992 QB1, is considered the prototype of this group, classical KBOs are often referred to as
cubewanos ("Q-B-1-os").
[49][50] The
guidelines established by the
IAU demand that classical KBOs be given names of mythological beings associated with creation.
[51]
The classical Kuiper belt appears to be a composite of two separate
populations. The first, known as the "dynamically cold" population, has
orbits much like the planets; nearly circular, with an
orbital eccentricity
of less than 0.1, and with relatively low inclinations up to about 10°
(they lie close to the plane of the Solar System rather than at an
angle). The second, the "dynamically hot" population, has orbits much
more inclined to the ecliptic, by up to 30°. The two populations have
been named this way not because of any major difference in temperature,
but from analogy to particles in a gas, which increase their relative
velocity as they become heated up.
[52]
The two populations not only possess different orbits, but different
colors; the cold population is markedly redder than the hot. If this is a
reflection of different compositions, it suggests they formed in
different regions. The hot population is believed to have formed near
Jupiter, and to have been ejected out by movements among the gas giants.
The cold population, on the other hand, has been proposed to have
formed more or less in its current position, although it might also have
been later swept outwards by Neptune during its
migration,
[2][53] particularly if Neptune's eccentricity was transiently increased.
[39]
Although the Nice model appears to be able to at least partially
explain a compositional difference, it has also been suggested the color
difference may reflect differences in surface evolution.
[39]
Resonances
When an object's orbital period is an exact ratio of Neptune's (a situation called a
mean motion resonance),
then it can become locked in a synchronised motion with Neptune and
avoid being perturbed away if their relative alignments are appropriate.
If, for instance, an object is in just the right kind of orbit so that
it orbits the Sun twice for every three Neptune orbits, and if it
reaches perihelion with Neptune a quarter of an orbit away from it, then
whenever it returns to perihelion, Neptune will always be in about the
same relative position as it began, because it will have completed 1½
orbits in the same time. This is known as the 2:3 (or 3:2) resonance,
and it corresponds to a characteristic
semi-major axis of about 39.4 AU. This 2:3 resonance is populated by about 200 known objects,
[54] including
Pluto together with its moons. In recognition of this, the members of this family are known as
plutinos.
Many plutinos, including Pluto, have orbits that cross that of Neptune,
though their resonance means they can never collide. Plutinos have high
orbital eccentricities, suggesting that they are not native to their
current positions but were instead thrown haphazardly into their orbits
by the migrating Neptune.
[55] IAU guidelines dictate that all plutinos must, like Pluto, be named for underworld deities.
[51]
The 1:2 resonance (whose objects complete half an orbit for each of
Neptune's) corresponds to semi-major axes of ~47.7AU, and is sparsely
populated.
[56] Its residents are sometimes referred to as
twotinos. Other resonances also exist at 3:4, 3:5, 4:7 and 2:5.
[57] Neptune possesses a number of
trojan objects, which occupy its
L4 and L5 points;
gravitationally stable regions leading and trailing it in its orbit.
Neptune trojans are often described as being in a 1:1 resonance with
Neptune. Neptune trojans typically have very stable orbits.
Additionally, there is a relative absence of objects with semi-major
axes below 39 AU that cannot apparently be explained by the present
resonances. The currently accepted hypothesis for the cause of this is
that as Neptune migrated outward, unstable orbital resonances moved
gradually through this region, and thus any objects within it were swept
up, or gravitationally ejected from it.
[58]
"Kuiper cliff"
Graph showing the numbers of KBOs for a given distance from the Sun. The
plutinos are the "spike" at 39 AU, whereas the classicals are between
42 and 47 AU, the twotinos are at 48 AU, and the 5:2 resonance is at 55
AU.
The
1:2 resonance
appears to be an edge beyond which few objects are known. It is not
clear whether it is actually the outer edge of the classical belt or
just the beginning of a broad gap. Objects have been detected at the 2:5
resonance at roughly 55 AU, well outside the classical belt; however,
predictions of a large number of bodies in classical orbits between
these resonances have not been verified through observation.
