The First Binary Centaur

By Ken Croswell

May 28, 2006

Image of the first binary centaur by Keith Noll, Harold Levison, Will Grundy, and Denise Stephens, using the Hubble Space Telescope.

A small world that cuts across the paths of Uranus, Neptune, and Pluto is the first binary centaur ever seen, say astronomers using the Hubble Space Telescope. Because centaurs can become comets, the discovery suggests that some comets may be double objects.

American astronomer Charles Kowal discovered the first centaur in 1977. He named it Chiron, after the mythological son of Saturn and grandson of Uranus. Chiron spends most of its time between the orbits of Saturn and Uranus, but when closest to the Sun it skirts inside the orbit of Saturn. Since Chiron's discovery, astronomers have spotted more than five dozen other centaurs--objects that cross the paths of the Sun's giant planets.

As centaurs orbit the Sun, the giant planets kick them around. These encounters might tear away companions, transforming all centaurs into single objects.

To test this idea, Keith Noll of the Space Telescope Science Institute in Baltimore; Harold Levison of the Southwest Research Institute in Boulder, Colorado; Will Grundy of Lowell Observatory in Flagstaff, Arizona; and Denise Stephens of Johns Hopkins University in Baltimore used the Hubble Space Telescope's High Resolution Camera to examine eight centaurs.

The astronomers discovered that one of the centaurs--designated (42355) 2002 CR₄₆--has a companion half its size. The companion is 1.47 � 0.04 magnitudes fainter than the main body. If both objects reflect 10 percent of the sunlight striking them, then the main body is 112 kilometers, or 70 miles, across, and its satellite is 56 kilometers, or 35 miles, across.

At the time of the observations, in January 2006, the two objects were 0.109 � 0.002 arcseconds apart, which corresponds to a separation of 1,330 � 130 kilometers. (One arcsecond is 1/3600 degree.)

The figure here, from Noll and his colleagues, shows the centaur's highly elliptical orbit around the Sun. The four solid lines show the orbits of the Sun's giant planets--Jupiter, Saturn, Uranus, and Neptune--while the dotted line shows the orbit of the centaur, which voyages from inside the orbit of Uranus to well beyond Pluto. At its nearest, 2002 CR₄₆ is 17.5 AU from the Sun, a bit closer than Uranus (19.2 AU); but at its most remote, the centaur is 58.8 AU from the Sun, far past Neptune (30.1 AU) and Pluto (39.5 AU). (One AU--astronomical unit--is the mean distance from the Sun to the Earth.) The centaur orbits the Sun every 236 years, so its period is just 12 years less than Pluto's.

Centaurs originate from the Edgeworth-Kuiper belt, a band of objects beyond Neptune's orbit that contains more than a thousand known members. The largest are Pluto and the still unnamed tenth planet. If an object in the Edgeworth-Kuiper belt migrates inward, it can become a centaur.

Because centaurs cross the orbits of the giant planets, they have short careers. Gravitational kicks from the giant planets can send them crashing into the Sun or into a planet, place them into the Oort cloud of comets, or eject them from the solar system. As a result, a typical centaur remains a centaur for just a few tens of millions of years.

The discovery of the binary centaur suggests that some comets may also be binaries. That's because, under the gravitational influence of the giant planets, some centaurs migrate closer to the Sun and become comets. In particular, Jupiter can perturb objects into orbits having periods of less than twenty years. Such comets are called Jupiter-family comets.

Astronomers have never seen a binary comet, but they have observed comets break up. Astronomers normally interpret such a breakup as the result of a single cometary nucleus that splits apart. But Noll's team raises the possibility that some comets may break up when the two components of a double nucleus drift apart.

David W. Hughes of the University of Sheffield in England has two opinions of the new work. "One is, I think it's very exciting that they've discovered the first binary centaur," he says. "It is very good news. If you've got a binary centaur, you've automatically got a method of measuring the mass of that object." That's because a satellite moves under the main body's gravitational pull, which depends on how much mass the main body has.

"But what it tells us specifically about comets I would think is extremely negligible," says Hughes. The problem: as Noll's team recognizes, all known centaurs are much larger than Jupiter-family comets. "Knowing how mountains are formed doesn't tell us how molehills are formed," says Hughes, "and it's the same thing here: knowing things about centaurs in essence tells us very little about the Jovian family comet production process, mainly because the things that lead to Jovian family comets are much smaller than anything that we can detect out there in the centaur group and also in the Edgeworth-Kuiper belt."

Noll and his colleagues will publish their discovery of the binary centaur in Icarus.

Meanwhile, the astronomers have used the Hubble Space Telescope to observe four more centaurs. One, designated (65489) 2003 FX₁₂₈, also proved to be double. Thus, out of the twelve centaurs the astronomers observed with the Hubble Space Telescope, two are double, suggesting a substantial binary fraction among centaurs.

Ken Croswell is an astronomer and the author of Magnificent Universe and Ten Worlds: Everything That Orbits the Sun.

"Magnificent Universe by Ken Croswell is elegant and eloquent."--Washington Post. See all reviews of Magnificent Universe here.

"On the basis of its striking design and photographs, this handsome, large-format volume is well worthy of praise. And astronomer Croswell's concise yet conversational, information-packed text wins it sky-high accolades in the narrative sphere as well."--Publishers Weekly, starred review. See all reviews of Ten Worlds here.

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