Nice model
Encyclopedia
The Nice model is a scenario for the dynamical evolution of the Solar System
. It is named for the location of the Observatoire de la Côte d'Azur, where it was initially developed, in Nice
, France. It proposes the migration
of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary gas disk
. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulation
s of the Solar System to explain historical events including the Late Heavy Bombardment
of the inner Solar System, the formation of the Oort cloud
, and the existence of populations of small Solar System bodies including the Kuiper belt
, the Neptune
and Jupiter
Trojan
s, and the numerous resonant trans-Neptunian object
s dominated by Neptune. Its success at reproducing many of the observed features of the Solar System means that it is presently widely accepted as the current most realistic model of the Solar System's early evolution, though it is not universally favoured among planetary scientists
. One of its limitations is reproducing the outer-system satellites and the Kuiper belt (see below).
in 2005 by an international collaboration of scientists: R. Gomes, Hal Levison
, Alessandro Morbidelli and Kleomenis Tsiganis. In these publications, the four authors proposed that after the dissipation of the gas and dust of the primordial Solar System disk, the four giant planets (Jupiter
, Saturn
, Uranus
and Neptune
) were originally found on near-circular orbits between ~5.5 and ~17 astronomical units (AU), much more closely spaced and more compact than in the present. A large, dense disk of small, rock
and ice
planetesimals, their total about 35 Earth masses, extended from the orbit of the outermost giant planet to some 35 AU.
This planetary system evolved in the following manner. Planetesimals at the disk's inner edge occasionally pass through gravitational encounters with the outermost giant planet, which change the planetesimals' orbits. The planets scatter inwards the majority of the small icy bodies that they encounter, exchanging angular momentum
with the scattered objects so that the planets move outwards in response, preserving the angular momentum of the system. These planetesimals then similarly scatter off the next planet they encounter, successively moving the orbits of Uranus
, Neptune
, and Saturn
outwards. Despite the minute movement each exchange of momentum can produce, cumulatively these planetesimal encounters shift (migrate
) the orbits of the planets by significant amounts. This process continues until the planetesimals interact with the inmost and most massive giant planet, Jupiter
, whose immense gravity sends them into highly elliptical orbits or even ejects them outright from the Solar System. This, in contrast, causes Jupiter to move slightly inward.
The low rate of orbital encounters governs the rate at which planetesimals are lost from the disk, and the corresponding rate of migration. After several hundreds of millions of years of slow, gradual migration, Jupiter and Saturn, the two inmost giant planets, cross their mutual 1:2 mean-motion resonance. This resonance increases their orbital eccentricities
, destabilizing the entire planetary system. The arrangement of the giant planets alters quickly and dramatically. Jupiter shifts Saturn out towards its present position, and this relocation causes mutual gravitational encounters between Saturn and the two ice giant
s, which propel Neptune and Uranus onto much more eccentric orbits. These ice giants then plough into the planetesimal disk, scattering tens of thousands of planetesimals from their formerly stable orbits in the outer Solar System. This disruption almost entirely scatters the primordial disk, removing 99% of its mass, a scenario which explains the modern-day absence of a dense trans-Neptunian
population. Some of the planetesimals are thrown into the inner Solar System, producing a sudden influx of impacts
on the terrestrial planet
s: the Late Heavy Bombardment
.
Eventually, the giant planets reach their current orbital semi-major axes, and dynamical friction
with the remaining planetesimal disc damps their eccentricities and makes the orbits of Uranus and Neptune circular again.
