Sedna, a Planet-Like Body Discovered at Fringes of Our Solar System

What's bigger than an asteroid, smaller than a planet, red all over and far, far away? The answer -- a mysterious planet-like body orbiting our Sun -- has been discovered by NASA-funded researchers led by an astronomer at the California Institute of Technology, Pasadena, Calif.

These three panels show the first detection of the faint distant object dubbed "Sedna." Imaged on November 14th from 6:32 to 9:38 Universal Time, Sedna was identified by the slight shift in position noted in these three pictures taken at different times. Image courtesy: NASA/Caltech.

The object is three times farther away from Earth than Pluto, making it the most distant known in the solar system.

"The Sun appears so small from that distance that you could completely block it out with the head of a pin," said Dr. Mike Brown, Caltech associate professor o f planetary astronomy and leader of the research team. The object, unofficially named "Sedna," is 13 billion kilometers (8 billion miles) away from Earth.

An artist's concept of the newly discovered planet-like object, dubbed "Sedna." The Sun appears as an extremely bright star instead of a large, warm disc observed from Earth. In the distance is a hypothetical small moon, which scientists believe may be orbiting this distant body. Image courtesy: NASA/JPL-Caltech.

This is likely the first detection of the long-hypothesized "Oort cloud," a faraway repository of small icy bodies that supplies the comets that streak by Earth.

Other notable features of Sedna include its size and reddish color; it is the second reddest object in the solar system, after Mars. At an estimated size of three-fourths the size of Pluto, it is likely the largest object found in the solar system since Pluto was discovered in 1930.

Brown, along with Drs. Chad Trujillo of the Gemini Observatory in Hawaii and David Rabinowitz of Yale University, New Haven, Conn., found the "planetoid" on November 14, 2003, using the 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory near San Diego. Within days, the object was observed by telescopes in Chile, Spain, Arizona and Hawaii, and soon after, NASA's new Spitzer Space Telescope looked for it.

Sedna lies extremely far from the Sun, in the coldest known region of our solar system, where the temperature never rises above minus 240 degrees Celsius (minus 400 Fahrenheit).

The artist's rendition shows "Sedna" in relation to other bodies in the solar system, including Earth and its Moon; Pluto; and Quaoar, a planetoid beyond Pluto that was until now the largest known object beyond Pluto. Image courtesy: NASA/JPL-Caltech.

The planetoid is usually even colder, because it approaches the Sun this closely only briefly during its 10,500 year orbit around the Sun. At its most distant, "Sedna" is 130 billion kilometers (84 billion miles) from the Sun. That is 900 times Earth's distance from the Sun.

Scientists used the fact that even the Spitzer telescope was unable to detect the heat of the extremely distant, cold object to determine that it must be no more than 1,700 kilometers (about 1,000 miles) in diameter, smaller than Pluto. By combining all available data, Brown estimates the size at about halfway between that of Pluto and Quaoar, the planetoid discovered by the same team in 2002. Until "Sedna" was detected, Quaoar was the largest known body beyond Pluto.

The extremely elliptical orbit of Sedna is unlike anything previously seen by astronomers; however, it resembles that of objects predicted to lie in the hypothetical Oort cloud. The cloud is thought to explain the existence of certain comets. It is believed to surround the Sun and extend outward halfway to the star closest to the Sun. But Sedna is 10 times closer than the predicted distance of the Oort cloud. Brown says this "inner Oort cloud" may have been formed by gravity from a rogue star near the Sun in the solar system's early days.

Brown explained, "The star would have been close enough to be brighter than the full Moon, and it would have been visible in the daytime sky for 20,000 years." Worse, it would have dislodged comets farther out in the Oort cloud, leading to an intense comet shower that could have wiped out any life that existed on Earth at the time.

Rabinowitz says there is indirect evidence that "Sedna" may have a moon. The researchers hope to check this possibility with NASA's Hubble Space Telescope.

Trujillo has begun to examine the object's surface with one of the world's largest optical/infrared telescopes, the 8-meter (26-foot) Frederick C. Gillett Gemini Telescope on Mauna Kea, Hawaii. He said, "We still don't understand what is on the surface of this body. It is nothing like what we would have predicted or what we can currently explain.

"Sedna" will become closer and brighter over the next 72 years before it begins its 10,500-year trip to the far reaches of the solar system and back again. "The last time "Sedna" was this close to the Sun, Earth was just coming out of the last ice age; the next time it comes back, the world might again be a completely different place," said Brown.