The Argument for Pluto: Why the Tiny Dwarf Is Still a Planet
Last week, the release of Mike Brown's new book, How I Killed Pluto and Why It Had It Coming (we published an excerpt), sparked a heated debate among professionals and amateurs alike that we haven't seen since, well, Pluto was delisted as a planet several years ago. Leading the charge was Laurel Kornfeld, a freelance writer and community activist who has made a name for herself on the Internet as the Pluto savior.
Kornfeld, who holds degrees in journalism, Middle East studies and English education from both Rutgers and Harvard, is working on a book that argues why Pluto should still be considered our ninth planet. An excerpt is published below. She also maintains a blog about Pluto.
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And of all planets in our solar system, Pluto is the strangest, the most different, the most enigmatic, one of the furthest away, the least known, the smallest, and the most fascinating. Combine all of these with a natural sentiment favoring the underdog, and the result is public fascination with this tiny underdog planet that by all odds, should not even have been discovered when it was, much less endowed with the honor of designation as a planet.
New discoveries in the last decade have changed the planetary landscape to the point that Pluto is no longer the furthest planet, even in our own solar system, or the smallest. But it is still the boundary of an unknown, unexplored frontier. The solar system, it turns out, is bigger and more diverse than we ever imagined. Moons of planets have been imaged and shown to be entire worlds in their own right, worlds that may even harbor microbial life in subsurface oceans. Our own Moon has been found to have water. And instead of being a lonely outpost at the solar system's edge, Pluto is now the gateway to yet a new region beyond Neptune, the Kuiper Belt. It is the next planet to be explored by a robotic flyby mission, New Horizons, launched in January 2006 and scheduled to rendezvous with its target in July 2015.
All of the above result in Pluto being, to many, the most fascinating of the planets, a class of objects that have themselves long inspired awe and wonder in generations of people. Those who argue that opposition to its demotion is based on sentiment ignore the fact that from the beginning, sentiments such as awe, wonder, and curiosity were what first motivated our ancestors' attempts to make sense of the universe. Science can never be completely separated from sentiment on any side of any debate.
Tiny Pluto has inspired powerful sentiments among many, both supporters and opponents of its planetary status. That sentiment is just as strong among professional astronomers as it is among lay people. To many, the little world that should be a gateway to new planet discoveries was instead used to slam the door shut on any such discoveries. It is as though we saw a door open and wanted to go through it to find new wonders only to have that door shut in our faces by those who want to limit the solar system to a small, manageable number of planets. No wonder many people responded with bafflement and even anger. Who are this group of so-called elites that purport to dictate arbitrary limits on what can be considered a planet? Why should their view take precedence over all others if it is based more on interpretation of the facts than on the facts themselves?
Many who had hoped to settle the issue of Pluto's status at the International Astronomical Union (IAU) General Assembly in 2006 remain baffled by an ongoing, intense public reaction refusing to accept the IAU dictate. Like a refrain, they constantly question, why do people care so much about Pluto, a tiny, frozen world 3.6 billion miles from the Sun?
Originally coined by Dr. Alan Stern to refer to objects large enough to be in hydrostatic equilibrium but not large enough to gravitationally dominate their orbits, the term dwarf planet was never intended to mean these objects are not planets at all. The problem arose because of a continuing insistence that there can be only two types of planets -- terrestrial or rocky planets, and jovians or gas giants. Dwarf planets do not fit into either category. Thus, some astronomers arrived at the erroneous conclusion that they -- and anything not a terrestrial or jovian -- are not planets at all.
But why must there be only two types of planets? Why can there not be three, four, five, or even fifty types? Some supporters of the position that dwarf planets are not planets create a false dichotomy by pitting the large moons of the gas giants against the dwarf planets. How can objects like Pluto and Eris be considered planets yet larger objects like Ganymede and Titan not be considered planets, they ask.
Once again, Dr. Stern has a practical, reasonable proposal. Spherical moons of planets should be designated as satellite planets. His proposal is not new. The large moons of the gas giants were often referred to as secondary planets during the 19th century. Secondary planets are objects whose secondary orbit is around the Sun and primary orbit is around another planet as opposed to primary planets, which orbit the Sun directly.