In some 50% of the initial models of Tsiganis et al., Neptune and Uranus also exchange places about a billion years (20%) into the life of the Solar System. However, the results only correspond to an even mass distribution in the protoplanetary disk, and match the masses of the planets, if the switch did take place.
a model of the evolution of the early Solar System is difficult, since the evolution cannot be directly observed. However, the success of any dynamical model can be judged by comparing the population predictions from the simulations to astronomical observations of these populations. At the present time, computer models of the Solar System that are begun with the initial conditions of the Nice scenario best match many aspects of the observed Solar System.
and on the terrestrial planets is part of the main evidence for the Late Heavy Bombardment (LHB): an intensification in the number of impactors, at about 600 million years after the Solar System's formation. The number of planetesimal
s that would reach the Moon in the Nice model is consistent with the crater record from the LHB.
groups in the L4 and L5 Lagrange points of Jupiter and Neptune. During this time, the Trojan co-orbital region is termed "dynamically open". Under the Nice model, the planetesimals leaving the disrupted disk cross this region in large numbers, temporarily inhabiting it. After the period of orbital instability ends, the Trojan region is "dynamically closed", capturing planetesimals present at the time. The present Trojan populations are then these acquired scattered planetesimals of the primordial belt. This simulated population matches the libration angle, eccentricity and the large inclinations of the orbits of the Jovian Trojans. Their inclinations had not previously been understood.
This mechanism of the Nice model similarly generates the Trojans of Neptune
.
A large number of planetesimals would have also been captured in the outer Main Belt, at distances greater than 2.6 AU, and in the region of the Hilda family. These captured objects would then have undergone collisional erosion, grinding the population away into smaller fragments that can then be acted on by the solar wind
and YORP effect; removing more than 90% of them according to Bottke et al. The size frequency distribution of this simulated population following this erosion are in excellent agreement with observations. This suggests that the Jovian Trojans, Hildas and some of the outer Main Belt, all spectral D-type asteroid
s, are the remnant planetesimals from this capture and erosion process, possibly also including the dwarf planet .
s captured by traditional mechanisms, such as drag or impacts from the accretion disks, would be lost during the interactions of the planets at the time of global system instability. In the Nice model, large numbers of planetesimals interact with the outer planets at this time, and some are captured during three-way interactions
with those planets. The probability for any planetesimal to be captured by an ice giant
is relatively high, a few 10−7. These new satellites could be captured at almost any angle, so unlike the regular satellites of Saturn
, Uranus
and Neptune
, they do not necessarily orbit in the planets' equatorial planes. Triton
, the largest moon of Neptune, can be explained if it was captured in a three-body interaction involving the disruption of a binary planetoid, of which Triton was the less massive member (Cuk & Gladman 2005). However, such binary disruption would not in general have supplied the large number of small irregulars. Some irregulars may have even been exchanged between planets.
The resulting irregular orbits match well with the observed populations' semimajor axes, inclinations and eccentricities, but not with their size distribution. Subsequent collisions between these captured satellites may have created the suspected collisional families
seen today. These collisions are also required to erode the population to the present size distribution.
There would not have been enough interactions with Jupiter
in the simulations to explain Jupiter's retinue of irregulars, suggesting either that a second mechanism was at work for that planet, or that the parameters of the Nice model need to be revised.
's outermost regions. Originally, the Kuiper belt
was much denser and closer to the Sun
, with an outer edge at approximately 30 AU. Its inner edge would have been just beyond the orbits of Uranus
and Neptune
, which were in turn far closer to the Sun when they formed (most likely in the range of 15–20 AU), and in opposite locations, with Uranus farther from the Sun than Neptune.
Some of the scattered objects
, including Pluto
, became gravitationally tied to Neptune's orbit, forcing them into mean-motion resonances. The Nice model is favoured for its ability to explain the occupancy of current
orbital resonance
s in the Kuiper belt, particularly the 2:5 resonance. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, capturing some of them into resonances and sending others into chaotic orbits. The objects in the scattered disc
are believed to have been placed in their current positions by interactions with Neptune's migrating resonances.
However, the Nice model still fails to account for many of the characteristics of the distribution. It is able to produce the hot population, objects in the Kuiper belt that have highly inclined orbits, but not the low-inclination
cold population.