[55]
Based on estimations of the primordial mass required to form
Uranus and Neptune, as well as bodies as large as Pluto (
see below),
earlier models of the Kuiper belt had suggested that the number of
large objects would increase by a factor of two beyond 50 AU,
[59]
so this sudden drastic falloff, known as the "Kuiper cliff", was
completely unexpected, and its cause, to date, is unknown. In 2003,
Bernstein and Trilling
et al. found evidence that the rapid
decline in objects of 100 km or more in radius beyond 50 AU is real, and
not due to observational bias. Possible explanations include that
material at that distance was too scarce or too scattered to accrete
into large objects, or that subsequent processes removed or destroyed
those that did.
[60] Patryk Lykawka of
Kobe University
has claimed that the gravitational attraction of an unseen large
planetary object, perhaps the size of Earth or Mars, might be
responsible.
[61][62]
Composition
The infrared spectra of both Eris and Pluto, highlighting their common methane absorption lines
Studies of the Kuiper belt since its discovery have generally
indicated that its members are primarily composed of ices: a mixture of
light hydrocarbons (such as
methane),
ammonia, and water
ice,
[63] a composition they share with
comets.
[64] The low densities observed in those KBOs whose diameter is known, (less than 1 g cm
−3) is consistent with an icy makeup.
[63] The temperature of the belt is only about 50 K,
[65] so many compounds that would be gaseous closer to the Sun remain solid.
Due to their small size and extreme distance from Earth, the chemical
makeup of KBOs is very difficult to determine. The principal method by
which astronomers determine the composition of a celestial object is
spectroscopy. When an object's light is broken into its component colors, an image akin to a rainbow is formed. This image is called a
spectrum.
Different substances absorb light at different wavelengths, and when
the spectrum for a specific object is unravelled, dark lines (called
absorption lines) appear where the substances within it have absorbed that particular wavelength of light. Every
element or
compound
has its own unique spectroscopic signature, and by reading an object's
full spectral "fingerprint", astronomers can determine what it is made
of.
Initially, such detailed analysis of KBOs was impossible, and so
astronomers were only able to determine the most basic facts about their
makeup, primarily their color.
[66] These first data showed a broad range of colors among KBOs, ranging from neutral grey to deep red.
[67] This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons.
[67]
This diversity was startling, as astronomers had expected KBOs to be
uniformly dark, having lost most of their volatile ices to the effects
of cosmic rays.
[68] Various solutions were suggested for this discrepancy, including resurfacing by impacts or outgassing.
[66]
However, Jewitt and Luu's spectral analysis of the known Kuiper belt
objects in 2001 found that the variation in color was too extreme to be
easily explained by random impacts.
[69]
Although to date most KBOs still appear spectrally featureless due to
their faintness, there have been a number of successes in determining
their composition.
[65] In 1996, Robert H. Brown
et al. obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of
Pluto, as well as Neptune's moon
Triton, possessing large amounts of
methane ice.
[70]
Water ice has been detected in several KBOs, including
1996 TO66,
[71] 38628 Huya and
20000 Varuna.
[72] In 2004, Mike Brown
et al. determined the existence of crystalline water ice and
ammonia hydrate on one of the largest known KBOs,
50000 Quaoar.
Both of these substances would have been destroyed over the age of the
Solar System, suggesting that Quaoar had been recently resurfaced,
either by internal tectonic activity or by meteorite impacts.
[65]
Mass and size distribution
Illustration of the power law.
Despite its vast extent, the collective mass of the Kuiper belt is
relatively low. The total mass is estimated to range between a 25th and
10th the mass of the Earth
[73] with some estimates placing it at one thirtieth of an Earth mass.
[74] Conversely, models of the Solar System's formation predict a collective mass for the Kuiper belt of 30 Earth masses.
[2]
This missing >99% of the mass can hardly be dismissed, as it is
required for the accretion of any KBOs larger than 100 km (62 mi) in
diameter. If the Kuiper belt had always had its current low density
these large objects simply could not have formed.
[2]
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 current influence is too weak to explain such a
massive "vacuuming", though the
Nice model
proposes that it could have been the cause of mass removal in the past.
Although 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.
[53]
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 between
N(
D) (the number of objects of diameter greater than
D) and
D,
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[75] give q = 4 ±0.5.
This implies that
Less formally, there are for instance 8 (=2
3) times more
objects in 100–200 km range than objects in 200–400 km range. In other
words, for every object with the diameter of 1,000 km (621 mi) there
should be around 1000 (=10
3) objects with diameter of 100 km (62 mi).
