Longer than humans have watched them, the planets have traced out the same well-worn paths through our solar system. But it wasn't always so. Many astronomers think that early on, Jupiter and the other gas-giants orbited more closely to the sun. Over time, gravitational interactions caused the massive planets to migrate outward. The idea seems plausible, but there hasn't been much direct proof of it.
One of the main pieces of circumstantial evidence is found in the Kuiper belt. This region of space is chock full of comets and other lightweight, icy bodies that now lie beyond Neptune. When the gas giants migrated outward from the sun, they disturbed the Kuiper belt objects and ultimately trapped quite a few of them at the so-called Trojan points in Jupiter's orbit, where the planet's gravity and the sun's gravity equalize.
New observations made by astronomer Franck Marchis at the University of California, Berkeley, and colleagues bear out this hypothesis. The team used the super-sensitive optics of the Keck telescope in Hawaii to observe the tailgaters, known as the Patroclus binary, that follow Jupiter and its Trojan points. By measuring their sizes, distance from each other, and orbital speeds, the researchers calculated their respective mass and density. The objects appear to be made of water ice and a thin layer of dust, closely resembling Kuiper belt residents, the team reports 2 February in Nature. "These bodies were supposed to be asteroids," says Marchis. "But the measurements we have show the density ... is similar to the density of comets."
The findings look solid, says astronomer David Jewitt of the University of Hawaii in Honolulu. But he cautions that the study doesn't necessarily support a particular scenario for the evolution of the solar system. Orbiting at Jupiter's distance--about 5 times farther out than Earth--the Patroclus binary could have formed in place and did not have to have "fallen in from the Kuiper belt to contain ice," he says. "Density is not destiny."