Pluto, seen in the illustration above from the perspective of one of its newly-discovered moons, is in the middle of a controversy regarding just what a “planet” should be. (credit: NASA, ESA, and G. Bacon (STScI))

Common sense planets

by Joseph Baneth Allen
Monday, November 21, 2005

What constitutes a planet?

It’s a question that a special committee of the International Astronomical Union (IAU) has been struggling to answer for nearly two years with no clear resolution in sight. Working diligently ever since the discovery of Sedna in the outer reaches of the solar system back in November 2003, the 19-member IAU committee has yet to reach a majority consensus on a simple definition that outlines the basic characteristics of a planet. What should have been a relatively simple question to answer is now threatening to turn all IAU members into very public laughingstocks.

Most elementary school children already know that a planet is a rocky and/or gaseous gravity-bound sphere that orbits a star. Members of the IAU special planet definition committee seem to be unable, or unwilling, to grasp this simple grade-school concept. The end result, after about two years of probably highly contentious meetings, is three possible definitions of what constitutes a planet. In very public airings of the committee’s very dirty laundry, it’s aptly apparent that no simple majority of the IAU can agree on just one definition of what a planet is.

Expanding the number of planets in our solar system is one of the proposals under consideration by the IAU. A planet-wannabe’s size counts in this proposed definition. If a piece of Solar System real estate has a diameter of around 1200 kilometers or greater, it gets rewarded with the scientific label of planet. If this definition were accepted, astronomical maps of the Solar System would have to be redrawn to include Sedna, Quaoar, 2003 UB313, and other trans-Neptunian planetary bodies that are larger and smaller than scrappy little Pluto. Ceres and other spherical asteroids could also be promoted from asteroid to planet status under this classification system. The number of planets in the Solar System could jump from the traditional nine to 20 or more.

Solar system maps would also have to be readjusted if the second proposal of demoting Pluto from the status of planet that it hass enjoyed for 75 years since its discovery. Pluto would become a Kuiper Belt Object (KBO) under this definition that defines a planet as being the dominant body in its immediate neighborhood. Planetary mass, size, and orbital dynamics are the properties that make up this second proposed definition. Charon, the moon gravitationally bound to Pluto, is conveniently overlooked in this definition. The global public outcry the IAU faced the last time it tried to demote Pluto to KBO status is also overlooked in this definition of what a planet is and isn’t.

The third planet definition proposal takes into account the special sentimental place in the public’s heart that the ninth planet enjoys in a most peculiar way. In this proposal, the scientific nomenclature of planet is done away with entirely to make room for subcategories of descriptions such as terrestrial, historical, and cultural planets. Imagine all the fun late night comedians will have if the IAU adopts this proposal.

So what constitutes a planet?

Astronomical photographs of the familiar planets of our old solar system perpetrate a comfortable lie: planets are giant round spheres that follow predictable Keplerian orbits around a common yellow dwarf star. Yet planets are not the perfect spheres of rock and atmospheric gases that we imagine them to be.

Our own Earth has a potbelly. Like many an aging baby-boomer, Earth has a bulge in its equatorial waistline due to the forces of gravity and axial spin. Jupiter’s clouds rotate in vastly opposite directions. Venus rotates backwards. Ganymede and Titan. both moons of gas giant planets, are larger than Mercury, the nearest planet to the Sun. Super-terrestrial rocky worlds and “hot Jupiters” are bound in weird orbits in other solar systems light-years away.

Defining what and what isn’t a planet depends on your view of what Pluto is. Tiny Pluto’s status as the ninth planet has always had one damning strike against it. Discovered by Clyde Tombaugh, a self-educated west Kansas farm boy born in Streator, Illinois, Pluto is truly America’s planet.

America’s planet

Clyde Tombaugh’s life’s story reads as if it came directly out of the pages of one of Horatio Alger’s many novels.

Despite having his hopes of attending college dashed when a hailstorm destroyed the family farm’s crops, Tombaugh continued to study on his own, teaching himself solid geometry and trigonometry. A turning point for him came when he turned 20 and decided to build a telescope. It wasn’t a great telescope. It was such a poor one that he decided to master optics and build himself a bigger and better telescope. He learned how to grind his own lenses and mirrors, and two telescopes later, he was satisfied with the results. His father took a second job to help him pay for the materials he needed to construct his telescopes.

Using his telescopes to study Mars and Jupiter, Tombaugh made highly detailed drawings of his observations since amateur astronomical cameras were still decades away from widespread use. He then took the bold step of sending his planetary drawings to the astronomers on staff at the Lowell Observatory in Flagstaff, Arizona. The astronomers at Lowell were so impressed with the young amateur's powers of observation they invited him to work at the Observatory.

Tombaugh’s highly detailed powers of observation led him to discover Pluto in 1930. America quickly embraced Pluto and Tombaugh as icons worthy of scientific superstardom, and the rest of the world quickly followed suit. Walt Disney even immortalized Tombaugh’s historic discovery in animation by naming Mickey Mouse’s faithful canine companion after the tiny world.

With his place in astronomical history seemingly assured, Tombaugh realized his dreams of higher education in 1932 when he entered the University of Kansas and earned his Bachelor of Science degree in 1936, eventually going on to earn a doctorate.

Common sense planets

Tombaugh’s triumphant discovery of Pluto has never set well in some scientific circles: the ninth planet is a tiny oddball discovered by a relative outsider, and elitist snobbery will always exist in scientific circles. Most IAU members give the appearance of not caring about a member of the solar system that still has great sway over the public imagination. Even though the bigger 2003 UB313 was discovered by another American astronomer, it hasn’t caught the public’s imagination the same way Pluto has. Pluto may not be massive enough to influence the orbits of Neptune and Uranus, but it was discovered by a young man whose life story should be held up to inspire children everywhere who want to be scientists, regardless of their socioeconomic status.

Pluto is the everyman’s planet. Members of the IAU special planet definition committee should stick to the classic textbook definition of what a planet is: a rocky and/or gaseous gravity-bound sphere that usually orbits a star. Sure, solar system maps will have to be expanded outward—yet that is the direction good science and common sense is supposed to take. Demoting Pluto to KBO status or eliminating the nomenclature of planet will only diminish the IAU in the eyes and hearts of the public.

Pluto should be a beginning to what the solar system truly is. Not an end.

A Florida-based freelance science writer, Joseph Baneth Allen has written for Popular Science, Omni, Astronomy, USA Today, Boys’ Life, Canadian Geographic, Final Frontier, Astronomy Now, American Profile, and many other magazines and newspapers.

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