The solar system shrank a little today after the world’s top astronomers voted to drop Pluto as a planet.

The International Astronomical Union, meeting in Prague, created a scientific definition of a planet for the first time.

The decision means that there are now eight planets known to orbit the Sun. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

A plan to create a new class of world called either plutons or plutonian objects was dropped. Instead, Pluto is demoted and joins its moon Charon and newly discovered 2003 UB313, nicknamed Xena, as what will be called dwarf planets. So too will Ceres, the largest of the asteroids that circle the sun between Mars and Jupiter.

The IAU’s resolution declares that a planet is a celestial body that (a) is in orbit around the Sun, (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) has cleared the neighbourhood around its orbit.

In other words, that it is large enough to have swept its region clear of smaller bodies. Robin Scagell, of the UK’s Society for Popular Astronomy commented after the vote: “All the illustrations of the solar system will have to be redrawn. But Pluto was always the odd one out, and this decision clears up the 76 years of doubt since Pluto was discovered.” Photo: IAU/Lars Holm Nielsen