A review of David A. Weintraub’s Is Pluto a Planet?
Posted by Darin Hayton on 12/18 at 01:25 AM
Spoiler: David Weintraub is committed to the idea that Pluto is a planet. Granted, he also thinks there are a handful of other objects that also merit that distinction, e.g., Ceres, Pallas, Vesta, Hygeia. He declares it at least eight times in the last three pages: Pluto is a planet (his emphasis). Not a minor planet. A planet. But also more than a planet. Far from being demoted, in Weintraub’s eyes, Pluto should be promoted: “Pluto meets all the requirements to be a planet, yet Pluto is much more than just a planet … Pluto is a planet and a Plutino and a KBO.”
Alright, now that we have that out of the way, let’s look a little more closely at Weintraub’s argument. He proceeds from the ancient Greek model that posited seven planets—the moon, sun, mercury, venus, mars, jupiter, and saturn—orbiting a central earth. He blames tradition (here tradition means intellectual lethargy and lack of ability to question accepted authority) and the Church for promoting and defending this model. Copernicus, of course, published a revised model that elevated the earth to the status of planet while reclassifying the sun as a star. Despite our preference for this model, it was more than 50 years before it gained much acceptance. Tycho Brahe, Johannes Kepler, and Galileo Galilei all play important roles in Weintraub’s story as stepping stones to the modern, heliostatic model.
The 17th century was, for Weintraub, and important century. Early in the century Galileo discovered four Jovian moons, which Weintraub claims were classified as planets. Later in the century, Christiaan Huygens and Jean-Dominique Cassini discovered five moons orbiting Saturn. He tells us that all of these were considered planets, giving a total of 16 planets by the end of the century.1 The other significant development in the 17th century was Kepler’s success in determining the laws of planetary motion.
The 18th century witnessed the formulation of the Titus-Bode rule and the discovery of Uranus. Weintraub makes much of the Titus-Bode rule as a means of distinguishing planets—objects orbiting the sun at distances that fit the Titus-Bode rule are planets. William Herschel discovered Uranus while trying to determine the distance of the stars. Then, in the middle of the 19th century John Adams and Urbain Le Verrier used the irregularities in Uranus’s orbit to calculate the position of an additional planet. J. G. Galle and H. L. d’Arrest, staff astronomers at the Berlin Observatory, first observed Neptune in September 1846. The 18th and 19th centuries also enjoyed the discovery of a number of other objects which, in the end, turned out to be imaginary.
At the beginning of the 20th century, William Pickering and Percival Lowell were on a mission to find a trans-Neptunian planet. Using what they thought were irregularities in the orbits of Uranus and Neptune, they calculated into existence a number of planets to account for the perturbations. In 1930, after Lowell had died, Clyde Tombaugh discovered Pluto, which was too small and too distant to have any effect on the orbits of the other planets. Over the course of the 20th century, a number of other objects and features of the solar system have been discovered: Kuiper Belt Objects, large asteroids in the belt between Mars and Jupiter, the Oort comet cloud.
For Weintraub, the point of this romp through history is to show that definition of planet has changed considerably over time and to argue that any attempt to define planets by physical characteristics has run into difficulties. Given that, he settles on a definition of a planet that he thinks is based solely in astrophysics:
Objects that orbit a star and are small enough not to generate heat through fusion but large enough to acquire a spherical shape under the force of gravity.
This definition allows for Pluto and Ceres and their relatives. Apparently, the IAU’s little caveat about clearing the area around it (a planet “has cleared the neighbourhood around its orbit”) is not important to Wientraub nor should the inclination of a planet’s orbit be limited to a certain number of degrees from the ecliptic.
Despite his claims to adhere only to astrophysics, Weintraub is more persuaded by tradition, as he reveals toward the end of his book: “Pluto has been a planet for three-quarters of a century” and should therefore remain a planet. Invoking history, however, is problematic. The moon was a planet for millennia, as was the sun. Sure, we’ve adjusted our understanding of the solar system such that we don’t consider them planets any longer. But the same could be said of Pluto. It looks more like a Kuiper Belt object than a planet, just like the sun looks more like a star than a planet. We reclassified the sun, why such reservation about reclassifying Pluto?
Weintraub’s book is worth reading. Not because he will convince you that Pluto should be a planet—you will already have decided that issue for yourself. It is worth reading because it shows just how emotional people get about the the planetary status of Pluto. It reveals a lot about the irrationality of science (at least astronomy). It is a nice reminder that scientists are no more rational than anybody else.
1A closer examination of the use of languages at the time would probably reveal that what was meant by “planet” varied with use. Galileo originally called the moons of Jupiter the “Medicean stars,” though he certainly didn’t think of them as stars in the same way that the sun was a star.⇑