Migration (astronomy)
The term planetary migration describes the change in orbit of a planet during the formation of a planetary system around a central star. However, since it is a theoretical model, there is no uniform definition. Planetary migration comes about through a complex interaction of a planet with its environment (other planets, planetesimals , gas from a protoplanetary disk ). Path changes that occur due to random events, for example due to collisions, are not included in the term.
introduction
The discovery of exoplanet systems in which Jupiter-like celestial bodies have orbits close to the star of only a few star radii (so-called " Hot Jupiters ", 51 Pegasi b has a major semi-axis of a = 0.05 AU ) has triggered a discussion about the formation model of planetary systems. Many astronomers are of the opinion that gas giants are a few astronomical units (AU) away from the central star, behind the so-called ice line . This is the distance from the central star from which hydrogen compounds can exist in solid form. In the process of creation, these planets must have moved towards the central star.
An attempt to explain that tries to get along without migration is, for example, the “Jumping Jupiter Theory”. This states that the simultaneous creation of several gas giants in a planetary system leads to gravitational interactions with one another. Simulations show that these processes would lead to unstable orbits, collisions between the planets, accretion by the protostar or even leaving the planetary system, which is why the formation of Hot Jupiters in this way is considered unlikely.
Another approach is planetary migration. In the development of a planetary system, this describes the interactions between the protoplanetary disk and the planet itself, which can lead to changes in orbit. There are three different ways in which the planetary disk can interact with the planet. These are divided into three types of migration, which are explained further in the section below. Although the migration theory can be used to explain the current position of the planets during the formation phase (e.g. in Jupiter ) and orbital enlargements (e.g. in Uranus and Neptune ), migration is only a theory that experts use is generally recognized, but has not yet been directly proven due to the lack of direct observation possibilities. The grand tack model offers such a scenario . In addition, as in the Nice model , the migration theory can be applied to the later Late Heavy Bombardment (LHB) or the origin of the Trojans and useful results can be drawn from simulations.
Theoretical considerations
Standard model of planet formation
Since migration is an effect that takes place in the late phase of planet formation, a rough overview of the standard scenario of planet formation that is widespread among astronomers should first be given here. According to the theory, the origin of the planets lies in the so-called large molecular clouds (GMC - "giant molecular clouds"), which mainly consist of gas (99% hydrogen, helium) and dust (silicates, carbon) and compression due to their own gravity until they finally fragment into smaller "cores". Such cores can reach dimensions of a few thousand AU and finally collapse according to the jeans criterion . A protostar arises in the middle of the cloud, which dominates the gravitational properties of the system. In particular, it is a central force field for the surrounding matter in which angular momentum conservation applies. This prevents, for example, all matter from simply falling into the star because it is attracted to it. Rather, a stable rotating disk ( protoplanetary disk ) develops from the cloud , in which angular momentum can be “transported” from the inside to the outside through the viscosity of turbulence and viscous friction. In our solar system, for example, Jupiter and Saturn carry 99% of the total angular momentum, while the sun has almost all of the mass. The inner parts of the disk move further inwards and are finally accreted by the star, while the outer parts can be spared from this fate. This creates a complex hydrodynamic system that enables sedimentation and drift of the solid bodies that are now growing ever stronger. With a size of a few meters to a few kilometers, one speaks of planetesimals . From this size, the planetesimals dominate the events in their environment through their own gravity, for example they capture surrounding, smaller planetesimals, more and more efficiently the larger they get (this is why this phase is called "runaway growth"). At some point a few so-called planetary embryos have formed in this way, which gravitationally dominate their environment and accret the matter and gas from the protoplanetary disk (in the case of gas giants) (so-called "oligarchic growth" and isolation of the embryos). However, the planets do not have to have originated in the place where we observe them today. For example, a Jupiter-like gas giant with 51 Pegasi b was observed only a few stellar radii away from the central star. The formation of such a massive object so close to a star would be very difficult to explain with these so-called in-situ theories, which is why it is assumed that the planets can experience changes in their orbit in the final phase of their formation under certain conditions. This phenomenon is known as planetary migration.
Types of Planetary Migration
The different types of migration are classified by most astronomers into three types:
Type 1
The object (planetesimal or planetary embryo) interacts with its self-generated density waves, which arise because the surrounding gas moves at a higher speed than Kepler's orbital speed . This accelerates the gas due to the gravitational effect of the protoplanet and pressure and density waves are created that move with the protoplanet. Because of the asymmetry on the side facing away from or facing the star, this results in a net force on the planet that changes its orbit.
Type 2
Protoplanets open a gap in the gas disk by accretion of surrounding matter, creating a region of lower density in the “feeding zone” of the planet. The protoplanet is trapped in this gap. As the gas moves inward in the course of the planet formation process, the gap follows and the protoplanet migrates inward.
Type 3
Instabilities in the planetary disc (interactions between the planets) lead to an orbit deviation within a few revolutions of the planet.
If a planet or planetesimal changes its orbit too much and is lost to the system (i.e. leaves the solar system or migrates inwards as a result of the slowing down of the orbital speed and falls victim to the star / protostar), this is called "violent migration".
