Planetary formation
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- See also: Protoplanetary disc
The currently accepted theory of planetary formation is known as the nebular hypothesis, and was first proposed in 1734 by Emanuel Swedenborg[1]. In 1755 Immanuel Kant, who was familiar with Swedenborg's work, developed the theory further. He argued that nebulae slowly rotate, gradually collapsing and flattening due to gravity and eventually forming stars and planets. A similar model was proposed in 1796 by Pierre-Simon Laplace. These can be considered early theories of cosmology.
While originally applied only to our own Solar System, this method of planetary system formation was subsequently believed by theorists to be at work throughout the universe; over 200 extrasolar planets have since been discovered in our galaxy.
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[edit] Overview of the solar nebula hypothesis
[edit] The original nebula
The hypothesis maintains that a planetary system begins as a large (typically ~10,000 AU in diameter), roughly spherical cloud of very cold interstellar gas, part of a larger molecular cloud. Such a nebula is just dense enough to begin contracting under the force of its own gravity, and its collapse may have been initiated by a pressure wave from a nearby event (such as a shock wave from a supernova) compressing the molecular cloud. The composition of such a nebula will reflect the composition of the resulting star; for our own Solar System the Solar Nebula is believed to have been comprised of about 98% (by mass) hydrogen and helium present since the Big Bang, and 2% heavier elements created by earlier generations of stars which died and ejected them back into interstellar space (see nucleosynthesis). The fraction of heavier elements is known as the cloud's metallicity; statistically, stars with larger metallicities (i.e. that formed from a cloud with more heavy elements) are more likely to possess planets. Once begun, the gravitational contraction of the solar nebula accelerates slowly but inevitably.
As it collapses, three physical processes shape the nebula: it heats up, its spin increases, and it flattens. The nebula heats up because atoms move more quickly as they fall deeper into the gravitational well and become denser, colliding more frequently: gravitational potential energy is converted to kinetic energy of the atoms, or thermal energy. Second, while initially imperceptible, the solar nebula had some small amount of net rotation (angular momentum), and because angular momentum is conserved, the nebula must rotate more quickly as it shrinks in size. Finally, the nebula must also flatten into a disk, called a protoplanetary disk, as collisions and mergers of blobs of gas average out their motions in favor of the direction of the net angular momentum.
[edit] The protostar
At the center of the solar nebula's gravity accumulates an increasingly dense protostar. During the process of planet formation in the disk, the protostar gradually compacts further, until after about 10-50 million years, it finally reaches the conditions of temperature and pressure needed to initiate hydrogen nuclear fusion, and a star is born. A young star of this kind (a T Tauri star) produces a stellar wind, much stronger than that of a fully formed star, which eventually blows the remaining gases out of the disk, and largely ending the accretion process (particularly for any gas giants). Like most processes in a star's life, the time spent in the protostar phase depends on mass: massive stars collapse more quickly.
The gas in the protoplanetary disk, meanwhile, gradually cools from the gravitational heating of its collapse, and as it cools, dust (metals and silicates) and ice (hydrogen compounds such as water, methane, and ammonia) grains condense out of the gas (solidify). These grains gently bump into neighboring grains (collide) and stick together electrostatically, beginning the accretion process. Gas atoms and molecules are present in great abundance, but cannot be accreted, because they are moving too quickly to be held electrostatically. Hydrogen and helium, 98% of the mass of the disk, remain gaseous throughout the solar nebula, never condensing.
[edit] Planetesimals
Initially-microscopic 'seeds' of solid material gradually increase in size and become planetesimals (pieces of planets). Initially such dust is spread throughout the disk, but it is expected to rain out into the disk midplane. Dust grains of different sizes fall down at different speeds, gathering more dust along the way. Larger grains may grow faster by clumping together randomly to produce fractal structures; such arrangements have more surface area for other grains to bump against and stick to. Once planetesimals become sufficiently massive, their gravity helps bring more grains into contact.
Planetesimals have a harder time growing above a few hundred kilometers in size, however. With significant mass, planetesimals now have gravitational interactions with each other, modifying their orbits from circular to more eccentric ones, particularly so for the lower mass planetesimals. With crossing orbits, planetesimals now sometimes collide violently, often shattering into smaller pieces again. Asteroids are understood to be left-over planetesimals, now gradually grinding each other down into smaller and smaller bits. Meteorites are therefore samples of planetesimals and give us a great deal of information about the formation of our solar system. Primitive-type meteorites are chunks of shattered low-mass planetesimals, where no gravitational differentiation took place, while processed-type meteorites are chunks from shattered massive planetesimals. Only the largest of planetesimals survive these high-energy collisions with lower mass planetesimals, and can continue to grow.
[edit] Non-uniform temperatures
The temperature in a protoplanetary disk is not uniform, and this is the key to understanding the differentiation between terrestrial and jovian planet formation. Inside the frost line, the temperature is too high (above 150 K) for hydrogen compounds to condense: they remain gaseous. The only grains available for accretion, then, are the heavier metal and silicate dust grains. Thus the planetesimals in this region are composed entirely of rock and metal, such as the asteroids, and make up the terrestrial planets.