The two populations not only possess different orbits, but different compositions; the cold population is markedly redder than the hot, suggesting it formed in a different region. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giant
s. The cold population, on the other hand, is believed to have formed more or less in its current position, although it may also have been later swept outwards by Neptune during its migration. Quoting one of the scientific articles, the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".
; those objects scattered to a lesser degree by the migrating Neptune formed the current Kuiper belt and scattered disc.
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. It is named for the location of the Observatoire de la Côte d'Azur, where it was initially developed, in Nice
Nice
Nice is the fifth most populous city in France, after Paris, Marseille, Lyon and Toulouse, with a population of 348,721 within its administrative limits on a land area of . The urban area of Nice extends beyond the administrative city limits with a population of more than 955,000 on an area of...
, France. It proposes the migration
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary gas disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulation
Dynamical simulation
Dynamical simulation, in computational physics, is the simulation of systems of objects that are free to move, usually in three dimensions according to Newton's laws of dynamics, or approximations thereto...
s of the Solar System to explain historical events including the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
of the inner Solar System, the formation of the Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
, and the existence of populations of small Solar System bodies including the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
, the Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
and Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
Trojan
Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
s, and the numerous resonant trans-Neptunian object
Resonant trans-Neptunian object
In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object in mean motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e.g. 1:2, 2:3 etc...
s dominated by Neptune. Its success at reproducing many of the observed features of the Solar System means that it is presently widely accepted as the current most realistic model of the Solar System's early evolution, though it is not universally favoured among planetary scientists
Planetary science
Planetary science is the scientific study of planets , moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation,...
. One of its limitations is reproducing the outer-system satellites and the Kuiper belt (see below).
Description
The original core of the Nice model is a triplet of papers published in the general science journal NatureNature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
in 2005 by an international collaboration of scientists: R. Gomes, Hal Levison
Harold F. Levison
Harold F. "Hal" Levison is a planetary scientist specializing in planetary dynamics. He argued for a distinction between what are now called dwarf planets and the other eight planets based on their inability to "clear the neighborhood around their orbits", although his proposal suggested the terms...
, Alessandro Morbidelli and Kleomenis Tsiganis. In these publications, the four authors proposed that after the dissipation of the gas and dust of the primordial Solar System disk, the four giant planets (Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
) were originally found on near-circular orbits between ~5.5 and ~17 astronomical units (AU), much more closely spaced and more compact than in the present. A large, dense disk of small, rock
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
and ice
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
planetesimals, their total about 35 Earth masses, extended from the orbit of the outermost giant planet to some 35 AU.
This planetary system evolved in the following manner. Planetesimals at the disk's inner edge occasionally pass through gravitational encounters with the outermost giant planet, which change the planetesimals' orbits. The planets scatter inwards the majority of the small icy bodies that they encounter, exchanging angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
with the scattered objects so that the planets move outwards in response, preserving the angular momentum of the system. These planetesimals then similarly scatter off the next planet they encounter, successively moving the orbits of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
, Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
outwards. Despite the minute movement each exchange of momentum can produce, cumulatively these planetesimal encounters shift (migrate
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
) the orbits of the planets by significant amounts. This process continues until the planetesimals interact with the inmost and most massive giant planet, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, whose immense gravity sends them into highly elliptical orbits or even ejects them outright from the Solar System. This, in contrast, causes Jupiter to move slightly inward.
