New Planet-Shaped Body Found in Our Solar System

John Roach
for National Geographic News
October 7, 2002
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Astronomers announced today the discovery of the largest object in the solar system since Pluto was named the ninth planet in 1930. The object is half the size of Pluto, composed primarily of rock and ice, and circles the sun once every 288 years.

Named Quaoar (pronounced KWAH-o-ar), the object resides in the Kuiper belt, a region of the sky beyond the orbit of Pluto and about 4 billion miles (6.5 billion kilometers) from Earth. The Kuiper belt is chock full of remnants from the planet-formation era of the solar system.

Scientists study the Kuiper belt to understand what the solar system was like when it formed. They have theorized for several years that objects the size of Quaoar and bigger exist in the region, but until now had not detected anything quite so large.

"My first reaction was 'wow, that is a bright object and looks like it could be really big,'" said Chad Trujillo, a researcher at the California Institute of Technology in Pasadena.

"My second reaction was I wonder why no one has seen this before. It is almost bright enough that a dedicated amateur could find it at a dark site with a 16-inch [41-centimeter] telescope."

Trujillo is working with colleague Mike Brown to survey the Kuiper belt with the 48-inch (122-centimeter) Oschin Telescope at the institute's Palomar Observatory. The telescope is equipped with a charge-coupled device (CCD—the same technology used in digital cameras), which is sensitive to faint objects.

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

"Quaoar is an awesome object that fits well with the existing picture of the Kuiper belt," said Dave Jewitt, a professor at the University of Hawaii's Institute for Astronomy in Honolulu and an expert in the study of Kuiper belt objects.

Sizing Up Quaoar

Since Quaoar was discovered on June 4, the researchers have measured it with a heat-sensitive telescope at the International Institute for Research in Millimeter Astronomy (IRAM) in France and the Hubble Space Telescope.

The 98-foot (30-meter) IRAM telescope measures the amount of heat emitted by an object. Larger objects emit more heat. This measurement is combined with an optical measurement to determine size.

"We found the object was about 1,200 kilometers [745 miles] in diameter," said Trujillo. "Just to be sure, and to see if it has any satellites around it, we imaged it with the Hubble Space Telescope and found it to be 1,250 kilometers [777 miles] in diameter."

No satellites were found.

Further studies of Quaoar with the Keck Telescope in Hawaii indicate the body has a strong water-ice feature. Trujillo suspects that it is also full of rock.

"As for an atmosphere, it's unlikely to have much of one," he said. "Even Pluto, which is eight times more massive—twice as large in diameter—only has a tenuous atmosphere, about a million times less pressure than the Earth's."

Trujillo and Brown also looked through archived images taken by a variety of instruments and found that Quaoar was previously imaged, but not detected, in the years 1982, 1996, 2000, and 2001.

These images allowed the researchers to determine that Quaoar's orbit is quite circular in contrast to Pluto's elliptical orbit, and that the plane of Quaoar's orbit is tilted by 7.9 degrees from the relatively flat orbital plane in which all the planets except Pluto are found.

Planet Material

Quaoar and other Kuiper belt objects are believed to be leftover remnants from when icy fragments of matter coalesced to form the outer planets billions of years ago.

"For some reason we don't know, the process was arrested mid-stride," said Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. "We find these things have been stranded in this stage for four billion years."

Astronomers have been racing to study Kuiper belt objects since the first was discovered in 1992 in order to gain insight to the process of planet formation. An estimated 50,000 objects greater than 62 miles (100 kilometers) in diameter are thought to exist in the belt. The existence of the Kuiper belt had been theorized for decades before its first objects were identified ten years ago.

"I am not at all surprised that 1,000- to 1,500-kilometer [621- to 932-mile] objects are now being found in the Kuiper Belt and have long thought that images could be found on existing sky survey photographs," said Brian Marsden of the Harvard-Smithsonian Center for Astrophysics.

Varuna, a Kuiper belt object discovered in 2000, is 1,000 kilometers in diameter and Ixion, which was discovered in 2001, is thought to be of similar size to Quaoar and Varuna, but its diameter has yet to be accurately measured.

The discovery of Quaoar also adds support to the argument Pluto itself is a Kuiper belt object rather than a planet, according to the researchers.

Pluto is believed to be composed of rock and ice like other Kuiper belt objects, which is quite different to Pluto's neighboring giant gas planets Neptune and Uranus. Pluto also orbits the sun outside the orbital plane of all the other planets, like Kuiper belt objects.

These similarities between Pluto and other Kuiper belt objects have led Jewitt, Marsden and others to argue that Pluto is a Kuiper belt object, casting Pluto's designation as a planet into doubt.

"Pluto is the largest known Kuiper belt object," said Jewitt. "Some people think of it as a planet as well. That's fine, of course, but the reasons for doing so are historical, or sociological at best."

Scientifically, argues Marsden, it does not make sense to maintain that there are nine planets, with Pluto one of them and some other objects not. "It does not bother me that objects can have dual status as a belt member and a small planet, but if dual status is not allowed, belt membership is the more important characteristic," he said.

Stern, who is also the principal investigator of NASA's New Horizons Mission, which will send a spacecraft to study Pluto and other Kuiper belt objects, suggests that the solar system is full of "planets," including Pluto and hundreds of other Kuiper belt objects.

"A reasonable estimate is that there are about 900 planets. All but eight of them are out there [in the Kuiper belt]," he said.

The New Horizons Mission launches in January of 2006 to conduct the first detailed studies on the complex geology of Pluto, the ninth and only named planet not visited by a spacecraft. It will then proceed to study a range of other Kuiper belt objects.

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