Shaking up the solar system

Reclassifying Pluto means opening the membership to as many as 53 planets

a NASA probe is on the way

Visions

August 20, 2006|BY A SUN REPORTER

Pluto's status as a planet was confirmed last week at a meeting in Prague, Czech Republic, of 2,500 astronomers who attended the triannual assembly of the International Astronomical Union, but a long debate about the nature of planets and their definition is likely to continue.

As some concerned astronomers have pointed out, the new definition threatens to significantly alter our sense of the solar system: eight large objects that orbit the sun - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - with a much smaller one, Pluto, way out on the edge of the neighborhood. Using the new definition, there could be as many as 53 planets.

Pluto wasn't discovered until 1930, but astronomers were already fairly certain that something large was out there because of gravitational effects on the orbits of two other planets. They called this mysterious body Planet X .

The discovery and naming of Pluto caused a popular sensation. Walt Disney named Mickey Mouse's dog, who first appeared in the 1930 cartoon The Chain Gang, Pluto after the new planet.

But astronomers were suspicious almost from the start that Pluto wasn't really large enough to be Planet X. As telescopes improved, the astronomers became aware of a large assortment of more than 1,000 icy bodies collectively called the Kuiper Belt circling the sun beyond Neptune where Pluto lay.

In 1978 they discovered that Pluto had a moon, Charon, and were able to use calculations of the gravitational effects of the two bodies on each other to determine that Pluto was much smaller than originally thought.

Critics of Pluto's status as a planet pointed out that it appeared to be smaller than seven moons in the solar system - Ganymede, Titan, Callisto, Io, Earth's moon, Europa and Triton.

The debate about what makes a planet really picked up steam last year when astronomers discovered a Kuiper Belt object larger than Pluto.

The romance of the discovery was somewhat undermined by the new body's geeky name - 2003 UB313.

Now, 2003 UB313 will be a planet, along with others orbiting the sun with gravitational forces large enough to be shaped into spheres.

We still don't know much about Pluto because it is located so far from Earth. But with any luck, much of the mystery will be lifted within a few years.

NASA's New Horizons spacecraft, a mission led by the Southwest Research Institute and the Johns Hopkins Applied Physics Laboratory, is already on its way to the tiny planet.

The mission launched Jan. 19, 2006. It will benefit from a gravity assist from Jupiter, and the closest approach to Pluto will be July 14, 2015.

Observations of Pluto will start five months prior to closest approach and will continue for at least a month after the encounter. New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to assess the global geology and other characteristics of both Pluto and Charon.

Does it matter that there is a lingering disagreement about whether Pluto is a planet? Many scientists say no. Such labels are simply mechanisms to help us organize our world, they explain.

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