The nebular theory of the formation of the solar system successfully accounts for four of these facts, while one can be considered to be just a coincidence. Which fact is a coincidence?

The nebular theory of the formation of the solar system is the generally accepted hypothesis of the formation of the solar system.

It is explained below.

The coincidental fact is the actual occurence that caused the cloud to collapse, which in turn resulted to the formation of the solar system

Explanation:

Nebular theory proposed that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. It is a generally acceptable hypothesis in the formation of solar systems. Although, the hypothesis have some irregularities.

Nebular theory proposed that the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc.

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies.

Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In another view, the giant planets. Examples of which are: Jupiter, Saturn, Uranus, and Neptune formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt, Kuiper Belt, and Oort Cloud.

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