On the human timescale, a star system seems eternal. It can last anywhere from a few million years to a few trillion. Even so, it can change in a hurry — especially when it’s young. A couple of years ago, for example, astronomers were using Spitzer Space Telescope to keep an eye on a system in the constellation Vela. The system is less than one percent the age of the Sun. It’s still surrounded by the building blocks of planets — chunks of rock and ice, plus clouds of dust. Astronomers hoped that watching the system would provide clues about how these materials come together to make planets. Spitzer had to avoid the star for a few months because it was lost in the glare of the Sun. When the telescope turned toward the system again, it detected a vast cloud of dust that hadn’t been there before. It probably was the aftermath of a collision between two asteroids, which pulverized both of them. Yet that act of destruction is also part of the process of making rocky planets like Earth. Smal
