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Listen to part of a lecture in an astronomy class. Professor: Today we're going to talk about the formation of the solar system. For centuries, scientists have debated how our solar system came to be. The most widely accepted theory is the nebular hypothesis. According to this idea, about 4.6 billion years ago, a vast cloud of gas and dust, called a solar nebula, collapsed under its own gravity. As it collapsed, it began to spin faster and flatten into a disk. Most of the material was pulled toward the center, forming the Sun. The remaining material in the disk clumped together to form planets. Now, one important piece of evidence for this theory is the fact that all planets orbit the Sun in the same direction and roughly in the same plane. This is consistent with the spinning disk model. However, there are some exceptions, like Uranus, which rotates on its side, and Venus, which rotates backward. These exceptions may be due to collisions with large bodies early in their history. So while the nebular hypothesis explains the general pattern, it doesn't account for every quirk.