Planetary migration
Encyclopedia
Planetary migration occurs when a planet
or other stellar satellite interacts with a disk of gas or planetesimal
s, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis
. The generally accepted theory of planet formation from a protoplanetary disk
predicts such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals. It has also become clear that terrestrial-mass planet
s may be subject to rapid inward migration if they form while the gas disk is still present. This may affect the formation of the cores of the giant planets (which have masses of the order of 10 Earth masses), if those planets form via the core accretion mechanism. Planetary migration is the most likely explanation for hot Jupiter
s, extrasolar planets with jovian
masses and orbits of only a few days.
to the surrounding gas in the protoplanetary disk
so that their orbits spiral gradually inwards towards the primary.
s gravitationally interact in a chaotic manner causing many planetesimals to be thrown into new orbits. This results in angular-momentum exchange between the planets and the planetesimals, and leads to migration (either inward or outward). Outward migration of Neptune
is believed to be responsible for the resonant capture of Pluto
and other Plutino
s into the 3:2 resonance
with Neptune.
s" form.
and the Oort cloud
. Sedna
may be the first known example of such oligarch planets. Even smaller objects would have been scattered even further out to become the Oort cloud
.
The migration of the outer planets is necessary to account for the existence and properties of the Solar System's outermost regions. Beyond Neptune
, the Solar System continues into the Kuiper belt
, the scattered disc
, and the Oort cloud
, three sparse populations of small icy bodies thought to be the points of origin for most observed comet
s. At their distance from the Sun, accretion was too slow to allow planets to form before the solar nebula
dispersed, and thus the initial disc lacked enough mass density to consolidate into a planet. The Kuiper belt lies between 30 and 55 AU from the Sun, while the farther scattered disc extends to over 100 AU, and the distant Oort cloud begins at about 50,000 AU.
Originally, however, the Kuiper belt was much denser and closer to the Sun: it contained millions of planetesimals, and had an outer edge at approximately 30 AU, the present distance of Neptune.
After the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with the large number of remaining planetesimals. After 500–600 million years (about 4 billion years ago) Jupiter and Saturn fell into a 2:1 orbital resonance
; Saturn orbited the Sun once for every two Jupiter orbits. This resonance created a gravitational push against the outer planets, causing Neptune to surge past Uranus and plough into the dense planetesimal belt. The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets' orbits outwards while they moved inwards. This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward.
This scattering scenario explains the trans-Neptunian populations' present low mass.
The outer two planets of the Solar System, Uranus
and Neptune
, are believed to have migrated outward in this way from their formation in orbits near Jupiter and Saturn to their current positions, over hundreds of millions of years. Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again.
In contrast to the outer planets, the inner planets are not believed to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts
.
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
or other stellar satellite interacts with a disk of gas or 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, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
. The generally accepted theory of planet formation from a protoplanetary 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...
predicts such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals. It has also become clear that terrestrial-mass 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 may be subject to rapid inward migration if they form while the gas disk is still present. This may affect the formation of the cores of the giant planets (which have masses of the order of 10 Earth masses), if those planets form via the core accretion mechanism. Planetary migration is the most likely explanation for hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s, extrasolar planets with jovian
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...
masses and orbits of only a few days.
Gas disk
Protoplanetary gas disks around young stars are observed to have lifetimes of a few million years. If planets with masses of around an Earth mass or greater form while the gas is still present, the planets are thought to transfer angular momentumAngular 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...
to the surrounding gas in the protoplanetary 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...
so that their orbits spiral gradually inwards towards the primary.
Planetesimal disk
During the late phase of planetary system formation, massive protoplanets and planetesimalPlanetesimal
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 gravitationally interact in a chaotic manner causing many planetesimals to be thrown into new orbits. This results in angular-momentum exchange between the planets and the planetesimals, and leads to migration (either inward or outward). Outward migration of 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...
is believed to be responsible for the resonant capture of 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...
and other Plutino
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...
s into the 3:2 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...
with Neptune.
Type I migration
Terrestrial mass planets drive spiral density waves in the surrounding gas or planetesimal disk. An imbalance occurs in the strength of the interaction with the spirals inside and outside the planet's orbit. In most cases, the outer wave exerts a somewhat greater torque on the planet than the interior wave. This causes the planet to lose angular momentum and the planet then migrates inwards on timescales that are short relative to the million-year lifetime of the disk.Type II migration
Planets of more than about 10 Earth masses clear a gap in the disk, ending Type I migration. However, material continues to enter the gap on the timescale of the larger accretion disk, moving the planet and gap inward on the accretion timescale of the disk. This is one hypothesis for how some or most "hot JupiterHot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s" form.
Gravitational scattering
Another possible mechanism that may move planets over large orbital radii is gravitational scattering by larger planets. In the case of our solar system, Uranus and Neptune may have been gravitationally scattered in close encounters with Jupiter and/or Saturn. Planetesimals that were present in the early formation of the solar system called oligarchs are much smaller than Uranus and Neptune and so are likely to have been scattered much further out and be roaming the space between the Kuiper beltKuiper 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...
and 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...
. Sedna
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...
may be the first known example of such oligarch planets. Even smaller objects would have been scattered even further out to become 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...
.
In the Solar System
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...
, the Solar System continues into 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 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...
, and 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...
, three sparse populations of small icy bodies thought to be the points of origin for most observed comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s. At their distance from the Sun, accretion was too slow to allow planets to form before the solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
dispersed, and thus the initial disc lacked enough mass density to consolidate into a planet. The Kuiper belt lies between 30 and 55 AU from the Sun, while the farther scattered disc extends to over 100 AU, and the distant Oort cloud begins at about 50,000 AU.
Originally, however, the Kuiper belt was much denser and closer to the Sun: it contained millions of planetesimals, and had an outer edge at approximately 30 AU, the present distance of Neptune.
After the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with the large number of remaining planetesimals. After 500–600 million years (about 4 billion years ago) Jupiter and Saturn fell into a 2:1 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...
; Saturn orbited the Sun once for every two Jupiter orbits. This resonance created a gravitational push against the outer planets, causing Neptune to surge past Uranus and plough into the dense planetesimal belt. The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets' orbits outwards while they moved inwards. This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward.
This scattering scenario explains the trans-Neptunian populations' present low mass.
The outer two planets of the Solar System, 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...
, are believed to have migrated outward in this way from their formation in orbits near Jupiter and Saturn to their current positions, over hundreds of millions of years. Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again.
In contrast to the outer planets, the inner planets are not believed to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts
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...
.