How do planets form?
It’s staggering to imagine a time when the Earth and its planetary siblings were nothing but cosmic dust. Yet astronomers agree that this was the state of things some 4.5 billion years ago. Our sun was but a fledgling protostar, continually amassing more matter via gravity and steadily cranking up its internal nuclear fusion. There was no solar system, only a giant, rotating cloud of particles called the solar nebula. To figure out how all that leftover gas and dust led to planets, astronomers have largely studied the structure of our own solar system for clues. They’ve also looked to distant, younger solar systems still in varying stages of development. With the formation of the sun, the remaining gas and dust flattened into a rotating protoplanetary disk. Within this swirling debris, rocky particles began to collide, forming larger masses that soon attracted even more particles via gravity. These particles contracted unde
