What happened to Pluto?
New telescope technologies began to reveal far-off objects even larger than Pluto – were these planets, or not? The International Astronomical Union (IAU) appointed a panel to decide on a new definition.
The panel proposed:
A planet is a celestial body that
(a) is in orbit around a star, and
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) is neither a star nor a satellite of a planet.
This gives us 12 "planets" in the current Solar System, adding Ceres (which originally was a planet), Charon (Pluto-Charon would be a "double-planet") and 2003 UB313 (also known as Xena). And who knows how many other "planets" remain to be discovered?
About 2,500 scientists meeting in Prague in August 2006 had the deciding vote. On the last day of the meeting, they added another condition for a celestial body to qualify as a planet:
(a) it must be in orbit around the Sun (or another star);
(b) it must be large enough that it takes on a nearly round shape;
(c) it has cleared its orbit of other objects.
Pluto, which was discovered in 1930 by the American Clyde Tombaugh, was automatically disqualified because its highly elliptical orbit overlaps with that of Neptune. It will join a new category of “dwarf planets.” Pluto is further away and much smaller than the eight other "traditional" planets. At just 2,360km (1,467 miles) across, it is smaller even than some moons in the Solar System. Since the early 1990s, astronomers have found several objects of similar size in an outer region of the Solar System called the Kuiper Belt. Xena, 3,000km (1,864 miles) across, is larger than Pluto.
The demotion is likely to upset the public, who have become accustomed to a particular view of the Solar System.
"I have a slight tear in my eye today, yes; but at the end of the day we have to describe the Solar System as it really is, not as we would like it to be," said Professor Iwan Williams, chair of the IAU panel.
3 September 2006