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The Nebula Hypothesis
Two theories are responsible for explaining how the Solar System formed. One is the nebula theory and the other is the protoplanet theory.
However, the nebula theory began with the German philosopher Immanuel Kant, who proposed in 1755 that the formation of the Solar System was the result of a nebula, which is a large dust cloud that collapsed to form into rings of matter. When the dust clouds grew sufficiently massive, stars formed and so did planets. This theory was expanded by Pierre Laplace in 1796 who said that the planets were formed by rings of matter that split away from a rotating nebulae by centrifugal forces. Soon after, the matter, through gravitational forces formed into a planet. This process repeated itself again and again, each time resulting in a planet. The leftover matter was the Sun.
There were some difficulties with this theory. For starters, if the rings of matter split away, wouldn't they just disperse into space and not form planets? The Sun's mass is the overwhelming amount in the Solar System, but it's angular momentum was not the largest. And, the planets did exist, their matter was not dispersed into space.
The protoplanet theory developed when the nebula theory could not explain these observations of the Solar System.
You can read more about Reflection Nebulae: Ghosts of the Galaxy
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The Protoplanet Hypothesis
For starters, not everything about the nebula hypothesis was incorrect. A modified version of the nebular hypothesis was developed independently by two astronomers Carl von Weizsacker and Gerard Kuiper. First the Solar System begins to form, as a rotating cloud, or nebula collapses. But instabilities develop in the nebula causing dust particles to pull together. Then the dust particles merge into billions of planetesimals about 10 meters in diameter. The planetesimals then collide and form protoplanets. Meanwhile, at the center of the nebular disk the protosun increases in mass and becomes a star by the process of hydrogen fusion. The Sun begins to radiate energy in the inner part of the Solar System. The remaining gas is blown away by solar winds.
The protoplanet explanation answered many question, however, the angular momentum distribution problem was not answered. Remember that the distribution of angular momentum in the Solar System expanded to the entire system, not just the Sun. So to explain the transfer of angular momentum from the Sun to the planets, scientists looked at the magnetic forces of the Sun and proposed that a braking action was taking place. The magnetic lines of force from the Sun transferred angular momentum from the spinning Sun to the planetary disk.
You can read about Planetoids at Facts, Definition and Information about Planetoids
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Examples of Protoplanets
Like any scientific theory, it has to be proven. In order to prove the Protoplanet Hypothesis of Solar System formation, astronomers needed to find protoplanets. Have any been spotted? The answer is yes. Three so far in the asteroid belt, and one located outside our Solar System, in the constellation Taurus.
Pallas, is considered to be a protoplanet. It is one of the largest asteroids in the asteroid belt, and orbiting the Sun at a distance between 2.1 and 3.4 Astronomical Units. It also has about 7 percent of the total mass of the entire belt; so for an asteroid/protoplanet it is large. It has diameter between 530–565 km (329 - 351 miles) It was discovered by the German astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802
Vesta, is an asteroid/protoplanet. It has a diameter of about 530 km. It comprises about 9% of the mass of the entire asteroid belt, it is the second most massive object in the belt). Vesta was the second asteroid discovered in by Olbers in March 1807.
Ceres has a diameter of about 950 km (590 mi), Ceres is by far the largest and most massive body in the asteroid belt, and contains almost a third (32%) of the belt's total mass. It was discovered in 1801 by Giuseppe Piazzi.
HL Tau B is thought to be a protoplanet and it is not in our Solar System. A team of astronomers located it in the constellation Taurus. A planet is forming in the dust and debris surrounding the star HL Tau. They named the protoplanet, named HL Tau B, and it may be the youngest one discovered so far. The protoplanet was discovered in 2008 by Dr. Jane Greaves of the University of St Andrews and her team and with the support of the British Royal Astronomical Society.
In the computer simulated image to the right, the bright circular spot on the upper right is the protoplanet.
To read about current protoplanet explorations see The Keck Telescopes--Progenitors of the 21st Century Giants
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The Comparative Sizes of a Protoplanet
Here are some comparative sizes of asteroids/protoplanets, to the Moon.
Sizes of the first ten Asteroids / Protplanets to be discovered compared to the Earth's Moon, all to scale. From left to right they are:1 Ceres (P), 2 Pallas(P), 3 Juno, 4 Vesta(P), 5 Astraea, 6 Hebe, 7 Iris, 8 Flora, 9 Metis and 10 Hygiea.
You can read more on asteroids at Asteroids: What are They Made Of?
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The Formation of the Solar System - Nebula Hypotheis
Image, Published under Wikimedia Commons Public domain, http://en.wikipedia.org/wiki/File:R136_HST_2009-12-15.jpg
The Formation of the Solar System - ProtoPlanet Hypotheis
Examples of Protoplanets
Image, Published by NASA, Under the Public Domain, http://en.wikipedia.org/wiki/File:Iau_dozen.jpg
Royal Astronomical Society (2008, April 2). Youngest Planet Ever Discovered Offers Unique View Of Planet Formation. ScienceDaily. Image by Greaves, Richards, Rice & Muxlow 2008, Published by http://www.sciencedaily.com/releases/2008/04/080402153613.htm
Comparative Sizes of a Protoplanet
Image, Published under Wikimedia Commons, http://en.wikipedia.org/wiki/File:Moon_and_Asteroids_1_to_10.svg