Scientists discover solar system's 'Planet X'

Researchers at Kobe University in western Japan say theoretical calculations using computer simulations led them to conclude it was only a matter of time before the mysterious 'Planet X' was found.

"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," Kobe University said in a statement.

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

Pluto disqualified

The research, led by Kobe University professor Tadashi Mukai along with researcher Patryk Lykawka, will be published in the April issue of the US-based Astronomical Journal.

Their 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 that the celestial body was merely a dwarf planet.

Astronomers concluded that Pluto's oblong orbit overlapped with that of Neptune, excluding it from being a planet.

Elliptical solar orbit

Instead it defined the solar system as consisting solely of the classical set of Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus and Neptune.

Planet X -- so called by scientists because it has yet to be found -- would have an elliptical solar orbit like Earth, the Kobe University team said, estimating its radius was 15 to 26 billion kilometres.

The team noted that more than 1,100 celestial bodies have been found in the outer reaches of the solar system since the mid-1990s.

Researchers have set up a theoretical model looking at how the remote area of solar system would have evolved over the past four billion years.

"In coming up with an explanation for the celestial bodies, we thought it would be most natural to assume the existence of a yet unknown planet," the university said in its statement.