Because this belt has more asteroids than ours, collisions are larger and more frequent, which is why Spitzer could detect the belt. Our present-day solar system is a quieter place, with impacts of the scale that killed the dinosaurs occurring only every 100 million years or so.

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We thought young stars, about 1 million years old, would have larger, brighter discs, and older stars from 10 to 100 million years old would have fainter ones. But we found some young stars missing discs and some old stars with massive discs.

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We had no idea that Spitzer would ever see light echoes. Sometimes you just trip over the biggest discoveries.

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