What makes the most sense, if we continue using the term planet, is keeping it broad to refer to objects in hydrostatic equilibrium that are not large enough to ever conduct fusion. A wide range of adjectives can easily distinguish among the many types of planets. Contrary to what may seem obvious, the term planet should not even require that the object in question orbit a star. A rogue planet is one that floats alone in space, one that at one time orbited a star but was somehow ejected from its solar system. There are also pulsar planets, which orbit the stellar remnants of massive stars. Just as we our universe is filled with stars, it is filled with planets -- primary planets, satellite planets, dwarf planets, sub-dwarf planets (a possible category for objects like Vesta and Pallas), and a wide variety of exoplanets, which orbit stars other than our Sun. This is a new chapter of the story Pluto "tells" us, provided we are open to it.
If we do not accept the concept of multiple categories of planets, we are left with a dilemma. Does an object become something else if it moves or migrates to another location? If an Earth-sized, spherical body once orbited a star but was then ejected from its solar system through gravitational interaction with another large body, does it stop being a planet once it no longer orbits its parent star? Neptune's moon Triton has features very similar to those of Pluto and Eris; some astronomers believe Triton once orbited the Sun directly and was subsequently captured by Neptune's gravitational pull. The fact that Triton orbits Neptune backwards -- opposite the direction in which Neptune orbits the Sun -- and is in an unstable orbit that will eventually result in its impacting Neptune -- strongly supports this capture theory.
Though it may have been captured by Neptune, Triton itself did not change as far as its structure and composition. Yet according to current usage, it went from being a planet to being a moon when Neptune pulled it into its orbit. Similarly, if a Mercury-sized object is discovered in the Kuiper Belt, according the IAU planet definition, it would not be considered a planet even if its composition were identical to that of the Mercury we know.
Even those who recognize only two categories of planet -- terrestrial and jovian -- are beginning to question lumping all four gas giants into the same category. While Jupiter and Saturn have compositions similar to the Sun and are largely made up of hydrogen and helium, Uranus and Neptune contain methane in their atmospheres and are believed to have icy cores of rock and metal. This knowledge has led some astronomers to distinguish the former as gas giants from the latter, which they label ice giants. The histories of the latter may be quite different from those of the former.
Ultimately, the 2006 IAU vote and the continued debate over Pluto are not about Pluto at all. They are about a clash between two very different but equally legitimate ways of understanding celestial bodies in the solar system -- and eventually in other solar systems as well.
Dynamicists focus on the dynamics, or interactions, among celestial bodies, whether they are planets, stars, or galaxies. Their emphasis is on the type and degree of influence one body has on another. Since large planets cause the most perturbations of other solar system bodies, including other planets, dynamicists studying the solar system view these as the objects on which to center their research. Small planets and asteroids, which are not large enough to perturb other bodies, are viewed by dynamicists as having barely more significance than debris.
Geophysicists, or planetary geologists, on the other hand, center their studies on individual celestial bodies. The size of these bodies is largely irrelevant, as is the question of whether they perturb other bodies. To a geophysicist, there are two significant thresholds relating to the size of celestial objects. For planets, that threshold is the minimum size at which an object is rounded by its own gravity, the state formally known as hydrostatic equilibrium. This is known as the Geophysical Planet Definition. For stars, the threshold is the minimum mass at which the interior of the object becomes hot enough to generate hydrogen fusion.
Neither approach to the study of celestial objects is wrong, as each provides insight into different aspects of these objects. Essentially, dynamicists focus on where these objects are and how they affect other objects around them while geophysicists center their research on what the individual objects themselves are.
To dynamicists in 2006, recognition of planet status for Ceres, Pluto and newly discovered Eris meant "opening the floodgates" to potentially hundreds of other similar small bodies in the Kuiper Belt, all of which would have to be given planet status too. That troubled those who genuinely believed that planets had to be "the big guys on the block," the few large enough to gravitationally dominate their orbits, and those who believed that the term planet would be devalued if not confined to a select few "special" objects. In contrast, it posed no problem for geophysicists who viewed all spherical objects orbiting the Sun as planets whether large or small, and favored using subcategories to distinguish between different planetary types. Some geophysicists even favor classifying the moons of planets as "satellite planets" if those moons are large enough to be spherical. Our solar system does not have two classes of planets, terrestrials and jovians, as previously thought, but three: dwarf planets are a third class, and they are the most numerous class.
Excerpted from a work in progress. Copyright 2010 by Laurel Kornfeld. Printed by arrangement with the author. All rights reserved.