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Hard-hearted astronomers

Alas, poor Pluto! The little runt among the nine planets thought to make up our solar system is no longer to be counted among the Sun's immediate family. The International Astronomical Union, at its recent meeting in Prague, decided that a celestial body must satisfy three criteria to be counted as a planet in the solar system: it must orbit the Sun; it must be sufficiently big so that its gravity pulls it into a nearly round shape; and it should have cleared other objects from the vicinity of its orbit. Pluto gets demoted because it does not meet the last criterion. After several decades of planet membership, Pluto's expulsion may well leave many teary-eyed. Even the Astronomical Union was not so hard-hearted as to deny Pluto a special place in its new scheme. Pluto is to be recognised as the prototype of a new category of objects that lie beyond the orbit of the planet Neptune. That said, Pluto's inclusion among the planets has often been questioned. For one thing, it is considerably smaller than our Moon; its discoverers at the Lowell Observatory in the United States reportedly claimed in 1930 that Pluto was much larger than Earth, ensuring its immediate acceptance as a planet. In addition, its highly elliptical orbit is inclined to the plane of the orbits of the other planets.

But astronomers did not want to spend time and energy clarifying what was a planet and what wasn't. That attitude changed last year when the California Institute of Technology astronomer Michael Brown claimed that the rocky object 2003 UB313, which he had discovered and named Xena, should be considered the tenth planet. It was, after all, bigger than Pluto. With dozens of such objects in the Kuiper Belt, the icy debris left behind after the big planets coalesced, the concern was that the family of planets might expand uncontrollably. But it has not been easy for the International Astronomical Union to come up with a suitable definition for planets. At one point during their deliberations at Prague, it looked as if the number of planets might immediately increase to 12. The definition that the Union ultimately adopted may well have capped the number of planets in the Solar System at eight. However, several astronomers have already begun petitioning for the reinstatement of Pluto as a full planet. Even though its categorisation may have changed, its fascination remains. In January this year, the world's first spacecraft to Pluto and its moons left Earth on a nine-year voyage. The U.S. space agency NASA's New Horizons spacecraft may subsequently be despatched to study other objects in the Kuiper Belt. "Even frigid, distant and tiny Pluto is a dynamic world where the processes of nature continuously change the surface and the atmosphere, creating an alien and exotic world that beckons us from Earth to visit, explore and learn," says NASA. Little Pluto, it seems, still has us in thrall.

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