New Planetoid Found in Solar System—Most Distant Yet

John Roach
for National Geographic News
March 15, 2004
Astronomers announced Monday the discovery of the most distant object ever found orbiting the sun: a shiny, red body of rock and ice about three-quarters the size of Pluto. The plantoid is so far out that it takes 10,000 years to circle the sun.

"There is absolutely nothing else like it known in the solar system," said Mike Brown. Brown is a planetary astronomer at the California Institute of Technology in Pasadena and leader of the research team.

Brown and his colleagues discovered the body on November 14, 2003, using the Samuel Oschin Telescope at Caltech's Palomar Observatory near San Diego. Within days of the planetoid's discovery, telescopes in Chile, Spain, Arizona, and Hawaii observed the object.

The object is officially known as 2003VB12, so designated for the date of its discovery. Brown and his colleagues are proposing to name it Sedna, in honor of the Inuit goddess who is said to have created all creatures of the sea.

"Sedna lives at the bottom of the Arctic Ocean, a very cold location, and we thought it appropriate for an object that's this cold and this far away to be named after an Inuit creation goddess," said Brown.

The researchers say temperatures on Sedna never warm above minus 400 degrees Fahrenheit (minus 240 degrees Celsius), making it the coldest known body in the solar system.

Sedna is usually much colder. Its highly elliptical orbit takes it as far as about 84 billion miles (130 billion kilometers) from the sun, which is 900 times Earth's distance from the sun. Currently Sedna is ten times closer to the sun than it would be at its farthest orbit. The body's orbit takes 10,500 years to complete.

The body is believed to be composed mostly of rock and ice and has an unusual shiny red color to it. Astronomers have also collected indirect evidence that it has a moon. Future observations with the Hubble Space Telescope should provide a definitive answer.

Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, said Sedna's discovery is "wonderful, not surprising but wonderful." Like Brown, Stern believes hundreds, maybe thousands, of objects like Sedna will eventually be discovered as astronomers probe the outer reaches of the solar system.

Oort Cloud

Brown and his colleagues say that Sedna is unlike anything else ever discovered. It lies in a region of the solar system that is beyond the Kuiper Belt, a group of icy remnants orbitting beyond Neptune and dating back to the planet-formation era of the solar system.

In recent years astronomers have discovered several big objects orbiting in the Kuiper Belt, including Quaoar in 2002, Ixion in 2001, and Varuna in 2000. Many astronomers also believe Pluto, the ninth planet, is a Kuiper Belt object.

Sedna, the researchers say, lies beyond the Kuiper Belt in a region of space previously thought empty.

"One of the things we've learned about the Kuiper Belt in the last decade is that it is has a very sharp edge to it, just beyond the outer extent of Pluto, and this object is well beyond that edge, so it makes much more sense to say it's something other than a Kuiper Belt object," Brown said.

According to Brown, Sedna resembles objects predicted to lie in the hypothetical Oort Cloud, which is believed to be a zone of early comets that extends around the sun halfway to the nearest star.

But Sedna is much closer to the sun than the predicted distance of the Oort Cloud. Brown said Sedna may have been formed by gravity from a rogue star near the sun early in the history of the solar system.

Not a Planet

Brown and his colleagues are not calling Sedna a planet. Like many other researchers, he believes that Pluto should never have been considered a planet either. Part of the confusion is that astronomers have never properly defined what a planet is.

"In my opinion, to be considered a planet you need to be considerably more massive than any other object in a similar location," Brown said.

By this definition, Brown does not consider Pluto to be a planet, because it sits well inside a group of other objects in the Kuiper Belt that are nearly as massive as Pluto. And while Sedna is currently all by itself, Brown expects to find many more similar objects in the region over the next decade. He prefers to call Sedna an Oort Cloud object.

According to Stern, the astronomer with the Southwest Research Institute, the broadest definition of a planet is anything massive enough for gravity to make round.

"When an object is tiny, its strength controls its shape, but if you keep piling on mass the strength [eventually] means nothing. Be it cotton or steel, if it is thousands of kilometers across, the gravity will take over — it is rounded by gravity," said Stern.

Stern prefers to classify objects such as Sedna, which is thought to be about 1,000 miles (1,600 kilometers) across, as "dwarf planets," in the same vein as dwarf stars and dwarf galaxies.

Regardless of what Sedna is called, both Brown and Stern expect many more of these bodies to be found in the years to come.

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