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Discovery: Largest Solar System Object Since Pluto
By Robert Roy Britt
Senior Science Writer
posted: 12:30 pm ET
07 October 2002


A newfound frozen world beyond the recognized nine planets of Earth's solar system is the largest object discovered since Pluto was spotted in 1930. Its discovery raises the prospects that yet another object as big or perhaps larger than Pluto might lurk out there.

The rock is about half Pluto's size. It orbits the Sun every 288 years, mostly beyond Pluto's orbit.

At 4 billion miles (6.5 billion kilometers) from the Sun, it is the most distant thing in the solar system photographed by an optical telescope. Its discovery will expand knowledge of the faraway sea of icy rocks in which it resides, astronomers said today.

The icy Kuiper belt object 2002 LM60, was dubbed "Quaoar" (pronounced kwa-whar) by its discoverers. With the help of NASA's Hubble Space Telescope, astronomers have determined that it is the largest body found in the solar system since the discovery of Pluto 72 years ago. Quaoar is about 800 miles (1300 km) in diameter and is about half the size of Pluto. Illustration Credit: NASA and G. Bacon (STScI) Science Credit: NASA and M. Brown (Caltech)

Quaoar's size compared with Pluto, Earth's Moon, and the Earth. Illustration Credit: NASA and A. Feild (STScI)

Credit: NASA and M. Brown (Caltech) Left: DSS image (~15' wide) with approximate path of KBO. Top Right: Composite of 16 separate exposures resulting in the trail of the KBO against the sky. Bottom right: Sum of 16 exposures registered on the KBO.

Starry Night Map: Quaoar can only be seen by experienced backyard astronomers with expensive equipment. But you can find its general location in southwest. It is below Han, a moderately bright star, and near HIP 80793, a star near the limit of naked-eye viewing under very dark skies. Pluto is nearby, but seeing it requires a telescope, too.
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The object has a provisional name of 2002 LM60. However, astronomers involved in the discovery at Caltech University in Southern California have dubbed it Quaoar (KWAH-o-ar), after the creation force of the Tongva tribe of the Los Angeles basin. Quaoar, as an official name, will have to await approval from the International Astronomical Union.

Record breaker

Fewer than 400 objects have been identified in a region of our solar system known as the Kuiper Belt, which extends beyond Pluto and at least one-fifth the way to the nearest star. Astronomers expect the belt to be laden with frozen rocks, perhaps billions of them the size of a city and hundreds of thousands as large as Massachusetts. None had been photographed before.

One big Kuiper Belt Object (or KBO), called Varuna, was found two years ago and is about 40 percent as big as Pluto. Varuna was the record-holder for size in the far-out belt until now.

Quaoar's co-discoverer, Caltech postdoctoral researcher Chad Trujillo, told last week he's "quite confident" Quaoar is larger than Varuna.

Quaoar was first detected in images taken June 4 with the Palomar Observatory's 48-inch Oschin Telescope.

Trujillo and Caltech associate professor Mike Brown then used the Hubble Space Telescope's new Advanced Camera for Surveys in July and August to make visual observations, determining Quaoar's diameter to be roughly 780 miles (1,250 kilometers). Superimposed on America, it would blot out several Midwestern states.

A technique involving the detection of thermal radiation sets the diameter higher but is not considered as reliable. Varuna's size was estimated by the thermal technique to be 560 miles (900 kilometers).

Quaoar also appears to be larger than Pluto's moon, Charon, which is 745 miles (1,200 kilometers) in diameter.

Improving picture

Harold Levison, a Southwest Research Institute (SwRI) scientist who was not involved in the discovery, said it was important though not surprising.

"Its importance lies in the fact that we will be able to actually study the object in detail," Levison said. "Other KBOs are just too faint to study. It has been quite frustrating." He added that the Kuiper Belt and its presumed pristine contents "will tell us how the solar system formed."

