Possible planet is found, way out

Astronomers spot Sedna, a small, shiny, red body orbiting far beyond Pluto

March 16, 2004|By Michael Stroh | Michael Stroh,SUN STAFF

There may be a 10th planet lurking in the far-flung suburbs of the solar system - or maybe not.

What astronomers do know is that they have discovered a mysterious red object circling the sun that's three times as far from Earth as Pluto and nearly as large.

That could make the object - named Sedna after an Inuit goddess - the most remote body in the solar system. It's also the largest chunk of anything to turn up there since American astronomer Clyde Tombaugh first spotted Pluto on a hazy photographic plate in 1930.

"There's absolutely nothing else like it in the solar system," said astronomer Michael Brown of the California Institute of Technology in Pasadena, who led the team that announced details of the discovery yesterday.

Astronomers said the discovery could offer new insight into a vast region beyond Pluto where thousands of icy rocks are thought to roam, a region some astronomers informally dub the "great deep."

The find also might reignite a scientific debate over what, exactly, it takes to be a planet - an argument that has perked up in recent years as astronomers find more planetlike objects circling our sun and stars elsewhere.

"We have just scratched the surface of what there is to explore in this solar system," says Alan Stern, a planetary scientist at the Southwest Research Institute in Colorado.

"I will be surprised if we do not find frozen Mars- and even Earth-sized objects eventually."

The Cal Tech team first spotted Sedna in November as part of a continuing search for objects in a region beyond Neptune known as the Kuiper Belt.

Working at the Palomar Observatory outside San Diego, the scientists made the discovery using an age-old technique: They snapped successive pictures of the same piece of sky and then compared the images to see what, if anything, had moved. Distant stars and galaxies appear fixed in space, while objects closer to home don't. The slower an object moves from one photograph to the next, the more distant it is.

In the solar system, says Brown, Sedna was "the most slowly moving object we've ever seen."

With help from NASA's Spitzer Space Telescope, designed to study frigid bodies in deep space, the team determined that Sedna is 8 billion miles from the sun.

Its highly elliptical orbit, which requires 10,500 Earth years to complete, takes it as far as 84 billion miles from the center of the solar system. Even now, says Brown, it is so distant that "if you were standing on the surface of Sedna today and you held a pin at arm's length, you could cover up the sun."

Brown said Sedna could even lie at the fringe of a long-hypothesized region of the outer solar system known as the "Oort cloud." Proposed in 1950 by Dutch astronomer Jan Hendrik Oort, the cloud is thought to be a repository of icy debris that serves as the breeding ground for comets that periodically streak through the inner solar system.

With a surface estimated at minus-400 degrees Fahrenheit, astronomers said, Sedna probably has no atmosphere because no gas can stay aloft at that temperature. The frosty climate, however, gave them the idea for a name - Sedna is the Inuit goddess of the arctic oceans.

Scientists still have many questions about the object. For example, there are tantalizing clues that Sedna might have a moon, Brown said. The object seems to rotate once every 40 days, suggesting it is under the gravitational sway of some other body. The team plans to look for more evidence with the Hubble Space Telescope.

Another mystery is Sedna's color: It appears to be nearly as red as Mars and very shiny. Although scientists suspect it's made of ice and rock, they can't explain its appearance.

It's also unclear how Sedna got there. One theory: The object once orbited closer to the sun and was tugged out to its current location under the influence of another star. The Cal Tech team estimated Sedna's diameter at roughly three-fourths that of Pluto. This raises the big question: Should Sedna be classified as a planet?

The issue is sticky because the International Astronomical Union, the group responsible for naming and classifying heavenly objects, has no official definition of planethood, according to Brian Marsden, an astronomer and IAU official.

"Planet may be even a word that has lost its usefulness," says Marsden. Debating what is and isn't a planet "is a semantic mess, and one does better to stay away from it."

But scientists can't stay away from the debate. Some propose that an object is a planet only if it looks and behaves like the inner eight - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - big objects with mostly circular orbits.

Pluto, which is small and has an highly elliptical orbit, has in recent years narrowly escaped being demoted from planethood to something like large-rockhood. That debate grew especially intense after astronomers in 2002 found a Kuiper Belt object, roughly 800 miles across, called Quaoar. The IAU said Quaoar didn't make the cut.

Other astronomers argue that just because an object is small or doesn't orbit the sun like the rest of the planets, that doesn't mean it shouldn't be a planet.

"It just makes no sense," says Stern. "If you find a new species of dog that's smaller than a Chihuahua, is it not a dog?"

If Stern were writing the textbooks, he says, our solar system would contain not nine planets, but 900 - roughly the number of significant objects astronomers have discovered in the Kuiper Belt and elsewhere.

Of Sedna, Stern says: "This is a dwarf planet. Get over it."

Baltimore Sun Articles
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.