The low rate of orbital encounters governs the rate at which planetesimals are lost from the disk, and the corresponding rate of migration. After several hundreds of millions of years of slow, gradual migration, Jupiter and Saturn, the two inmost giant planets, cross their mutual 1:2 mean-motion resonance. This resonance increases their orbital eccentricities
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
, destabilizing the entire planetary system. The arrangement of the giant planets alters quickly and dramatically. Jupiter shifts Saturn out towards its present position, and this relocation causes mutual gravitational encounters between Saturn and the two ice giant
Ice giant
Ice Giant or Ice giants may refer to:* A type of gas giant composed largely of materials less volatile than hydrogen and helium* The frost giants of Norse mythology, see also Hrimthurs...
s, which propel Neptune and Uranus onto much more eccentric orbits. These ice giants then plough into the planetesimal disk, scattering tens of thousands of planetesimals from their formerly stable orbits in the outer Solar System. This disruption almost entirely scatters the primordial disk, removing 99% of its mass, a scenario which explains the modern-day absence of a dense trans-Neptunian
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...
population. Some of the planetesimals are thrown into the inner Solar System, producing a sudden influx of impacts
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...
on the terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
s: the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
.
Eventually, the giant planets reach their current orbital semi-major axes, and dynamical friction
Dynamical friction
Dynamical friction is a term in astrophysics related to loss of momentum and kinetic energy of moving bodies through a gravitational interaction with surrounding matter in space...
with the remaining planetesimal disc damps their eccentricities and makes the orbits of Uranus and Neptune circular again.
In some 50% of the initial models of Tsiganis et al., Neptune and Uranus also exchange places about a billion years (20%) into the life of the Solar System. However, the results only correspond to an even mass distribution in the protoplanetary disk, and match the masses of the planets, if the switch did take place.
Solar System features
Running dynamical models of the Solar System with different initial conditions for the simulated length of the history of the Solar System will produce the various populations of objects within the Solar System. As the initial conditions of the model are allowed to vary, each population will be more or less numerous, and will have particular orbital properties. ProvingMathematical proof
In mathematics, a proof is a convincing demonstration that some mathematical statement is necessarily true. Proofs are obtained from deductive reasoning, rather than from inductive or empirical arguments. That is, a proof must demonstrate that a statement is true in all cases, without a single...
a model of the evolution of the early Solar System is difficult, since the evolution cannot be directly observed. However, the success of any dynamical model can be judged by comparing the population predictions from the simulations to astronomical observations of these populations. At the present time, computer models of the Solar System that are begun with the initial conditions of the Nice scenario best match many aspects of the observed Solar System.
The Late Heavy Bombardment
The crater record on the MoonMoon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and on the terrestrial planets is part of the main evidence for the Late Heavy Bombardment (LHB): an intensification in the number of impactors, at about 600 million years after the Solar System's formation. The number of planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
s that would reach the Moon in the Nice model is consistent with the crater record from the LHB.
Trojans and Main Belt asteroids
During the period of orbital disruption following Jupiter and Saturn reaching the 2:1 resonance, the combined gravitational influence of the migrating giant planets would have quickly destabilized any pre-existing TrojanTrojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
groups in the L4 and L5 Lagrange points of Jupiter and Neptune. During this time, the Trojan co-orbital region is termed "dynamically open". Under the Nice model, the planetesimals leaving the disrupted disk cross this region in large numbers, temporarily inhabiting it. After the period of orbital instability ends, the Trojan region is "dynamically closed", capturing planetesimals present at the time. The present Trojan populations are then these acquired scattered planetesimals of the primordial belt. This simulated population matches the libration angle, eccentricity and the large inclinations of the orbits of the Jovian Trojans. Their inclinations had not previously been understood.
This mechanism of the Nice model similarly generates the Trojans of Neptune
Neptune Trojan
Neptune trojans are Kuiper belt object-like bodies in solar orbit that have the same orbital period as Neptune and follow roughly the same orbital path...
.
A large number of planetesimals would have also been captured in the outer Main Belt, at distances greater than 2.6 AU, and in the region of the Hilda family. These captured objects would then have undergone collisional erosion, grinding the population away into smaller fragments that can then be acted on by the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
and YORP effect; removing more than 90% of them according to Bottke et al. The size frequency distribution of this simulated population following this erosion are in excellent agreement with observations. This suggests that the Jovian Trojans, Hildas and some of the outer Main Belt, all spectral D-type asteroid
D-type asteroid
D-type asteroids have a very low albedo and a featureless reddish electromagnetic spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors...
s, are the remnant planetesimals from this capture and erosion process, possibly also including the dwarf planet .
