Obscure object of affection

Last August, the press had a feeding frenzy over a normally unnoticed event: the general assembly of the International Astronomical Union. The subject was not a huge discovery - alien life, or the asteroid that will extinguish humanity - but an administrative rejigging of long-familiar knowledge.

The discussion, ostensibly a debate about what constitutes a planet, was personalised into an argument about one minute world, Pluto, which turned out to have an unexpected reservoir of public affection. The media regarded the IAU's decision to relegate it to a new category of "dwarf planet" with universal dismay.

This book was completed before that discussion took place. But David Weintraub sets the debate in its full context, and his views will be of interest to anyone who wants to know how our view of the universe around us has changed over time.

As he notes, the ancients identified seven planets - the Moon, the Sun, Mercury, Venus, Mars, Jupiter and Saturn - which roamed the night sky while the "fixed stars" stayed put. In the 16th and 17th centuries, the number of planets altered as new knowledge emerged. The Sun was reclassified as a star, the Earth was redefined as a planet, and four new planets were added to the total when Galileo discovered what we now regard as the largest satellites of Jupiter. Later, satellites, including the Moon, were placed in their own category, which now bulges with hundreds of members.

Weintraub points out that the recent ruckus over classification is trivial compared with these realignments. Another big rethink occurred in the 19th century. The solar system had already been reordered in 1781 when Uranus was discovered. But Uranus is unquestionably a planet. More of a problem was the discovery of Ceres, the first asteroid, in 1801, and the realisation that it was one of hundreds of similar objects. As not all could be called planets, terms such as "minor planet" and "asteroid" were coined.

The recent fuss about Pluto reflects a similar growing awareness that it too is far from unique. It is only one five-hundredth as massive as the Earth and far smaller than the Moon. And we now know that it is only one of hundreds of similar objects in the outer solar system.

Weintraub, who views science as a human activity, argues that we cannot escape our history and should continue to class Pluto as a planet, as we have more or less for the past 77 years. In his fascinating account, he cites some hideous schemes that have been proposed for reclassifying objects in the sky, including one in which the word "star" would be replaced by "fusor".

The IAU did not agree with Weintraub, but the solution it has produced is a graceful one. To be a planet, an object must orbit the Sun and have enough mass to form a more or less spherical shape, which rules out asteroids. But to be a full planet, not a dwarf, it must dominate its orbit in terms of gravitation. The Earth and Jupiter, say, do this; but the four dwarfs so far defined (Pluto, Ceres, Pluto's satellite Charon and the Pluto-like object Eris) do not. This definition will need more work. It avoids saying anything about planets of other stars, of which hundreds are now known, or about planets not orbiting a star, of which we now believe many must exist.

But it seems to be fair. After all, people have always used adjectives with the word "planet", calling Mars a terrestrial planet and Uranus a gas giant planet. This new system formalises an arrangement that has long existed.

Pluto never was a planet in the same way that Venus or Neptune are. And a dwarf planet is still a planet.

Martin Ince is contributing editor of The Times Higher .