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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.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.Currently, astronomers are studying planets forming in other Solar Systems.