When musician and amateur astronomer William Herschel first spotted the planet Uranus through his homemade telescope back in 1781, he believed he had discovered a comet. But right from the start, he knew it was no ordinary comet: it had no tail, and everyone knew comets had tails.

Herschel was both wrong and right. He was wrong to believe that no planets existed beyond the original five that the ancient Greeks had known. But he was right to believe that only comets have tails (at least when they're within close distance of the sun) and that planets never do.

That's the way it works in our solar system anyway. But since astronomers began discovering worlds orbiting other stars in the 1990s, it's become clear that our family of planets isn't the only possible kind. The first planets ever found outside of our solar system, called exoplanets, shattered that belief. The new worlds were huge, like Jupiter, but hugged their stars so tightly that they completed a full orbit in just a few days.