What Defines a Planet? Astronomers Disagree

An image of the extrasolar planet candidate.

This artificial-color image, snapped by the Hubble space telescope, is an infrared-light view of the brown dwarf star 2M1207. The possible planet is the magenta-colored spot at lower right. It's estimated to be about five times the size of Jupiter. NASA, ESA, G. Schneider hide caption

toggle caption NASA, ESA, G. Schneider
Possible planet image from ESA

The bright object in the center of this composite image is the brown dwarf 2M1207. The possible planet candidate is the fainter object at bottom left. ESA hide caption

toggle caption ESA

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Astronomers have been racing to take the first photo of a planet beyond our solar system, and they may have finally done it. Some infrared pictures were recently snapped by two teams of researchers working with a large ground telescope in Chile and the Hubble space telescope. The images, taken four months apart, show a celestial body that's roughly five times the size of Jupiter. It orbits far away from a small, dim star.

But whether other scientists accept this as the first planet ever caught on camera depends a lot on how the word "planet" is defined. Amazingly, astronomers can't agree on a precise definition for this seemingly simple term.

"There's certainly a debate about whether something is a planet or not," says Charles Beichman of the California Institute of Technology in Pasadena. The recent photos bring that debate into sharp focus.

Most people have an intuitive sense that planets are spheres that orbit a star. But even within our own solar system, that notion runs into problems. Consider Pluto. Smaller than Earth's moon, Pluto long had a place as the teeny but respectable ninth planet from our sun. But in 1992, astronomers realized this "planet" was just one of thousands of tiny frozen bodies orbiting in a group beyond Neptune. Suddenly Pluto didn't seem so special.

Similar ambiguities bedevil weird planets detected around distant stars. Ten years ago, scientists learned to hunt for other solar systems using indirect clues, like the way a star wobbles under a planet's gravitational tug. These tricks have revealed dozens of new "planets" around other stars, but scientists haven't been able to see them and they know little about them.

Many of these new planets are giants almost as massive as a kind of small, failed star called a brown dwarf. Unlike planets, brown dwarfs emit a faint light. But they still could get easily confused with big planets, because brown dwarfs sometimes orbit other stars.

And sometimes a huge planet orbits a brown dwarf that's not much bigger than the planet itself. That's the kind of system seen in the new photos, where the central star is just five times bigger than the planet. "The really major question that particular system raises is whether we want to call that a planet or not," says Alan Boss, an astronomer at the Carnegie Institution of Washington. In our solar system, the sun is many, many times larger than its biggest planet. "That's why we, in a very simple-minded way, think of planets as small objects in orbit around large objects."

Boss heads a working group of the International Astronomical Union that's drafted one possible definition for the term "planet," though it's a work in progress and not universally accepted. Scientists disagree on so many issues. Is a planet just an object of a certain size? Does it have to be a sphere? Can it float freely through space, or must it orbit a star?

Despite this uncertainty, Beichman thinks a decent case could be made for considering the recent sightings to be the first glimpse of a planet. Astronomers still have to take some more observations this spring, to confirm that the object is really orbiting the star and is not just some random object in the photo's background. But even if it turns out to be what they think it is, many astronomers will still be waiting for the day when telescopes spy a more familiar-looking planet in a solar system that's reminiscent of our own.



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