Leading astronomers are to vote on a proposal that would redefine what a planet is and could see the demotion of Pluto.
About 2,500 astronomers from 75 countries are in Prague for the annual assembly of the International Astronomical Union and on Thursday they could condemn Pluto, discovered in 1930 and the most recent addition to the solar system, to the status of a "dwarf".
The IAU was looking to retain Pluto's planetary status and bring in a further three similar-sized celestial bodies into the solar system, the new members being known as "Plutons", meaning an object like Pluto.
The rationale was their initial draft definition of a planet: Any object larger than nearly 800km in diameter that orbits the sun, has a mass roughly one-12,000th that of Earth and has enough self-gravity to pull itself into a round shape.
However, that proposal appears to have been dropped under pressure from opponents, and Earth's neighbourhood could officially shrink to eight planets from the traditional nine.
Junichi Watanabe, a Japanese astronomer and a member of the IAU's planet definition committee, said: "There would be only eight planets, plus the dwarf planets."
After days of often heated debate, scientists opposing the proposal won some key concessions.
A planet, they insist, must be the dominant object in its area. That would draw a sharp distinction between the eight "classical planets" and Pluto, which would be known as a "dwarf planet".
Pluto's largest moon, Charon, the asteroid Ceres, which was a planet in the 1800s before it got demoted, and a recently discovered object known as 2003 UB313 and nicknamed Xena, also would be dwarfs.
The precise wording of the definition remained a work in progress on Wednesday.
However, if astronomers agree that a planet must have "orbital dominance" in its own neighbourhood, the new guidelines would eliminate Pluto and the trio of tentative candidates as proper planets.