Image: Patroclus and Menoetius
An artist's conception shows Patroclus and Menoetius in the foreground, with Jupiter and its moons barely visible in the background. The two bodies had been considered rocky asteroids, but scientists now say they appear to be captured comets.
updated 2/1/2006 4:34:45 PM ET 2006-02-01T21:34:45

Two objects lurking near Jupiter and once considered rocky asteroids have turned out to be comets made up mostly of ice and dirt.

Using the Keck II Laser Telescope in Hawaii, astronomers found that the two objects, 617 Patroclus and its companion, Menoetius, had a density of only 0.8 grams per cubic centimeters — only a third that of rock.

Most likely, the researchers say, Patroclus and Menoetius are comets, which are typically composed mainly of water ice and therefore much less dense than asteroids.

The findings could mean that many or most of asteroidlike objects hovering around Jupiter and known as Trojans are actually comets that originated much farther from the sun and were captured by the giant gas planet when the solar system was still young.

The findings are detailed in Thursday's issue of the journal Nature.

Patroclus and Menoetius are the only known binary objects around Jupiter. The two bodies orbit around each other while floating 465 million miles (750 million kilometers) from Jupiter in one of gas planet’s two so-called Lagrange points. At these points, the gravitational field of Jupiter and the sun are perfectly balanced, and objects can be captured and brought to relative rest. Jupiter has two Lagrange points, one in front and the other behind as the planet orbits the sun.

Patroclus and Menoetius are estimated to be about 76 miles (122 kilometers) and 70 miles (112 kilometers) wide, respectively. The two objects are not the first to be mistaken for asteroids: in 1999, astronomers determined that C/199 J3 was also a comet.

Because most comets are thought to form in the Kuiper Belt, a distant region of the solar system outside the orbit of Neptune, the researchers think Patroclus and Menoetius formed about 650 million years after the formation of the solar system.

“It’s our suspicion that the Trojans are small Kuiper Belt Objects,” said study leader Franck Marchis, an astronomer at the University of California at Berkeley.

According to one hypothesis proposed by the researchers, Jupiter captured the comets at a time when the large gas planets were orbiting much closer to the sun.

During this early period in the solar system, the gas planets were enveloped by billions of large asteroids called planetesimals. It’s thought that interactions with planetesimals caused the large gas planets to migrate outwards to their present positions. As the planets migrated, the swarming planetesimals were tossed around like confetti.

The majority of them would have been hurled into the outer reaches of the solar system to form the Kuiper Belt, while a smaller number would have been captured in the Lagrange points of Jupiter and the other gas planets.

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