Another KBO, called Ixion and found last year, has been estimated to be 745 miles wide (1,200 kilometers) -- nearly the same size as Quaoar. But that calculation was based on an estimate for how much sunlight Ixion might reflect, a figure called albedo. Researchers used a 4 percent albedo, once considered the standard for KBOs. The number is based on what scientists know about asteroids and comets they've actually seen.

Quaoar's albedo has been measured, not estimated, and it is 12 percent. Varuna's is 7 percent, and the only other measured KBO albedo is for an object that reflects 10 percent of sunlight.

"The standard assumption of 4 percent albedo, where the Ixion estimate comes from, is much too dark," Trujillo said. "Assuming a mean albedo of 10 percent, Ixion is likely to be 760 kilometers (470 miles) in diameter."

All this means that astronomers are getting a better handle on what's out there and what the objects look like on their surfaces, which will eventually lead to a better understanding of what the rocks are composed of.

"The main practical importance is that these bigger, brighter objects are easier to study," said David Jewitt, a University of Hawaii expert on KBOs. "We can obtain physical data that are impossible on the fainter KBOs."

More Plutos?

Quaoar is roughly a billion miles farther out than Pluto right now. But because Pluto's 248-year orbit around the Sun is not circular, parts of it extend beyond the orbit of Quaoar.

The discovery came in a survey of 1/15 of the sky, and the regions chosen were thought to be likely to harbor such objects, Trujillo explained. Based on this, he speculates that there might be eight objects in our solar system roughly half the size of Pluto.

Levison, of the SwRI and other astronomers have speculated for some time that another full-size Pluto might be out there.

"As time goes on we will find larger and larger objects as we cover more of the sky," Levison said. "How large is difficult to say. I think Pluto-size objects at least. I don't think they will find anything as large as Mars or even the Moon."

Trujillo agrees cautiously, adding that "anywhere from no more Plutos to three more Plutos is in the realm of possibility."

Either way, the discovery strengthens the case made by many astronomers that Pluto should never have been labeled as a planet.

"Quaoar definitely hurts the case for Pluto being a planet," said Brown, Quaoar's co-discoverer. "If Pluto were discovered today, no one would even consider calling it a planet because it's clearly a Kuiper Belt Object."

Not Planet X

Caltech astronomer Charlie Kowal first photographed Quaoar in the early 1980s. Kowal was looking for the so-called Planet X, the alleged 10th planet that some suspect might exist in the outer solar system. Kowal never found Planet X, and he didn't recognize Quaoar for what it is. His photographic plates proved useful, however, to Brown and Trujillo, who did find Quaoar on them and used the information to help determine the object's precise path.

"Its orbit is known better than any other Kuiper Belt Object," Trujillo said.

So what about Planet X?

"We have only looked at about 1/15th of the entire sky so far in our survey," Trujillo said. "If there is a Planet X out there, we will have a good chance at finding it as our survey continues. But I would say that we have ruled out very little so far, simply because the area we have searched is small."

Quaoar's orbit is close to the plane in which most of the other planets orbit, the researchers say. Pluto's odd orbit is elongated and also inclined about 17 degrees to the main solar system plane.

Quaoar spins about an axis, as expected, but the astronomers don't yet have enough data to determine the spin rate. They know, however, that Quaoar's spin axis is tilted about 7.9 percent compared to the plane of its orbit.

According to legend, Quaoar, came down from heaven and, after making order of chaos, laid out the world on the back of seven giants. He then created the lower animals, and then humans.

Right now, Quaoar's namesake is high in the sky. The KBO can be observed as a point of light by experienced backyard astronomers with 16-inch or larger telescopes and electronic CCD cameras, which record dim light over long exposures to render visible images. Quaoar and Pluto are coincidentally in the same region of sky as seen from Earth now.

The discovery was announced at a meeting of the Division of Planetary Sciences of the American Astronomical Society in Birmingham, Alabama.

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