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The Nebular Hypothesis
Transcript of The Nebular Hypothesis
The Nebular Hypothesis
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.
How Does A Star Form?
The formation of giant planets is a more complicated process. It is thought to occur beyond the so-called snow line, where planetary embryos are mainly made of various ices.
The star formation process naturally results in the appearance of accretion disks around young stellar objects.At the age of about 1 million years, 100% of stars may have such disks. This conclusion is supported by the discovery of the gaseous and dusty disks around protostars and T Tauri stars as well as by theoretical considerations.
The physics of accretion disks encounters some problems. The most important one is how the material, which is accreted by the protostar, loses its angular momentum. One possible explanation suggested by Hannes Alfven was that angular momentum was shed by the solar wind during its T Tauri phase. The momentum is probably transported to the outer parts of the disk, but the precise mechanism of this transport is not well understood. Another possible process for shedding angular momentum is magnetic braking, where the spin of the star is transferred into the surrounding disk via that star's magnetic field. The process or processes responsible for the disappearance of the disks are also poorly known.
Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star.
The protoplanetary disk is an accretion disk which proceeds to feed the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage, formation of small dust grains made of rocks and ices is possible.
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. However, 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.
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 it accelerates and proceeds in a runaway manner.
According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen. They are gravitationally unstable, and matter coalesces to smaller denser clumps within, which then proceed to collapse and form stars.
How does a giant planet form?
Is there evidence?
There is evidence that the nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg. Immanuel Kant, who was familiar with Swedenborg's work, developed the theory further in 1755. He argued that gaseous clouds—nebulae, which slowly rotate, gradually collapse and flatten due to gravity and eventually form stars and planets.
A similar model was proposed in 1796 by Pierre-Simon Laplace. It featured a contracting and cooling protosolar cloud—the protosolar nebula. As the nebula contracted, it flattened and shed rings of material, which later collapsed into the planets.
The fall of the Laplacian model stimulated scientists to find a replacement for it. During the 20th century many theories were proposed including the planetesimal theory of Thomas Chamberlin and Forest Moulton (1901), tidal model of Jeans (1917), accretion model of Otto Schmidt (1944), protoplanet theory of William McCrea (1960) and finally capture theory of Michael Woolfson.
Observations of these disks show that the dust grains inside them grow in size on short (thousand-year) time scales, producing 1 centimeter sized particles.
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 around 100 million years to form.