美天文学家发现冥王星两颗新卫星，冥王星卫星数改写为3颗！

Imagine a pinball machine on multi-ball mode—only the balls are the size of planets. The early solar system was such a rough-and-tumble place.

That chaotic environment produced the occasional literally Earth-shattering collision. The moon, for instance, is believed to have formed from debris stirred up by a Mars-size object careening into our young planet.

Now, new research indicates that Pluto may have suffered a similar fate. According to planetary scientist Robin Canup, all three of Pluto’s satellites could trace back to a single impact.

An impact has long been suspected for the birth of Charon, the largest Plutonian moon, but Canup ran impact simulations to show that Nix and Hydra, two much smaller satellites discovered in 2005, may be debris from the same collision. Her study appears in The Astronomical Journal. ["On a Giant Impact Origin of Charon, Nix and Hydra"]

A simulated low-speed crash between comparably sized planetoids yields a Pluto and a Charon, plus an icy debris disk that could coalesce into a Nix and a Hydra. That means that the small satellites should still be pretty icy—a prediction that may be tested as soon as 2015, when NASA’s New Horizons probe reaches the area. If the probe finds that Nix and Hydra are mostly ice, it'll show Canup’s simulation to be a smashing success.

—John Matson