Nebular hypothesis
The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting the Sun which clumped up together to form the planets. The theory was developed by Immanuel Kant and published in his Universal Natural History and Theory of the Heavens (1755) and then modified in 1796 by Pierre Laplace. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular theory is the solar nebular disk model (SNDM) or solar nebular model.[1] It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the original nebular theory are echoed in modern theories of planetary formation, but most elements have been superseded.
According to the nebular theory, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). These clouds are gravitationally unstable, and matter coalesces within them to smaller denser clumps, which then rotate, collapse, and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk (proplyd) around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star usually takes approximately 1 million years to form, with the protoplanetary disk evolving into a planetary system over the next 10–100 million years.[2]
The protoplanetary disk is an accretion disk that feeds the central star.[3] Initially very hot, the disk later cools in what is known as the T Tauri star stage; here, formation of small dust grains made of rocks and ice is possible. The grains eventually may coagulate into kilometer-sized planetesimals. If the disk is massive enough, the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes approximately 100 million to a billion years.[2]
The formation of giant planets is a more complicated process. It is thought to occur beyond the frost line, where planetary embryos mainly are made of various types of ice. As a result, they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. Some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk.[4] The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses (ME) it accelerates and proceeds in a runaway manner. Jupiter- and Saturn-like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. Ice giants such as Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared.[2]
Formation of planets[edit]
Rocky planets[edit]
According to the solar nebular disk model, rocky planets form in the inner part of the protoplanetary disk, within the frost line, where the temperature is high enough to prevent condensation of water ice and other substances into grains.[64] This results in coagulation of purely rocky grains and later in the formation of rocky planetesimals.[c][64] Such conditions are thought to exist in the inner 3–4 AU part of the disk of a Sun-like star.[2]
After small planetesimals—about 1 km in diameter—have formed by one way or another, runaway accretion begins.[20] It is called runaway because the mass growth rate is proportional to R4~M4/3, where R and M are the radius and mass of the growing body, respectively.[65] The specific (divided by mass) growth accelerates as the mass increases. This leads to the preferential growth of larger bodies at the expense of smaller ones.[20] The runaway accretion lasts between 10,000 and 100,000 years and ends when the largest bodies exceed approximately 1,000 km in diameter.[20] Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals.[20][65] In addition, the influence of larger bodies stops further growth of smaller bodies.[20]
The next stage is called oligarchic accretion.[20] It is characterized by the dominance of several hundred of the largest bodies—oligarchs, which continue to slowly accrete planetesimals.[20] No body other than the oligarchs can grow.[65] At this stage the rate of accretion is proportional to R2, which is derived from the geometrical cross-section of an oligarch.[65] The specific accretion rate is proportional to M−1/3; and it declines with the mass of the body. This allows smaller oligarchs to catch up to larger ones. The oligarchs are kept at the distance of about 10·Hr (Hr=a(1-e)(M/3Ms)1/3 is the Hill radius, where a is the semimajor axis, e is the orbital eccentricity, and Ms is the mass of the central star) from each other by the influence of the remaining planetesimals.[20] Their orbital eccentricities and inclinations remain small. The oligarchs continue to accrete until planetesimals are exhausted in the disk around them.[20] Sometimes nearby oligarchs merge. The final mass of an oligarch depends on the distance from the star and surface density of planetesimals and is called the isolation mass.[65] For the rocky planets it is up to 0.1 ME, or one Mars mass.[2] The final result of the oligarchic stage is the formation of about 100 Moon- to Mars-sized planetary embryos uniformly spaced at about 10·Hr.[21] They are thought to reside inside gaps in the disk and to be separated by rings of remaining planetesimals. This stage is thought to last a few hundred thousand years.[2][20]
The last stage of rocky planet formation is the merger stage.[2] It begins when only a small number of planetesimals remains and embryos become massive enough to perturb each other, which causes their orbits to become chaotic.[21] During this stage embryos expel remaining planetesimals, and collide with each other. The result of this process, which lasts for 10 to 100 million years, is the formation of a limited number of Earth-sized bodies. Simulations show that the number of surviving planets is on average from 2 to 5.[2][21][62][66] In the Solar System they may be represented by Earth and Venus.[21] Formation of both planets required merging of approximately 10–20 embryos, while an equal number of them were thrown out of the Solar System.[62] Some of the embryos, which originated in the asteroid belt, are thought to have brought water to Earth.[64] Mars and Mercury may be regarded as remaining embryos that survived that rivalry.[62] Rocky planets which have managed to coalesce settle eventually into more or less stable orbits, explaining why planetary systems are generally packed to the limit; or, in other words, why they always appear to be at the brink of instability.[21]
Meaning of accretion[edit]
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 protostar is still contracting, gaseous material may still be falling onto it, accreting on its surface from the disk's inner edge.[40] In an accretion disk, there is a net flux of mass from larger radii toward smaller radii.[23]
However, that meaning should not 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 protostar in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high-energy collisions between sizable planetesimals.[20]
In addition, the giant planets probably had accretion disks of their own, in the first meaning of the word.[81] The clouds of captured hydrogen and helium gas contracted, spun up, flattened, and deposited gas onto the surface of each giant protoplanet, while solid bodies within that disk accreted into the giant planet's regular moons.[82]