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 finding could mean that many or most of asteroid-like
objects hovering around Jupiter and known as Trojans are actually comets that
originated much farther from the Sun and which were captured by the giant gas
planet when the solar system was still young.
The findings were detailed in the Feb. 2 issue of the
journal Nature.
Patroclus and Menoetius are the only known binary objects
around Jupiter. The pair orbit around each other while floating 465 million
miles (750 million kilometers) from Jupiter in one of gas planet's two stable
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, 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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