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This is SCIENCE IN THE NEWS in VOA Special English. I'm Bob Doughty.

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And I'm Faith Lapidus. This week -- the debate over what is a planet.

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People might think they know a planet when they see one. But even the people who study planets get into arguments. Those disputes have only intensified, now that the International Astronomical Union has voted to define a planet for the first time.

The vote in Prague on August twenty-fourth leaves our solar system with eight planets instead of nine -- take Pluto off the list. But astronomy has gained a new kind of object: the dwarf planet.

Astronomers say new discoveries made the need for a scientific definition of a planet increasingly urgent. New bodies with orbits more distant from the Sun than Neptune and Pluto are being discovered all the time.

But one object changed a balance of ideas that held together for more than seventy years.

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In January of last year, a team of astronomers in the United States discovered a large object farther away than Pluto. Scientists have discovered other very distant objects in our solar system.

But the team -- Mike Brown, Chad Trujillo and David Rabinowitz -- later found that the object was bigger than Pluto. It does not have an official name yet; for now it is known as Xena [ZEE-nuh].

Xena presented a problem: If Pluto was big enough to be considered a planet, then what did that make Xena?

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Some astronomers have never considered Pluto a planet. They see it as just one of many objects in the Kuiper [KY-per] Belt. The Kuiper Belt is an area where comets and other small icy bodies come from.

But whatever people said about Pluto, one thing was clear: the existing model of the solar system was no longer working.

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"Planet" comes from a Greek word meaning "wanderer." Planets are wanderers in a sky filled with stars that appear fixed in their positions.

No one knows when humans first recognized the movement of planets. But people used to think of the Moon and the Sun as planets because they appeared to orbit Earth.

By the fifteenth century, Nicolas Copernicus changed the model of thinking about the solar system. The Polish astronomer established that Earth was, in fact, one of the planets orbiting the Sun.

Later, the German astronomer Johannes Kepler believed there was an importance to the number of planets. He suggested that their distances from the sun were related to the forces that create musical notes.

There were six known planets -- Mercury, Venus, Earth, Mars, Jupiter and Saturn.

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Then, in seventeen eighty-one, astronomer William Herschel discovered a seventh planet. Herschel was a native of Germany but had moved to Britain.

The planet was Uranus [YER-uh-nuhs]. This huge world, like Jupiter and Saturn, is mostly gas. It is also extremely cold, with Celsius temperatures of two hundred degrees below zero at its cloud tops.

For his discovery, Herschel was named the Astronomer Royal of Britain. He and his sister, Caroline, continued to make important astronomical discoveries.

The discovery of Uranus showed that the solar system was not unchanging, as many people believed at that time. But this was only the beginning of a new period of discovery.

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On January first, eighteen-oh-one, Giuseppi Piazzi discovered an object between the orbits of Mars and Jupiter. At first, the Italian astronomer thought it was a comet, an icy, dusty ball with a long gas tail. But he soon came to think it might be a planet.

What Piazzi found was Ceres -- the first recorded asteroid, and the largest one known. Asteroid means "star-like." Asteroids are mostly rock or metal, or a combination.

The astronomers who met in Prague agreed that objects like asteroids will now be described collectively as "small solar-system bodies." The orbits of these bodies can bring them very close to the sun. Sometimes they cross paths with Earth's orbit.

Piazzi's discovery of an asteroid changed the model of the solar system again. And there were more changes to come.

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Scientists suspected that the orbit of Uranus was being influenced by the gravity of another planet-sized object. Progress in mathematics led Urbain Jean Joseph Leverrier to discover the orbit of this body.

In eighteen forty-six, the French astronomer wrote Johan Gottfried Galle at the Berlin Observatory. He told Galle where to point his telescope to find the new planet. In about half an hour, Galle found Neptune.

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With Neptune, the discovery of the so-called classical planets came to a close. These planets move around the sun in nearly circular orbits. They include the rocky worlds of Mercury, Venus, Earth and Mars. They also include the huge gas planets Jupiter, Saturn, Uranus and Neptune.

But the American astronomer Percival Lowell was not satisfied. He believed there was evidence of yet another planet -- Planet X. From his observatory near Flagstaff, Arizona, he searched and searched beyond the orbit of Neptune.

Lowell died in nineteen sixteen, but the observatory continued his work. Success came in February of nineteen thirty when Clyde Tombaugh [TOM-baw] discovered the planet that would be called Pluto.

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Astronomers still know little about Pluto because of its great distance. It takes Pluto close to two hundred fifty years to circle the sun. Pluto is only a point of light even to the largest telescopes.

Percival Lowell had expected Planet X to be massive. But for many years Pluto was believed to be a little smaller than Earth. Then, in nineteen seventy-eight, American astronomers discovered that Pluto had a moon. The discovery led scientists to further shrink their estimates of the size of Pluto. Suddenly this "planet" was smaller than our own Moon.

In January of this year, the American space agency NASA launched the first spacecraft to study Pluto. The robotic ship, called New Horizons, will not arrive until two thousand fifteen.

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Earlier this year, the International Astronomical Union created a committee to propose a definition of a planet. The first proposal would have given us twelve planets.

There would have been eight "classical planets." And there would be a new group called "plutons," or Pluto-like planets. Among them would have been Charon -- Pluto's moon. Charon does not exactly orbit Pluto. They both orbit a common center of gravity between them.

Xena and Ceres would have also been called plutons.

But after heated debate that proposal was rejected. Instead, the astronomers voted to have eight "planets" and a group called "dwarf planets."

For now, the dwarf planets are Pluto, Xena and Ceres. But other objects are expected to join the list before long.

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The International Astronomical Union was established in nineteen nineteen. The group holds a general assembly meeting every three years.

Two thousand five hundred of its nine thousand members attended the meeting last month in the Czech capital. Two weeks of discussions took place. But the vote was left to a reported four hundred twenty-four astronomers who stayed to the end.

The definition they agreed to sets three requirements for a planet. It must orbit the sun. It must have enough mass so that its own gravity makes it nearly round. And it must have "cleared the neighborhood around its orbit."

Astronomers who supported this definition say they do not consider the area around Pluto's orbit cleared. They say Pluto has not gathered up enough nearby material and they note that its orbit crosses Neptune's orbit.

Yet some people say that by this reasoning, the continued presence of Pluto could mean that Neptune should no longer be called a planet.

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Many astronomers criticize the definition, which considers planets and dwarf planets as two separate classes of objects. Astronomy historian Owen Gingerich led the committee that proposed the idea for twelve planets. The Harvard University professor says it does not make sense to say that a dwarf planet is not a planet.

And other issues remain, like what to call huge objects that orbit stars other than the Sun.

Yet science is a process of continual discovery and change. For now, it means we have to remember that our solar system -- officially, at least -- has only eight planets. But to future generations, today's thinking might seem like the ideas of the first people who looked up and saw stars wandering in the sky.

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SCIENCE IN THE NEWS was written by Mario Ritter and produced by Brianna Blake. I'm Bob Doughty.

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And I'm Faith Lapidus. You can download transcripts and audio files of our programs at WWW.51VOA.COM. And listen again next week for more news about science in Special English on the Voice of America.