Planet Projected at Solar System's Edge

Feb. 28, 2008 -- Scientists at a Japanese university said Thursday they believed another planet up to two-thirds the size of the Earth was orbiting in the far reaches of the solar system.

The researchers at Kobe University in western Japan said calculations using computer simulations led them to conclude it was only a matter of time before the mysterious "Planet X" was found.

"Because of the very cold temperature, its surface would be covered with ice, icy ammonia and methane," Kobe University professor Tadashi Mukai, the lead researcher, told AFP.

The study by Mukai and researcher Patryk Lykawka will be published in the April issue of the Astronomical Journal.

"The possibility is high that a yet unknown, planet-class celestial body, measuring 30 percent to 70 percent of the Earth's mass, exists in the outer edges of the solar system," said a summary of the research released by Kobe University.

"If research is conducted on a wide scale, the planet is likely to be discovered in less than 10 years," it said.

Planet X--so called by scientists as it is yet unfound--would have an oblong elliptical solar orbit and circle the sun every thousand years, the team said, estimating its radius was 15 to 26 billion kilometers.

The study comes two years after school textbooks had to be rewritten when Pluto was booted out of the list of planets.

Pluto was discovered by the American astronomer Clyde Tombaugh in 1930 in the so-called Kuiper belt, a chain of icy debris in the outer reaches of the solar system.

In 2006, nearly a decade after Tombaugh's death, the International Astronomical Union ruled the celestial body was merely a dwarf planet in the cluttered Kuiper belt.

The astronomers said Pluto's oblong orbit overlapped with that of Neptune, excluding it from being a planet. It defined the solar system as consisting solely of the classical set of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.