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At least since the earliest recorded writings, people – scientists, astronomers, philosophers, and almost everyone else – have been searching for the answer to how the universe was formed. There is no authoritative model for how the universe was formed, but scientists have agreed on a most popular model.
The most popular theory is called the nebular hypothesis. It did fall out of favor for a number of years, but now it is back as the definitive model. According to the nebular hypothesis, our Solar System began around 4.6 billion years ago when part of a molecular cloud of interstellar gas, which was filled with particles of ice, dust, rock, and other particles, collapsed. These clouds collapsed from some kind of turbulence that caused it to heat up and eventually turn into a star.
Most of the cloud formed the Sun. Other material from the cloud flattened around the Sun forming a planetary disc. The material from the planetary disc, also known as the solar nebula, went to form the planets and other objects in our Solar System. Some of the material forms a solid object, which gets larger as other particles collide with it and stick together. Eventually, the object gets large enough to attract more dust and ice with its gravitational influence. These balls then form the cores of the planets. If one of these balls leaves the Sun early enough, it turns into a comet. Because of radiation and collision with other objects, these balls of ice, rock, and dust turned into very different planets. Astronomers believe that it took millions of years for the planets to form. Asteroids and other planetesimals are “failed planets” – objects formed from the solar nebula that never got large enough to turn into planets. Currently, astronomers are studying planets forming in other Solar Systems.
Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to explain away. One of these problems is the planets’ axial tilts. According to the nebular theory, they are supposed to have the same ecliptic planet, but the inner planets and outer planets have radically different axial tilts. With technological advancements allowing astronomers to study extrasolar planets, scientists have noticed irregularities that cast doubt on the nebular hypothesis. Some of these irregularities are hot Jupiters that orbit their stars in just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but so far it has not answered all the questions.
Astronomy Cast has an episode on where baby stars come from.