After another few million years, radiation from the young Sun and nearby stars dispersed the disk’s remaining gas, leaving behind only solid objects, which continued to collide, shatter, and merge to form the planets, asteroids, and other bodies we see today. Within a million years after the proto-Sun formed, collisions among planetesimals created larger bodies called planetary embryos, which were roughly as massive as Mars. Astronomers call these bodies planetesimals. Although the process remains poorly understood, solid objects miles or more across eventually populated the disk. Astronomers think small rocky and icy grains within the solar nebula began sticking together, growing into even larger objects. Our solar system began to form about 4.6 billion years ago. This protoplanetary disk may extend more than 100 times Earth’s distance from the Sun. The glowing central mass becomes a newborn star, while farther out in the disk solid particles of rock and ice collide and merge to build up ever-larger objects. As gas falls toward the center, it heats up and rotates faster, flattening into a disk. They’re always in motion, and now and then part of a cloud begins to collapse in response to its own gravity. Stars form inside vast, cold clouds of gas and dust, stellar factories called molecular clouds. This picture has been much refined, but it captures the essential idea – planetary systems are really a side effect of making stars. From this, early astronomers supposed that the Sun formed at the center of a large, flat cloud of gas and dust now called the solar nebula and that the planets formed farther out. All of the planets circle the Sun in the same direction, and their orbits all lie in nearly the same plane. The layout of our solar system provided the first clues for how planetary systems come to be.
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