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What is a planet? It seems such a simple question, but the answer keeps getting more and more confused. On the one hand, the line between planets and lesser bodies is notoriously hazy. Just last year astronomers identified a body larger than Pluto in the outermost solar system, rekindling the old debate over whether Pluto really qualifies as a planet and, if it does, why large asteroids do not. Even newspapers and museums have jumped into the fray. Less well known, though, is the muddle at the upper end of the planetary scale: the blurring of the divide between planets and stars.
The distinction used to be straightforward: a star shines by its own light, whereas a planet merely reflects the light of the star it orbits. In the more rigorous parlance of physics, stars are massive enough to undergo stable hydrogen fusion in their interiors over a sustained period, making them self-luminous. They form out of collapsing clouds of interstellar gas. Planets, on the other hand, are too puny and cold to initiate fusion. They are thought to congeal out of the debris floating around newborn stars; in short, they are leftover scraps of the star formation process....
