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Before 1995, most people believed that the only planets in the Universe were found in our Solar System. Since 1995, hundreds of “exoplanets,” or planets outside of our Solar System, have been discovered orbiting other stars. The Earth and our Solar System are not as unique as they were once thought to be. Planet formation is now considered to be very common in the Universe, and planets can form in the wake of the formation of any star. Star formation begins in giant gas clouds, and probably 99.9 percent of the material in these clouds goes into making the star. Only about 0.1 percent of the material in the original gas cloud is left for planet formation. This leftover material orbits the star and various forces cause the materials to begin crashing into one another. Over time, this process leads to the formation of very large objects—what we know as planets. Sometimes rocky like our Earth, sometimes gassy like Jupiter, these planets gather mass as other floating debris crashes onto their surfaces. In the early days of our Solar System, the Earth was constantly bombarded with floating debris. Over time, things settled down and the Earth cooled, making it the perfect place for life to form.

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