Experts say that the change is necessary to end the debate over whether or not Pluto should be stripped of its planet status, after the discovery of many similar sized objects beyond the ninth planet’s orbit.
A seven member panel of astronomers, historians and a science writer gathering under the auspices of the International Astronomical Union may have found an answer that will satisfy everyone: A planet, they decreed, is any star-orbiting object so large that its own gravity pulls in its rough edges, producing a near-perfect sphere.
That definition excludes 200,000 small, odd-shaped rocks, comets and asteroids that wander around the sun, but it also means Pluto remains a planet.
The new definition also includes three other big space rocks, including one currently considered an asteroid and another long described as a moon of Pluto. Also on the list is an icy body beyond Pluto, which would belong to a class of planets to be known as "plutons".
"In a day and a half of hammering it out, we came up with this unanimous recommendation," said Owen Gingerich, chairman of the IAU's "planet definition committee" and an emeritus historian of astronomy at Harvard University.
Richard Binzel, a committee member and asteroid specialist at the Massachusetts Institute of Technology, said that after a discouraging first day of deliberations, the members were surprised to find a definition that appealed to all of them.
"We want this definition to apply not only in our solar system, but in other solar systems as well," Mr Binzel said.
Vote
The definition will be put to the vote of astronomers from around the world on August 24 at the IAU's meeting in Prague. If successful the definition would constitute the first official recognition of new planets since Pluto's discovery in 1930.
Mr Gingerich has already received backing from 10 of the group's division chairmen. There is nothing binding about the vote, but the IAU is considered the world's authoritative source on the naming of heavenly bodies.
Finding a solution to the "Pluto problem" has proven a surprisingly emotional task for the usually staid astronomical profession – the public, and some astronomers, have grown attached to the tiny planet, but will people feel warm towards the new ones?
Ceres, Charon and Xena
The three new planets encompassed by the definition would be the asteroid Ceres, Pluto's moon Charon and an object beyond Pluto called 2003 UB313, unofficially known as Xena.
For Ceres, it would be a belated promotion, actually declared a planet when first discovered in 1801 further findings of asteroids between the orbits of Mars and Jupiter led astronomers to class it with those smaller objects. But according to a study by the Hubble Space Telescope it is round, placing it within the new definition.
At the moment Charon is considered to be Pluto's moon, but it too will get a promotion under this new definition because many astronomers believe the two bodies comprise a "double planet" system. Pluto and Charon orbit each other, and their common centre of gravity lies outside Pluto - unlike any other planet-moon system.
The existence of Xena was announced last year. Hubble Space Telescope images have shown the object is at least as big as Pluto.