There may soon be three new planets in our solar system. The expansion would be the result of a proposed new definition of what a planet is. The change is controversial, but if adopted by astronomy's governing body, it could eventually mean there will be many more than 12 planets.
Distant Pluto, the last official planet to be discovered, may keep its rank. For years, astronomers have wanted to demote it from planetary status because it was not quite like the eight classical planets closer to the sun. Its tilted orbit and its icy composition set it apart from the rocky planets such as Earth and the gas giants Saturn and Jupiter.
But a new definition of planets proposed at the International Astronomical Union meeting in Prague would retain Pluto's standing and add three more bodies.
They are Pluto's companion Charon, the asteroid Ceres, and a more distant body discovered last year that is not yet named, but known temporarily as UB 313.
"This is a revolution in our understanding of the solar system," explained Massachusetts Institute of Technology scientist Richard Binzel, a member of the astronomical union's planet definition committee. He says it took two years of tough deliberations to reach the new definition.
The revision is necessary, says Binzel, because of improved observations that make better distinctions within our solar system. This includes the 1992 discovery that there are icy bodies in the region beyond Pluto called the Kuiper Belt.
"We now know that out beyond Neptune and, in fact, out beyond Pluto, is a zone of bodies, literally thousands of bodies, that are basically a third region of our solar system. In that region, we know that there are many Pluto-like objects remaining to be discovered," Binzel said.
The long accepted view of a planet has been anything that orbits a star and is not itself a star. This broad meaning included asteroids and many other smaller objects as minor planets.
The new definition is much more exclusive. Binzel and his colleagues eliminated the minor planet category and included only those bodies orbiting stars that have enough gravity to pull themselves into a sphere. They do not include moons of planets.
"After we made that definition, we stopped and looked back and said, 'Okay, where is Pluto?' Pluto falls well above the boundary. It's a spherical object and, by gosh, Pluto makes the list," Binzel says.
Charon also makes the list because it is not a moon of Pluto as once thought. But in recognition that Pluto, Charon, UB 313 and other similar bodies are different in composition and angle of orbit, the new planet definition calls them plutons.
The change is controversial. Astronomer Scott Sheppard of the Carnegie Institution in Washington argues that plutons should not be considered planets because, unlike the eight major ones, their orbits are not independent. Instead, they are attuned to that of Neptune. Furthermore, he says their gravitational fields do not dominate their local environment.
"Pluto and UB 313 and the rest of the thousands of Kuiper Belt objects beyond Neptune, since they formed and evolved in a very similar manner, should be grouped together. So I would personally consider Pluto and the rest of the Kuiper Belt objects not really planets, but the third domain of the solar system or something like that," Sheppard says.
The astronomer who discovered UB 313 last year, Michael Brown of the California Institute of Technology, takes a middle view. He maintains that size should be considered.
"Pluto has been called a planet for so long that I think we are never going to not call Pluto a planet," Brown said. "If Pluto is going to be called a planet, then anything larger than Pluto is a planet. Things that are smaller I think we just call them typical members of the Kuiper Belt and they do not join this very special class of things that are planets."
His view would maintain his discovery as a planet because it is bigger than Pluto.
The new planet definition must now be approved by a majority of the nearly 2,500 astronomers attending the Prague meeting from 75 nations. But Scott Sheppard is not sure that would settle the issue.
"I think no matter what the International Astronomical Union decides, it will still be a debate for years to come," Sheppard said.