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Solar nebula is a form of accretion to create planets.


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Solar nebula is believed to be a gaseous cloud from which Earth's solar system formed. This solar nebular hypothesis was first proposed in 1755 by Immanuel Kant. Kant argued that solar nebulae slowly rotate, gradually collapsing and flattening due to gravity and eventually form stars and planets. A similar model was proposed in 1796 by Pierre-Simon Laplace.

Overview of the solar nebula hypothesis: The original nebula.

Solar Nebula.
A planetary disk forming in the Solar Orion Nebula.

The hypothesis maintains that the Solar System began as a large (about 100 AU diameter and 2-3 times the current mass of the Sun), roughly spherical cloud of very cold interstellar gas, the solar nebula, part of a larger Molecular cloud. The nebula was 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) pushing into the molecular cloud. The composition of the solar nebula was the same as the composition of the Sun today: 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). 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.

Solar nebula: The protosun.

At the center of the solar nebula's gravity accumulates an increasingly dense protosun. During the process of planet formation in the disk, the protosun 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 the Sun is born. As is typical of protostar or young star (a T Tauri star), the young Sun produces a solar wind much stronger than the present solar wind, which eventually blows the remaining gases out of the disk, and largely ending the accretion process (particularly for the jovians). Like most processes in a star's life, the time spent in the protosun 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.

Solar nebula: planetesimal.

Initially-microscopic 'seeds' of solid material gradually increase in size and become planetesimals (pieces of planets). As they grow larger, in fact, they grow more quickly: larger objects have more surface area for other grains to bump against and stick to, and as planetesimals become significantly 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.

Solar nebula: Non-uniform temperatures.

The temperature in the protoplanetary disk was not uniform, however, 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.

Solar nebula: 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. The moons of the jovian planets are 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 explains why 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 Sun's strong solar wind blows the remaining gas and dust out of the disk and into interstellar space.

The differentiation among the jovians is also understood in this model. The first jovian planetesimal to reach the critical mass required to capture helium gas and subsequently hydrogen gas is most interior one, because orbital speeds are higher, the density in the disk is higher, and collisions happen more frequently. Thus Jupiter is the largest jovian because it swept up hydrogen and helium gas for the longest period of time, and Saturn is next. These composition of these two are dominated by the captured hydrogen and helium gases (about 97% and 90% by mass, respectively).

The Uranus and Neptune planetesimals reach the critical size significantly later, and thus captured less hydrogen and helium, which presently makes up about only about 1/3 of their total mass.

Solar nebula: Heavy Bombardment.

Finally, long after the solar wind cleared the gas out of the disk, the jovian planets (particularly Jupiter and Neptune) gradually swept the disk clean of leftover planetesimals, "Clearing the neighbourhood" either by slinging them in the distant outer reaches of the Oort cloud (as far as 50,000 AU), or continually nudging their orbits into collisions with other planets (or into more stable orbits like the asteroid belt). This period of heavy bombardment lasts several hundred million years, and is evident in the cratering still visible on geologically dead bodies of the solar system. Planetesimals impacting Earth are thought to have brought the Earth its water and other hydrogen compounds. Although not widely accepted, some believe life itself may have been deposited on Earth in this way (known as the panspermia hypothesis).

Even more important, the bombardment and collisions of planetesimals and protoplanets can explain unusual moons, moon orbits, axial tilts, and other discrepancies from the originally very orderly motions. A giant impact of a Mars-sized protoplanet is suspected of being responsible for Earth's unusually large moon, whose composition and density is similar to the Earth's mantle, and could simultaneously have altered Earth's rotation axis to its present 23.5º from its orbital plane.

In the solar nebula model, the only other way terrestrial planets can get moons is by capturing them. Mars' two tiny low-altitude moons are clearly asteroids, and other examples of captured satellites abound in the jovian systems.

Jupiter's regular gravitational interactions (see orbital resonance) are also responsible for preventing the material which once inhabited the asteroid belt from accreting into another probably sizable terrestrial planet. Most of that material has long since been thrown into eccentric orbits and collided with something else; the total mass of the asteroid belt is now less than a tenth of Earth's Moon!

Solar system features explained by theory

The nebular hypothesis effectively explains all the major features of our solar system:

  1. 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).
  2. all major differences between terrestrial and jovian planets (mass, distance from Sun, composition, moon and ring systems).
  3. small bodies (asteroids and comets, both short- and long-period).
  4. exceptions to the trends (terrestrial moons, axial tilts, non-coplanar jovian moons, Triton).

Challenges to the solar nebula hypothesis.

The current challenges for the nebular hypothesis include explaining:

  1. missing mass in Kuiper belt.
  2. capture process for Triton.
  3. sideways rotation of Uranus.
  4. discovered "hot Jupiter" exoplanets.
  5. discovered exoplanets in binary and trinary stellar systems.

Solar nebula: 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.




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