The solar system comprises the Sun, nine major planets, some 100,0000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. While the major planets lie within 40 astronomical units (AU) of the Sun, the outermost boundary of the solar system stretches to 1 million AU, one third the way to the nearest star. It is believed that the solar system was formed through the collapse of a spinning cloud of interstellar gas and dust.
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