The world's astronomers are shaking up the solar system by proposing to raise the number of planets to 12 from nine, and opening the way for dozens more to follow in a startling new vision of our cosmic neighbourhood.
The discovery of numerous small objects circling the sun had triggered a quandary that threatened to boot Pluto out of the classical solar system and forced the International Astronomical Union to redefine what exactly a planet is.
After two years of debate, the IAU's official proposal will be put to its convention in Prague today.
The good news is, Pluto is in.
So are Charon, an object caught in a continuing dance with Pluto to create what is now considered a “double planet”; the small “dwarf planet” of Ceres that sits between Mars and Jupiter; and the so-called “10th planet,” 2003 UB313, currently nicknamed Xena.
The scientists behind the proposal say they let physics do the talking. If an orbiting object has a gravitational pull strong enough to drag its shape into a rough sphere, then it's powerful enough to be called a planet.
But the man who discovered the “10th planet” says the new definition is lengthy, muddled and unscientific, and will only make things more confusing.
Mike Brown, of the California Institute of Technology in Pasadena, said that if the definition is applied equally, it's not just his discovery that could be written into the history books — a whopping 53 round objects are already known to be circling the sun.
“To me, the word ‘planet' always meant something special. Nine was special. Maybe 10. Fifty-three? No,” Dr. Brown said in a telephone interview. “In some ways it drains the excitement of what I thought was an exciting find. Turns out it was the 12th planet. Who knew?”
Under the IAU proposal, orbiting objects would be divided into two categories: planet (from the Greek word for wanderer) and the newly created pluton.
A planet orbits around a star, but cannot actually be a star, and must be massive enough for its own gravity to pull it into a nearly spherical shape. The new plutons are small planets that have orbits of more than 200 years, meaning they orbit beyond Neptune.
An IAU review committee will make the final decisions on classification.
If the new definitions are approved, it means the solar system would consist of Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313.
Richard Binzel, a member of the IAU planet-definition committee, said the new categories will go to a full vote of the IAU next Thursday.
“It's a whole new frontier,” said Dr. Binzel, a professor of planetary science at the Massachusetts Institute of Technology. “We think there are dozens of Plutos out there. Our solar system is full of new things to discover.”
For the people searching the night skies, though, the IAU proposal could take away all the fun. As UB313 discoverer Dr. Brown said, who wants to discover planet 45? And why does a scientifically stringent definition need a committee to have final say on classifications?
“You can have a scientific definition and let the scientists decide what fits and what doesn't. No other definition has a committee to decide if you fit it,” Dr. Brown said.
“We've needed a scientific definition for many years now. They started down the right path. In the end, I think they blew it. I think maybe the scientists will think one thing, like there are 53 planets, and society will generally ignore them.”
In the end, this issue falls a long way down the list of astronomers' concerns, said University of Western Ontario professor Phil Stooke. Astronomers will remain more concerned with the bigger picture, he said, including which of those whizzing cosmic bodies might actually collide with the Earth.
“In the world of astronomy, it's tiny, but in the public eye it's big,” Dr. Stooke said of the definition.
“It might not be what astronomy needs, but it might be something the public benefits from. It makes them look at the solar system differently.”
Meanwhile, Dr. Brown's planet should receive its proper name at a later date. Persephone is the leading contender.
