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How Many Planets Are in the Solar System?

The most important question has to be: What makes a planet, anyway?

PlanetMaker

PLANET MAKER. In this artwork showing the early solar system, an enormous dust disk stretches around the Sun and serves as a breeding ground for new planets as material clumps together.

NASA/JPL-Caltech

You might think astronomers know the solar system, the region of space immediately closest to home, pretty well. And they do. But they might not know its whole story.

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