If
q is 4 or less, the law would imply an infinite mass in the Kuiper belt. The true function
N(
D) obviously assumes only integer values, so the fractional values it gives for
N at large
D cannot be accurate. More accurate models find that the "slope" parameter
q is in effect greater at large diameters and lesser at small diameters.
[75] It seems that
Pluto
is somewhat unexpectedly large, having several percent of the total
mass of the Kuiper belt. It is not expected that anything larger than
Pluto exists in the Kuiper belt, and in fact most of the brightest
(largest) objects at inclinations less than 5° have probably been found.
[75]
Of course, only the magnitude is actually known, the size is inferred assuming
albedo (not a safe assumption for larger objects).
Since January 2010, the smallest Kuiper belt object discovered to date spans 980 m across.
[76]
Scattered objects
Comparison of the orbits of scattered disc objects (black), classical
KBOs (blue), and 2:5 resonant objects (green). Orbits of other KBOs are
gray.
The scattered disc is a sparsely populated region, overlapping with the Kuiper belt but extending as far as 100 AU and farther.
Scattered disc objects
(SDOs) travel in highly elliptical orbits, usually also highly inclined
to the ecliptic. Most models of Solar System formation show both KBOs
and SDOs first forming in a primordial comet belt, whereas later
gravitational interactions, particularly with Neptune, sent the objects
spiraling outward, some into stable orbits (the KBOs) and some into
unstable orbits, becoming the scattered disc.
[8]
Due to its unstable nature, the scattered disc is believed to be the
point of origin for many of the Solar System's short-period comets.
Their dynamic orbits occasionally force them into the inner Solar
System, becoming first
centaurs, and then short-period comets.
[8]
According to the
Minor Planet Center,
which officially catalogues all trans-Neptunian objects, a KBO,
strictly speaking, is any object that orbits exclusively within the
defined Kuiper belt region regardless of origin or composition. Objects
found outside the belt are classed as scattered objects.
[77]
However, in some scientific circles the term "Kuiper belt object" has
become synonymous with any icy minor planet 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-disc region). They often describe scattered
disc objects as "scattered Kuiper belt objects".
[78] Eris, which is known to be more massive than Pluto, is often referred to as a KBO, but is technically an SDO.
[77]
A consensus among astronomers as to the precise definition of the
Kuiper belt has yet to be reached, and this issue remains unresolved.
The
centaurs,
which are not normally considered part of the Kuiper belt, are also
believed to be scattered objects, the only difference being that they
were scattered inward, rather than outward. The
Minor Planet Center groups the centaurs and the SDOs together as scattered objects.
[77]
Triton
Neptune's moon Triton
During its period of migration, Neptune is thought to have captured
one of the larger KBOs and set it in orbit around itself. This is its
moon
Triton, which is the only large moon in the Solar System to have a
retrograde orbit;
it orbits in the opposite direction to Neptune's rotation. This
suggests that, unlike the large moons of Jupiter, Saturn, and Uranus,
which are thought to have coalesced from spinning discs of material
encircling their young parent planets, Triton was a fully formed body
that was captured from surrounding space. Gravitational capture of an
object is not easy; it requires some mechanism to slow the object down
enough to be snared by the larger object's gravity. Triton may have
encountered Neptune as part of a binary (many KBOs are members of
binaries; see
below); ejection of the other member of the binary by Neptune could then explain Triton's capture.
[79]
Triton is only slightly larger than Pluto, and spectral analysis of
both worlds shows that they are largely composed of similar materials,
such as
methane and
carbon monoxide. All this points to the conclusion that Triton was once a KBO that was captured by Neptune during its
outward migration.
[80]
Largest KBOs
Artistic comparison of
Eris,
Pluto,
Makemake,
Haumea,
Sedna,
2007 OR10,
Quaoar,
Orcus, and
Earth.
Since the year 2000, a number of KBOs with diameters of between 500
and 1,500 km (932 mi), more than half that of Pluto, have been
discovered.
50000 Quaoar, a classical KBO discovered in 2002, is over 1,200 km across.
Makemake and
Haumea, both announced on July 29, 2005, are larger still. Other objects, such as
28978 Ixion (discovered in 2001) and
20000 Varuna (discovered in 2000) measure roughly 500 km (311 mi) across.
[2]
Pluto
The discovery of these large KBOs in similar orbits to Pluto led many to conclude that, bar its relative size,
Pluto
was not particularly different from other members of the Kuiper belt.
