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The beginning of wisdom

Is Pluto a Planet? A Historical Journey Through the Solar System

Princeton University Press: 2006. 248 pp. $27.95, £17.95 0691123489 | ISBN: 0-691-12348-9

Pluto carries much the same sentimental, emotional and historical overload as Father Christmas. Even the name for this frozen dwarf, suggested in 1930 by 11-year-old Venetia Burney in Oxford, UK, is evocative, having mythological connections. And one of man's best friends appears as Disney's Pluto of the same vintage. It all makes for a heady cocktail.

After 76 years, astronomers still argue over the status of Pluto (represented here with its moon Charon). Credit: D. VAN RAVENSWAAY/SPL

Discovered by accident by Clyde Tombaugh, Pluto is a legacy of the obsession of Percival Lowell (he of the martian canals). It might otherwise not have been discovered for another 50 or 60 years, in which case the question of whether Pluto is a planet might never have arisen.

The discovery of Pluto's relatives, the trans-Neptunian objects (TNOs), or Kuiper Belt objects as they are also known, is reminiscent of the history of the asteroid belt. Ceres, located in 1801, was hailed as the missing planet between Mars and Jupiter that was long predicted by the Titius–Bode law. But its trivial size and the rapid discovery of Pallas, Juno and Vesta in the vicinity suggested that the true situation might be more complex. It was not until 1845 that other bodies were discovered, and better telescopes led to a deluge, relegating the four new planets to the status of minor planets or asteroids. As they did not display a common origin point, even the notion that they were remnants of a shattered planet vanished.

Why, then, did it take so long to discover the predicted swarm of TNOs beyond Neptune? Probably because Pluto has such an eccentric and inclined orbit that no one was dedicated enough, or had sufficient funding or energy, to repeat Tombaugh's arduous search technique. Although Pluto's moon, Charon, was found in 1978, it took David Jewitt and Jane Luu until 1992, searching with modern instruments, to discover the next body in the Kuiper Belt — a classic case of finding a needle in a haystack. Now we are overwhelmed with TNOs.

The story is well told in Is Pluto a Planet? by David Weintraub, which provides a readable historical account of our knowledge of the Solar System and the concept of what has been considered to be a planet. Fashions have changed with time, and the number of planets has ranged from 6 to 24. Specialists will note that the author passes over the questionable acquisition by Johannes Kepler of Tycho Brahe's data, which Kepler himself admitted to “usurping”. And Uranus was discovered after a careful search of the heavens by William Herschel, arguably not by accident, but it was probably inevitable. I also found no mention of Burney, who now forms part of the mythology.

A concluding appendix gives a useful discussion of the properties of Pluto and Charon, preceded by the views of various astronomers and planetary specialists on what should constitute a planet. As always, these reveal as much about the commentator as the problem. I found the most perceptive was from Brian Marsden: “It has rarely been scientifically useful to use the word [planet] without some qualification.”

Towards the end of this interesting book, Weintraub surprisingly concludes, despite the close analogy between the discovery of the asteroid and Kuiper belts, that we should retain Pluto as a planet by using the three physical parameters of orbital characteristics, mass and roundness. Although he admits that “we know that Pluto earned its status as a planet by accident”, this suggestion accords Pluto a kind of royal status. The application of his criteria leads him to a total of 24 planets. This number is bound to grow and the classification is too broad to be scientifically or even culturally useful. As The New York Times has remarked, “too many planets numbs the mind”.

Trying to define a planet runs into the philosophical difficulty of attempting to classify any set of randomly assembled items. A bewildering array of objects formed in the early solar nebula, including dust, asteroids, Trojans, centaurs, comets, TNOs and our eight planets, ranging from tiny Mercury to mighty Jupiter. All differ from one another in some salient manner, as do the 160 satellites. The key question is how did they form and evolve, not what pigeonhole can they be forced into. So the decision of the International Astronomical Union that there are eight major planets and a host of minor planets seems an appropriate compromise. But even so, qualifying terms, such as ice giants and Earth-like planets, will always be needed. We need, then, to recall the wise words of Confucius: “The beginning of wisdom is to call things by their right names.”

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Taylor, S. The beginning of wisdom. Nature 444, 1006–1007 (2006).

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