Is Pluto A Planet?

Title: Is Pluto A Planet? A Historical Journey Through The Solar System

Author: David A. Weintraub

Publisher: Princeton University Press 2007 (First)

ISBN: 978-0-691-12348-6

Pages: 241

The status of Pluto was a raging debate among astronomers from the last decade of 20^th century. As knowledge on the outermost regions of solar system began accumulating, and more and more Pluto-like objects were observed by the end of the millennium, the newest planet’s fate hung in balance. The issue was settled by the general body meeting of International Astronomical Union (IAU) held at Prague in Aug 24, 2006. By a majority vote of the members present, the IAU demoted Pluto to a minor planet, making the total count of planets in the solar system to eight. This book, though published a year later than IAU’s fateful decision, does not include the crucial last scene and is a glaring omission. The book ends with the author’s speculations about the rightful status of Pluto and the opinions of other prominent astronomers. Perhaps future editions of the work would settle the issue so that the reader will not be in any doubt.

As suggested by the title, the book provides a nice historical journey through the origin of the concepts of solar system and other heavenly bodies in ancient societies. Ancient Greek astronomers like Aristarchus, Eratosthenes and Hipparcus produced accurate reports on celestial bodies. Hipparcus’ calculations were very precise regarding the duration of the year and earth’s diameter. He could predict eclipses with an accuracy of one hour! Aristotle proposed the geocentric model of the universe and Ptolemy’s mathematical basis for it helped it to stand critical scrutiny for the next 1500 years. Aristotle was held in supreme authority by the church in the middle ages and no challenge to conventional wisom was allowed. Things began to change in the Renaissance era. Machiavelli’s diatribes against Aristotelian order in politics and Vesalius’ advances in medical concepts paved the way for a break with the past. Crowning it all came Martin Luther’s Protestant Reformation which challenged church authority. Copernicus introduced his heliocentric ideas during this time, which was opposed by philosophers who stood on the right side of the church fathers. Ludovico delle Colombe summarized biblical phrases to ‘prove’ that Copernicus was wrong, which makes interesting reading today.

1. You fixed the earth on its foundation, never to be moved (Psalms 104:5)
2. He stretches out the North over empty space, and suspends the earth over nothing at all (Job 104:7)
3. Tremble before him, all the earth; he has made the world firm, not to be moved (1 Chronicles 16:30)
4. Who has cupped in his hand the waters of the sea, and marked off the heavens with a span? Who has held in a measure the dust of the earth, weighed the mountains in scales and the hills in a balance? (Isaiah 40:12)
5. Stone is heavy, and sand a burden, but a fool’s provocation is heavier than both (Proverbs 27:3)
6. The sun rises and the sun goes down; then it presses on to the place where it rises (Ecclesiastes 1:5)

Galileo helped astronomy leapfrog into the modern world. He observed four moons of Jupiter, which were also considered as planets then. Cassini and Huygens discovered the five moons of Saturn and the number of planets were steadily going up. There were no clear definition of a planet at that time, rather than the ‘wandering star’ assigned from time immemorial. The general consensus was that it should orbit the sun. But, discovery of Halley’s comet complicated matters. Here was a celestial body observed from ancient times, which is clearly not a planet, but revolving around the sun! A bailout from the dilemma was facilitated by the formulation of Titius-Bode rule which predicted the orbits of planets around the sun which surprisingly corresponded to the known planets then. By the help of this rule, the number of planets again came down to six. The rule predicted a planet at 19.6 AU from the sun, which was realized when Herschel observed Uranus in 1781. The rule also predicted a planet in between Mars and Jupiter. The discovery of asteroids in the early 19^th century made some astronomers designate them as planets since they obeyed the Titius-Bode rule. Diligent astronomers detected an unexplained deviation in Uranus’ orbit at that time. Mathematicians, most prominent among them being John Couch Adams and Urbain Le Verrier theorized this to be due to the gravitational tug of an unknown planet and calculated the orbit of this new planet. Neptune was observed in 1846 at the precise location predicted by the mathematicians.

Astronomers again found discrepancies in Neptune’s orbit and suspected this to be due to the influence of another undiscovered planet. An American millionaire, Percival Lowell started search for such a planet, founding an own observatory, which later shot to prominence as Lowell Observatory. The search for the trans-Neptunian planet bore fruit in 1930 when Clyde Tombaugh discovered Pluto in that observatory. However, Pluto’s observed size was far too small to produce the deviations in Neptune’s orbit. Later, it became clear that the Neptunian discrepancies were due to observational errors. Pluto’s position became dubious. After 1992, a flood of Pluto-like objects were ushered in, with the discovery of minor planets like Varuna, 2003UB313, which is even larger than Pluto. Such objects in the Kuiper Belt are called Kuiper Belt Objects now.

Weintraub doesn’t clearly state his position on the question of Pluto’s planethood. He lists three conditions to be satisfied by a planet.

1. the object must be too small to generate, or to have ever generated, energy through nuclear fusion
2. the object must be big enough to be spherical
3. the object must have a primary orbit around a star

Many asteroids like Ceres and minor planets like Varuna satisfy these conditions and the author is equivocal in granting the status of planets to all of them.

The book is impressive to read in the first part. After the discovery of Pluto, the reading gets tougher and less interesting. The author struggles to move the discussion forward and his reliance on scientific papers, listing out all the others doesn’t help matters a bit.

The book is recommended

Rating: 3 Star