Newton's concept of a clockwork universe isn't what it used to be. Uranus is tilted 90 degrees on its side, Pluto follows a highly elliptical orbit, and new Earth-crossing asteroids appear out of nowhere each month. Then there's Neptune's moon Triton, a body 40 percent larger than Pluto that travels in a retrograde orbit--in the opposite direction of all the planets and major satellites in the solar system.
For decades, astronomers have been trying to figure out what could have caused Triton's errant course. The most popular hypothesis had been that it was pushed out of whack by a collision early in its history. The problem is that the numbers don't add up. Anything large enough to affect the orbit of Triton by that much also would have pulverized it.
Now, a computer simulation suggests Triton may have started out moving in a conventional direction but with a binary companion. Planetary scientists Craig Agnor of the University of California, Santa Cruz, and Douglas Hamilton of University of Maryland, College Park, report in today's issue of Nature that the pair may have wandered too close to Neptune, whose powerful gravity grabbed Triton when it was moving in a retrograde direction around its companion. This slowed Triton enough, relative to Neptune’s gravity, to unlock it from its partner and yank it into orbit around the planet. Without its companion, chances are that Triton would have collided with Neptune or been flung away.
The fate and whereabouts of Triton's former partner remain uncertain. "We really don't know what happened," Agnor says, "but it's probably gone. Objects that cross with the outer planets tend to get flung out of the solar system."
Other planets may have nabbed moons in much the same way, Agnor says. Astronomers have also discovered more and more binary planetoids, both in the asteroid and Kuiper belts. "Similar objects have probably been around for billions of years, and their prevalence indicates that the binary-planet encounter that we propose for Triton's capture is not particularly restrictive," Hamilton says.
The computer model is very innovative because it shows how binaries probably helped grow the lunar population of the solar system, says Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. "If you only had a few of these systems rattling around the outer solar system, the chances of a Triton encounter would be minuscule," he says. "This basically clinches the need for hundreds to thousands of Pluto-like dwarf planets, which is extremely exciting."