Outside the frost line, hydrogen compounds such as water, methane and ammonia are able to solidify into 'ice' grains, and accrete. Rock and metal grains are also available, but are vastly outnumbered (and outweighed) by the hydrogen compounds, which are much more abundant everywhere. Thus the planetesimals in this region are icy bodies with small amounts of rock and metal mixed in. The Kuiper Belt and Oort Cloud objects, comets, Neptune's huge moon Triton, and probably Pluto and its moon Charon, are all examples of these 'dirty snowball' planetesimals. Due to the greater amount of solid materials available, as well as less frequent collisions and lower velocities (being in much larger orbits), the largest of these planetesimals grow so massive (about 10 times the mass of the Earth) their gravity begins to collect and retain helium and even hydrogen gases. Once that starts, they grow rapidly, as hydrogen and helium are 98% of the disk, and collecting these gases increases their mass and consequently the size of their gravitational net.
[edit] Jovian planetesimals
Soon the jovian planetesimals are nothing like the icy bodies they came from, but are more or less dominated by the hydrogen and helium gas they have captured, huge gaseous clouds with dense cores. These jovian gas balls then, in close analogy to the solar system itself, gradually collapse gravitationally, heating up, rotating more quickly, and flattening. Some moons of the jovian planets may be formed in an analogous process to the planets themselves, coalescing from condensed grains in the disks which formed as the gas giant protoplanet collapsed. This can explain why, in our own Solar System, the jovian planets all have many moons and rings in the same plane, and why jovian planets rotate quickly. The growth of the jovians ends when the young star's strong stellar wind blows the remaining gas and dust out of the disk and into interstellar space.
In the simplest possible terms, the innermost giant protoplanetary core forms where the disk density is highest and dynamical times (the typical timescale for collisions) are shortest; hence this body reaches the critical mass for gas capture earliest, and in the densest regions of the disk, and so has longest to accrete the surrounding gas. In our own Solar System Jupiter was the largest protoplanetary core beyond the frost line, and so fulfilled this role, becoming the largest planet in that system. In reality, the process may be more complicated, with planetary migration and turbulence muddying this picture; compared to the extrasolar planetary systems observed to date, the distribution of the planets in our own system may even be considered somewhat unusual.
[edit] Giant Impacts
Finally, after the stellar wind has cleared the gas out of the disk, a large population of protoplanets and planetesimals may be left over. Over a period of 10-100 million years, these protoplanets - typically with a mass between that of the Moon and several Earths - will perturb each other until orbit-crossing occurs, leading to collisions. The bodies resulting from these collisions will be the final planets of that system. Such a collision, between the proto-Earth and a Mars-sized protoplanet, is believed to have formed the Earth's moon. The process is highly random; a forming terrestrial system near-identical to that which produced the inner planets may easily end with fewer or more planets than we observe in our Solar System.
The smaller planetesimals, being vastly more numerous, remain within the planetary system for much longer. These bodies may be swept up by the planets that have formed in a process known as "clearing the neighbourhood", either by slinging them in the distant outer reaches of the system (the Oort Cloud in our Solar System), or continually nudging their orbits into collisions or stable orbits with other planets. This period of bombardment lasts several hundred million years, and may leave evidence of cratering which is still visible on geologically dead bodies. In some respects, as long as there are small rocky or icy bodies available to the system, it may be argued that this stage of formation never really 'finishes', as the threat of asteroid collisions with Earth or the recent impact of comet Shoemaker-Levy 9 upon Jupiter ably demonstrates.
In our own Solar System it is believed this episode of formation was exceptionally strong due to a 2:1 resonance crossing of Jupiter and Saturn, catastrophically disturbing a large outer planetesimal disk, and the process is known as the Late Heavy Bombardment.
[edit] Solar system features explained by theory
The nebular hypothesis effectively explains all the major features of our solar system:
- regular motions of the planets and moons (all revolve in the nearly same plane, in nearly circular orbits, in same direction the Sun rotates, and nearly all rotate in the nearly same direction too)
- all major differences between terrestrial and jovian planets (mass, distance from Sun, composition, moon and ring systems)
- small bodies (asteroids and comets, both short- and long-period)
- exceptions to the trends (terrestrial moons, axial tilts, non-coplanar jovian moons, Triton)
[edit] Challenges to the hypothesis
The current challenges for the nebular hypothesis include explaining:
- missing mass in Kuiper Belt
- capture process for Triton
- sideways rotation of Uranus
- discovered "hot Jupiter" exoplanets
- discovered exoplanets in binary and trinary stellar systems
[edit] The meaning of accretion
Use of the term accretion disk for the protoplanetary disk leads to confusion over the planetary accretion process.
The protoplanetary disk is sometimes referred to as an accretion disk, because while the young T Tauri-like protosun is still contracting, gaseous material may still be falling onto, accreting on, its surface from the disk's inner edge.
However, that meaning should not to be confused with the process of accretion forming the planets. In this context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the protosun in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high energy collisions between sizable planetesimals.
In addition, the jovians probably had accretion disks of their own, in the first meaning of the word. The clouds of captured hydrogen and helium gas contract, spin up, flatten, and deposit gas onto the surface of each jovian protoplanet, while solid grains within that disk accrete into planetesimals and eventually forming the jovian moons.
[edit] See also
- Formation and evolution of the Solar System
- History of Earth
- Asteroid Belt, Kuiper Belt, and Oort Cloud
- Bok globule, Herbig-Haro object
- T Tauri star
[edit] References
- ^ Swedenborg, Emanuel. 1734, (Principia) Latin: Opera Philosophica et Mineralia (English: Philosophical and Mineralogical Works), (Principia, Volume I)
[edit] External links
- SPACE.com: Discovery Hints at a Quadrillion Space Rocks Beyond Neptune (Sara Goudarzi) 15 August 2006 06:13 am ET