Simulation results
Solar system
The theory of planet formation suggests that giant planets develop in round and coplanar orbits. At the moment the eccentricities of Jupiter are 6%, Saturn's 9% and Uranus ' 8% and the mutual orbital inclination (angle between the normal vectors of the orbital planes) is a maximum of 2 ° with respect to Jupiter's orbit. Existing models have not yet been fully successfully applied to the solar system . However, they show that calculations beginning with coplanar quasi-circular orbits deliver the currently valid conditions, especially with regard to the 1: 2 orbital resonance between Jupiter and Saturn. This occurred during the migration of the giant planets as a result of the interactions with the planetesimal disk. The calculations reproduce all relevant parameters, such as large semi-axes , eccentricities and mutual inclination.
The orbital distribution of objects beyond Neptune's orbit that can be observed today suggests that this is a result of Neptune's planetary migration from 20 AU to 30–35 AU during the expansion of the protoplanetary disk. In the course of the migration, both eccentricities and the orbital tilt of the planets decreased due to the interactions with the particles of the disk due to so-called dynamic friction. If the orbits of the planets are very close to one another, resonances (mean motion resonance; MMR) can occur when the orbits are changed if the ratios of the orbital times form small ratios. These cause fixed eccentricity ratios between the resonant planets.
The best known example of this is the 1: 2 resonance between Jupiter and Saturn. After the slow migration period when both passed the 1: 2 MMR, their eccentricities quickly changed to the values observed today. These jumps can be explained by the fact that both planets jumped over the resonance stage without being caught. This disturbance was transferred to the gas giants Uranus and Neptune and caused an increase in orbit there too, which is dependent on the respective masses. This led to chaotic and overlapping path conditions for a short period of time after the path resonance was exceeded. The gas giants drifted outward (radially away from the Sun) and carried small chunks of rock inward, moving towards Jupiter and Saturn. The bombardment of boulders resulted in the orbits of Jupiter and Saturn becoming smaller again due to deceleration ( conservation of angular momentum ). This rapid migration phase ended as soon as Jupiter and Saturn reached the resonance threshold again and their orbits stabilized there. The results of these developments are the values observed today for eccentricity, orbital inclination and semi-axes of these planets.
These results show that one should never consider migration or resonance separately, but always include the entire system in order to explain these processes.
Loss of a fifth gas planet?
Jupiter would have disturbed the orbits of the inner planets if it had migrated slowly into the inner solar system. However, it could have entered a new orbit if it had catapulted a gas giant from the solar system from the mass of Uranus or Neptune.
Late Heavy Bombardment
Around 700 million years after the formation of the planets, there was a very high rate of impact on the planets (and moons), the so-called " Late Heavy Bombardment " (LHB). Probably the trigger of the LHB is the rapid migration of the giant planets (Jupiter, Saturn). This caused a destabilization of the orbits of smaller objects (planetesimals), whereby they got into the interior of the solar system and caused impacts.
Jupiter's trojan
The origin of the Trojans is quite controversial. One theory says that these celestial bodies were formed at a greater distance from Jupiter, were then "collected" during the Jupiter migration and now move on orbits close to Jupiter.
Planets in the disk can also be caught in resonances if their major semi-axes change at different speeds. This was the first time that M. Melita and M. Woolfson explained the relationships between the orbital times of the main planets in the solar system. They dealt with the change in the major semiaxes of the planets, which are influenced by gas aggregation and dynamic friction.
Kuiper belt
Observations at the Very Large Telescope (VLT) of the European Southern Observatory ESO in Chile show that the “ice-cold” Kuiper Belt Objects (KBOs) were formed on the edge of the planetary system closer to the sun. Two dynamic subgroups of KBOs, the “hot” and “cold” Cubewanos , stand out, which differ in their surface colors (different chemical composition of the objects) but have similar trajectories so that different regions of origin are assumed. This observation supports the theory that the two outer planets Uranus and Neptune were formed closer to the sun and only then "migrated" to their present greater distances. During this migration they dragged the “Hot” Cubewano population into the Kuiper Belt, so to speak.
Individual evidence
- ↑ a b K. Tsiganis, R. Gomes, A. Morbidelli, HF Levison: Origin of the orbital architecture of the giant planets of the solar system. In: Nature. 2005, 435, 459-461 ( doi: 10.1038 / nature03539 ).
- ^ W. Kley: On the migration of a system of protoplanets. In: Monthly Notices of the Royal Astronomical Society. 2000, Vol. 313, No. 4, pp. L47-L51 ( doi: 10.1046 / j.1365-8711.2000.03495.x ).
- ^ A b G. Wuchterl: From Clouds to Planet Systems. Formation and Evolutions of Stars and Planets. 2004 ( PDF ).
- ↑ C. Terquem, JCB Papaloizou: migration and the formation of the system of hot super-Earths and Neptune. In: The Astrophysical Journal. 2007, Vol. 654, No. 2, pp. 1110-1120. arxiv : astro-ph / 0609779 .
- ↑ Stefan Deiters: Solar System. Were there once five gas giants? In: Astronews.com. November 16, 2011, accessed November 20, 2011.
- ↑ Laura Hennemann: Solar system. The outcast planet. In: astronomie-heute.de. November 15, 2011, accessed on November 20, 2011. Source there: arxiv : 1109.2949v1 .
- ^ R. Gomes, HF Levison, K. Tsiganis, A. Morbidelli: Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. In: Nature. 2005, 435, 466-469 ( doi: 10.1038 / nature03676 ).
- ↑ A. Morbidelli, HF Levison, K. Tsiganis, R. Gomes: The chaotic capture of Jovian Trojan asteroids during the early dynamical evolution of the Solar System. In: Nature. 2005, 435, 462.