Volume: 05, Issue: 01 08/30/2006 
Montage of planets. Image credit: NASA/JPL.
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Hubble Space Telescope view of Pluto. The larger view includes Charon. Image credit: ESA/NASA.
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Pluto's Interior. Image credit: Lunar and Planetary Institute.
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Other Articles in This Issue:
New Horizons Continuing on to Pluto, Planet or Not
Atlantis Headed Back to Launch Pad
NASA Galaxy Hunter: Huge Black Holes Stifle Star Formation
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Pluto No Longer a Planet

The International Astronomical Union, wrapping up its meeting in Prague, Czech Republic, has resolved one of the most highly-debated topics in the cosmos by approving a specific definition that gives our solar system eight planets, instead of the nine most of us grew up memorizing. NASA has already visited all eight planets that retain their official title: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In addition the agency has its New Horizons spacecraft en route to Pluto, which the International Astronomical Union has re-assigned to a new category of celestial objects, to be called “dwarf planets.”

“NASA will, of course, use the new guidelines established by the International Astronomical Union,” said Dr. Paul Hertz, Chief Scientist for the Science Mission Directorate at NASA Headquarters. “We will continue pursuing exploration of the most scientifically interesting objects in the solar system, regardless of how they are categorized.”

Ceres, which orbits in a belt between Mars and Jupiter and is the largest known asteroid, is one of those interesting objects. In 2007, NASA will launch the Dawn spacecraft on a mission to study Ceres, which the astronomers have placed in the dwarf planet category, alongside Pluto. The dwarf planet family also includes 2003 UB313, nicknamed “Xena.” When Dr. Mike Brown of Caltech and his colleagues announced in the summer of 2005 they had discovered the object, which is bigger and farther away than Pluto, many astronomers decided it was time to figure out once and for all, “What exactly is a planet, anyway?”

The International Astronomical Union has decided that, to be called a planet, an object must have three traits. It must orbit the sun, be massive enough that its own gravity pulls it into a nearly round shape, and be dominant enough to clear away objects in its neighborhood.

To be admitted to the dwarf planet category, an object must have only two of those traits – it must orbit the sun and have a nearly round shape. Moons do not count as dwarf planets. In addition to Pluto, Ceres, and 2003 UB313, the International Astronomical Union has a dozen potential dwarf planets on its watchlist.

What’s to become of the other objects in our solar system neighborhood, the ones that are not planets, not dwarf planets, and not moons? The organization has decided that most asteroids, comets, and other small objects will be called “small solar-system bodies.”

Despite the establishment of these three distinct categories, there are bound to be gray areas. As technologies improve and more objects are found, the International Astronomical Union will set up a process to decide which categories are most appropriate for specific objects.

Even before the discovery of Xena, not all was calm in the planetary world. There was debate after Clyde Tombaugh discovered Pluto in 1930. With its small size, distant location, and odd orbit, some questioned whether Pluto was really a planet or just an icy remnant of the planet forming process.

That issue has been resolved by the International Astronomical Union. Among those most keenly following the debate – Mike Brown, who was waiting word on Pluto and the object he found, Xena.

“I’m of course disappointed that Xena will not be the tenth planet, but I definitely support the IAU in this difficult and courageous decision,” Brown said. “It is scientifically the right thing to do, and is a great step forward in astronomy.”

Although the revamping of our solar system might seem unsettling, it is really nothing new. In fact, when Ceres was first discovered in 1801, it was called a planet, as were several similar objects found later. But when the count kept growing, astronomers decided “enough is enough” and the demoted Ceres and its siblings, placing them in a new category, called “asteroids.”

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