Not only did these objects approach Pluto in size, but many also
possessed satellites, and were of similar composition (methane and
carbon monoxide have been found both on Pluto and on the largest KBOs).
[2] Thus, just as
Ceres was considered a planet before the discovery of its fellow
asteroids, some began to suggest that Pluto might also be reclassified.
The issue was brought to a head by the discovery of
Eris, an object in the
scattered disc far beyond the Kuiper belt, that is now known to be 27% more massive than Pluto.
[81] In response, the
International Astronomical Union (IAU), was forced to
define what a planet is for the first time, and in so doing included in their definition that a planet must have "
cleared the neighbourhood around its orbit".
[82]
As Pluto shared its orbit with so many KBOs, it was deemed not to have
cleared its orbit, and was thus reclassified from a planet to a member
of the Kuiper belt.
Although Pluto is currently the largest KBO, there are two known
larger objects currently outside the Kuiper belt that probably
originated in it. These are Eris and Neptune's moon
Triton (which, as explained above, is probably a captured KBO).
As of 2008, only five objects in the Solar System (Ceres, Eris, and
the KBOs Pluto, Makemake and Haumea) are listed as dwarf planets by the
IAU. However,
90482 Orcus, 28978 Ixion and
many other Kuiper-belt objects are large enough to be in hydrostatic equilibrium; most of them will probably qualify when more is known about them.
[83][84][85]
Satellites
Of the four largest TNOs, three (Eris, Pluto, and Haumea) possess
satellites, and two have more than one. A higher percentage of the
larger KBOs possess satellites than the smaller objects in the Kuiper
belt, suggesting that a different formation mechanism was responsible.
[86]
There are also a high number of binaries (two objects close enough in
mass to be orbiting "each other") in the Kuiper belt. The most notable
example is the Pluto–Charon binary, but it is estimated that around 11%
of KBOs exist in binaries.
[87]
Exploration
Artist's conception of New Horizons at Pluto
On January 19, 2006, the first spacecraft mission to explore the Kuiper belt,
New Horizons, was launched. The mission, headed by
Alan Stern of the
Southwest Research Institute, will arrive at
Pluto
on July 14, 2015, and, circumstances permitting, will continue on to
study another as-yet-undetermined KBO. Any KBO chosen will be between 40
and 90 km (25 to 55 miles) in diameter and, ideally, white or grey, to
contrast with Pluto's reddish color.
[88] John Spencer, an astronomer on the
New Horizons
mission team, says that no target for a post-Pluto Kuiper belt
encounter has yet been selected, as they are awaiting data from the
Pan-STARRS survey project to ensure as wide a field of options as possible.
[89] The Pan-STARRS project, partially operational since May 2010,
[90] will, when fully online, survey the entire sky with four 1.4 gigapixel digital cameras to detect any moving objects, from
near-Earth objects to KBOs.
[91] To speed up the detection process, the New Horizons team established
Ice Hunters, a
citizen science project that allowed members of the public to participate in the search for suitable KBO targets;
[92][93][94] the project has subsequently been transferred to another site,
Ice Investigators,
[95] produced by
CosmoQuest.
[96]
The debris discs around the stars
HD 139664 and
HD 53143. The black central circle is produced by the camera's
coronagraph, which hides the central star to allow the much fainter discs to be seen.
Other Kuiper belts
By 2006, astronomers had resolved dust discs believed to be Kuiper
belt-like structures around nine stars other than the Sun. They appear
to fall into two categories: wide belts, with radii of over 50 AU, and
narrow belts (like our own Kuiper belt) with radii of between 20 and 30
AU and relatively sharp boundaries.
[97] Beyond this, 15–20% of solar-type stars have an observed
infrared excess that is believed to indicate massive Kuiper-belt-like structures.
[98] Most known
debris discs around other stars are fairly young, but the two images on the right, taken by the
Hubble Space Telescope
in January 2006, are old enough (roughly 300 million years) to have
settled into stable configurations.
The left image is a "top view" of a
wide belt, and the right image is an "edge view" of a narrow belt.
[97][99] Supercomputer
simulations of dust in the Kuiper belt suggest that when it was
younger, it may have resembled the narrow rings seen around younger
stars.
[100]
