Astronomers decide Pluto 'not a planet'

Pluto has traditionally been regarded as the 9th planet in our solar system.

Pluto was stripped of its status as a planet today when astronomers from around the world redefined it as a "dwarf planet", leaving just eight major planets in the solar system.

With one vote, toys and models of the solar system became instantly obsolete, forcing teachers and publishers to scramble to update textbooks and lessons used in classrooms for decades.

"Pluto is dead," Mike Brown of the California Institute of Technology bluntly told reporters on a teleconference.

Discovered in 1930 by the American Clyde Tombaugh, the icy rock of Pluto has traditionally been considered the ninth planet, farthest from the sun in the solar system.

However, the definition of a planet, approved after a heated debate among 2,500 scientists from the International Astronomical Union (IAU) meeting in Prague, drew a clear distinction between Pluto and the other eight planets.

The need to define what is a planet was driven by technological advances enabling astronomers to look further into space and measure more precisely the size of celestial bodies.

"This is all about the advancement of science changing our thinking as we get more information," said Richard Binzel, professor of Planetary Sciences at The Massachusetts of Technology and a member of the planet definition committee.

"The significance is that new discoveries and new science have told us that there is something different about Pluto from the other eight planets and as science learns more information, we get new results and new considerations."

OBLONG ORBIT

Brown added impetus to the decades-old debate on the definition of a planet when he discovered UB313 in 2003. Xena, as it is nicknamed, is larger than Pluto, instantly creating a buzz over whether a new planet had been discovered.

The scientists agreed that, to be called a planet, a celestial body must be in orbit around a star while not itself being a star.

It must be large enough in mass for its own gravity to pull it into a nearly spherical shape and have cleared the neighborhood around its orbit.

Pluto was disqualified because its oblong orbit overlaps with Neptune's. Xena also does not make the grade of being a planet, and will also be known as a dwarf planet.

"It's an issue mainly for the public, not really for scientists. Some people may be upset, but we've long regarded it (Pluto) as a minor planet," said Richard H. Miller of the University of Chicago.

The agreed-upon definition -- the first time the IAU has tried to define scientifically what a planet is -- comes in sharp contrast to the draft sent around to delegates at the General Assembly last week.

That document, which kept Pluto as a planet and would have added three others, touched off a revolt that grew with each day. Some delegates appeared downright hostile to the notion, saying the committee was going overboard.

Tombaugh's widow Patricia said the discoverer, like any good scientist, would have accepted the demotion as inevitable.

"Clyde would have said, 'Science is a progressive thing and if you're going to be a scientist and put your neck out, you're apt to have it bitten upon,'" the 94-year-old said from her home in Las Cruces, New Mexico.

She added that a small amount of her husband's ashes were now on a spacecraft bound for Pluto.

The new definition creates a second category called "dwarf planets", as well as a third category for all other objects, except satellites, known as small solar system bodies.

"We are just defining a new class of planets and I think it's very appropriate. We are finding more planets in our solar system, and some are larger than Pluto," said Philip Diamond, a professor at the University of Manchester and a delegate attending the IAU meeting.

"I think what we have done is a good thing, we have actually expanded the number of planets in our solar system, but just spread them over two categories."

From now on -- or 'at least for the time being' joked one delegate -- traditional planets will be restricted to eight: Mercury, Venus. Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

- REUTERS