Outer-system satellites
Any original populations of irregular satelliteIrregular satellite
In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....
s captured by traditional mechanisms, such as drag or impacts from the accretion disks, would be lost during the interactions of the planets at the time of global system instability. In the Nice model, large numbers of planetesimals interact with the outer planets at this time, and some are captured during three-way interactions
Three-body force
A three-body force is a force that does not exist in a system of two objects but appears in a three-body system. In general, if the behaviour of a system of more than two objects cannot be described by the two-body interactions between all possible pairs, as a first approximation, the deviation is...
with those planets. The probability for any planetesimal to be captured by an ice giant
Ice giant
Ice Giant or Ice giants may refer to:* A type of gas giant composed largely of materials less volatile than hydrogen and helium* The frost giants of Norse mythology, see also Hrimthurs...
is relatively high, a few 10−7. These new satellites could be captured at almost any angle, so unlike the regular satellites of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, they do not necessarily orbit in the planets' equatorial planes. Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
, the largest moon of Neptune, can be explained if it was captured in a three-body interaction involving the disruption of a binary planetoid, of which Triton was the less massive member (Cuk & Gladman 2005). However, such binary disruption would not in general have supplied the large number of small irregulars. Some irregulars may have even been exchanged between planets.
The resulting irregular orbits match well with the observed populations' semimajor axes, inclinations and eccentricities, but not with their size distribution. Subsequent collisions between these captured satellites may have created the suspected collisional families
Collisional family
In astronomy, a collisional family is a group of objects that are thought to have a common origin in an impact . They have similar compositions, and most share similar orbital elements....
seen today. These collisions are also required to erode the population to the present size distribution.
There would not have been enough interactions with Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
in the simulations to explain Jupiter's retinue of irregulars, suggesting either that a second mechanism was at work for that planet, or that the parameters of the Nice model need to be revised.
Formation of the Kuiper belt
The migration of the outer planets is also necessary to account for the existence and properties of the Solar SystemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
's outermost regions. Originally, the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
was much denser and closer to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, with an outer edge at approximately 30 AU. Its inner edge would have been just beyond the orbits of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, which were in turn far closer to the Sun when they formed (most likely in the range of 15–20 AU), and in opposite locations, with Uranus farther from the Sun than Neptune.
Some of the scattered objects
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...
, including Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
, became gravitationally tied to Neptune's orbit, forcing them into mean-motion resonances. The Nice model is favoured for its ability to explain the occupancy of current
Resonant trans-Neptunian object
In astronomy, a resonant trans-Neptunian object is a trans-Neptunian object in mean motion orbital resonance with Neptune. The orbital periods of the resonant objects are in a simple integer relations with the period of Neptune e.g. 1:2, 2:3 etc...
orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
s in the Kuiper belt, particularly the 2:5 resonance. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, capturing some of them into resonances and sending others into chaotic orbits. The objects in the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...
are believed to have been placed in their current positions by interactions with Neptune's migrating resonances.
However, the Nice model still fails to account for many of the characteristics of the distribution. It is able to produce the hot population, objects in the Kuiper belt that have highly inclined orbits, but not the low-inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
cold population.
The two populations not only possess different orbits, but different compositions; the cold population is markedly redder than the hot, suggesting it formed in a different region. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s. The cold population, on the other hand, is believed to have formed more or less in its current position, although it may also have been later swept outwards by Neptune during its migration. Quoting one of the scientific articles, the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".
Scattered disc and Oort cloud
Those objects scattered by Jupiter into highly elliptical orbits formed the Oort cloudOort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
; those objects scattered to a lesser degree by the migrating Neptune formed the current Kuiper belt and scattered disc.