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Lofty ambition: Defining what exactly is a planet
San Francisco Chronicle


August 23, 2005

If Planet Earth is a planet, why not Planet Pluto or Planet X?

When astronomers announced amid a flurry of headlines last month that they had discovered the solar system's long-sought "10th planet," an old argument flared once again:

Just what makes a planet a planet, anyway, and why should some big chunk of rock and ice flying more than 9 billion miles away from us be entitled to bear the honorable title of planet?

Right now, this newfound object, even larger and more distant than Pluto, bears only the code name 2003 UB313, but its discoverers are calling it a planet and already have given it a name - which they won't reveal unless it wins official approval by the International Astronomical Union, the 85-year-old Paris-based organization whose mission is "to promote and safeguard the science of astronomy in all its aspects."

And the IAU is already mired in the puzzle of how to classify all 160 giant "exoplanets" that have been discovered in the past 10 years flying in orbit around stars far, far out in the galaxy - some of them, perhaps, parts of solar systems of their own. The new planet - if the IAU agrees it is a planet - caps a search for an unknown "Planet X" that the aristocratic Bostonian Percival Lowell, nearly 100 years ago, believed must exist beyond the orbit of Neptune. At his Lowell Observatory in Flagstaff, Ariz., he assigned a young astronomer named Clyde Tombaugh to find his Planet X, and Tombaugh instead discovered Pluto in 1930 - a ninth planet, all right, but a lightweight object far too small to fit Lowell's prediction.

Now Michael Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory in Hawaii and David Rabinowitz of Yale have found what they believe is a true planet, larger and more massive and farther away than Pluto. But others insist that if the Brown team's object is truly a planet, then many other objects flying in distant orbits around the sun - and even at least one large asteroid flying in solar orbit between Mars and Jupiter - should also be called planets.

Way out beyond the true planet Neptune lies a crowded, jostling swarm of rocks and unborn comets large and small in a far-off region of the solar system known as the Kuiper Belt. Hundreds of thousands of them fly in immensely long, looping orbits away from the sun and their paths - like runners on a track - carry them roughly in the same plane as our familiar planets: Earth, Mars, Jupiter and all the others.

Neptune lies some 2.8 billion miles from us, whereas Pluto is so distant that its orbit carries it as far as 4.6 billion miles from the sun - which puts it well within the Kuiper Belt and causes many astronomers to call it a Kuiper Belt Object, or KBO, and not a planet at all. And its tilted orbital path is puzzling, too, angling at some 40 degrees above the plane of the Kuiper Belt and all Earth's planets, according to the Brown team's observations.

Brown and his colleagues are on a constant hunt for large objects in Pluto's neighborhood, and three years ago they found one 4 billion miles away. They named it Quaoar (pronounced Kwah-wahr) and called it a KBO. Last year, they discovered an even stranger object orbiting the sun 8 billion miles away, far more distant than Pluto. They named it Sedna, and Brown called it a "planetoid," but not a planet.

That was last year, and Brown declared on his Web site that "historically, then, Pluto, too, should no longer be considered a planet."

He's changed his mind, however, and in a telephone interview about his group's newest find, this is what he said:

"I had long been a fan of the eight-planet side, but I came to an epiphany. My mind has been changed, and I now say that Pluto is a planet because for 75 years we've said it's a planet - it's part of our culture - and to call our new object something else takes away all the excitement that young people can feel about the discovery of unknowns in the solar system."

Brown said he owes his "epiphany" to University of California astronomy professor Gibor Basri, who has proposed a simple definition for a planet: It must be a round object with a mass no larger than 13 Jupiters; it must be orbiting another object so massive that its interior burns with thermonuclear fusion - as is true of the sun and most other stars. Any object more massive than 13 Jupiters, Basri says, would make the object a "brown dwarf" star, and certainly not a planet.

By Basri's definition, Pluto is indeed a planet, and Brown said his team is pursuing its hunt for other more distant orbiting planets.


Distributed by Scripps Howard News Service,

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