August 22, 2006

Scientists Create Big Bang with Planet Definition

By Alan Crosby

PRAGUE -- In coming up with the definition of a planet, astronomers hoped to bring order to the way we look at solar systems. Instead, they created a big bang of their own.

A committee formed by the International Astronomers Union (IAU) has proposed that to be called a planet, a celestial body must be in orbit around a star while not itself being a star.

It also must be large enough in mass for its own gravity to pull it into a nearly spherical shape.

The new definition -- the first time the IAU has tried to define scientifically what a planet is -- means three new planets will be added to the current nine, and opens the door to dozens more which are seen fitting the description.

Reaction to the proposal, which will be voted on this Thursday by some 2,500 astronomers and scientists, has hit the IAU's annual conference in Prague like a meteor.

"The desire to put everything in neat little boxes is admirable but can be overdone, particularly when the boxes become contorted to include some objects and exclude others. The universe is too complex and too fascinating to fit everything into neatly described categories," said Paul Weissman, an IAU delegate from the Jet Propulsion Laboratory in the United States.

"Often, the most interesting objects are the ones that refuse to be categorized simply. Anally pursuing such a goal is stamp collecting, not science."

The need to define what it takes to be a planet stems from technological advances that enable astronomers to look further into space and to measure more precisely the size of celestial bodies in our solar system.

Under the new definition, schoolchildren will be relieved to know that, just as they were taught, Pluto will remain a planet. But it would also fall into a newly created category called plutons, which are distinguished from classical planets in that they take longer than 200 years to orbit the sun.

Pluto would be joined in this new category by two other celestial bodies, Xena and Charon, while another, Ceres, would be known as a dwarf planet.

"The proposed change will lead to confusion and near constant turmoil for no really good reason. The main beneficiaries of the (definition) if approved, are likely to be publishing companies and textbook authors," one IAU delegate said.

Not everyone agrees.

"The definition seems to be reasonable. It addresses whether asteroids and Kuiper Belt objects (including Pluto) are planets in a rational way," said Paul Withers, a research associate professor in the Center for Space Physics at Boston University.

"The debate is likely to continue after the IAU meeting, even if the resolution is approved ... In the end, what we call these objects in our solar system doesn't really matter. What matters is what they tell us about the formation and history of our little corner of the universe."

Geologists have also entered the fray, complaining at the use of plutons as a new category -- a term they already use to describe a body of igneous rock that solidified below the Earth's surface.

"Well, if we are changing the dictionary definition of a planet, why stop there. I guess they could change other terms such as plutons as well," quipped one delegate.