The Case for Pluto

By their very nature, blogs are intended to address controversial issues and stimulate discussion, so I really appreciate everyone who has written so far to share their thoughts. If had to venture a guess, I suspect that the large majority (and perhaps all) of the several hundred astronomers at the IAU who voted to exclude Pluto based their decision strictly on scientific reasoning. But given the tense and changing international climate, and how the US government is perceived around the world, I can understand why some American astronomers at the IAU might have genuinely perceived an anti-American bias whether or not such a bias actually existed.

Pluto advocates are being accused of misrepresenting the IAU's planet definition in order to advance their agenda. But I can understand why they are upset with the IAU's definition. Among other reasons, the definition went against astronomical precedent by including a clause pertaining to whether an object can clear out its neighborhood. Here's why I think that was a mistake, and why I don't think Pluto should suffer the ignominy of having the word "dwarf" officially and permanently attached to its status.

Clearly, Pluto is a different type of object than the 8 solar system planets in the official IAU definition. But if we look at those 8, we see an extreme range of diversity as well. Mercury and Jupiter differ in mass by a factor of 5,750, and in volume by 25,000. Their compositions could hardly be more different. Jupiter's composition is more like that of a star; it's a giant ball of mostly hydrogen and helium. It also has a family of at least 63 moons, and several tenuous rings. In contrast, Mercury is a ball made of heavy elements, with no appreciable atmosphere and zero moons. Mercury is more than 13 times closer to the Sun. About all that Jupiter and Mercury have in common is that they are spherical objects orbiting the Sun. So if astronomers are comfortable lumping Jupiter and Mercury into the same category, it's not at all obvious that Pluto should be excluded from this club.

Yes, Pluto is embedded in the Kuiper Belt, and it certainly does not dominate its region of space, but its spherical shape, multiplicity of moons, and an atmosphere are properties that give it some commonality with the official 8 solar system planets. Let's consider three other classes of astronomical objects — galaxies, stars, and black holes — and we'll see an even greater diversity of characteristics, and we will see that these categories are not based on an object's location or gravitational influence.

First, let's examine galaxies. On one extreme, we see enormous elliptical-shaped galaxies that are much larger than our Milky Way, such as M87, which anchors the giant Virgo Cluster. Some of these assemblages contain trillions of stars, and have converted almost all of their gas into stars. Astronomers also see stately spirals like the Milky Way and Andromeda, which contain hundreds of billions of stars, and which are still actively churning out new stars. At the bottom end, astronomers studying Sloan Digital Sky Survey data have found about 10 new dwarf galaxies orbiting the Milky Way. These galaxies contain only a few tens of millions of stars, and appear to be even more dominated by dark matter than larger galaxies. Almost all of these dwarfs are destined to be tidally shredded and devoured by the Milky Way. But despite their vast range in sizes, shapes, stellar populations, and locations, astronomers have no problem lumping all of these objects together under the term "galaxies" because they share certain properties — they are collections of large numbers of stars that are spread out over a very large volume of space. We might call a small galaxy a "dwarf galaxy," but it's still a "galaxy."

Stars range in mass from blazing beacons such as Eta Carinae, which contains roughly 100 solar masses and shines with the luminosity of about 5 million Suns, to red dwarfs, which have only about one-twelfth of a solar mass and shine with the feeble glow of about 1/100,000th of a Sun. Some stars are only the size of Jupiter, others have swelled to such enormous diameters that their outer envelopes would extend to the orbit of Jupiter if they replaced the Sun. Some stars are located in dense star clusters, and others have been flung out of their parent galaxies into the lonely depths of intergalactic space. Some stars are solitary, but many others reside in binary or higher-order multiple systems. But despite their extraordinary wide range of sizes, masses, luminosities, and locations, astronomers lump them together because all of these objects fuse lighter elements into heavier elements in their cores. So a red-dwarf star is still a "star." (For the sake of brevity and simplicity, I’m leaving out collapsed stars such as neutron stars and white dwarfs, and brown dwarfs.)

Black holes exhibit an even wider range of masses, about as extreme as it gets in the universe. At the center of large galaxies such as M87 are monsters that pack a billion or more solar masses into a volume of space no larger than our solar system. Some of these monsters accrete matter at such furious rates that they become incredibly active, producing jets of particles traveling at near light-speed across thousands of light-years of space. These black holes are expected to live for 10100 years, a span of time so immense that no human mind can fully grasp its meaning. At the other extreme, many physicists think that tiny black holes were forged in the very early universe, and some might be created even today when ultrahigh-energy cosmic rays slam into Earth's atmosphere. These black holes have the mass of a heavy atomic nucleus, and live for a minuscule fraction of a second before exploding into a shower of subatomic particles and radiation (the so-called Hawking radiation). Yet because all of these objects share similar properties (the presence of event horizons and singularities), astronomers call all of them "black holes."

Seen in this context, it makes perfect sense to call all spherical objects (from hydrostatic equilibrium) that directly orbit stars "planets," and this is close to the position that the world's largest body of planetary scientists strongly endorsed a few weeks ago. If astronomers don't make a distinction based on location, mass, and size for galaxies, stars, and black holes, why should planets be different? Why should it matter whether a spherical object orbiting a star can clear out its zone of space? A galaxy is a galaxy whether it dominates its cluster or is being devoured by a bigger galaxy. A star is a star whether it's alone, inside a cluster, or part of a binary. Just because a 50-solar-mass star can't eject a red-dwarf binary companion doesn't mean that we stop calling it a star! As described above, astronomers have historically categorized almost all classes of objects by their properties, not their locations. So if Mercury and Jupiter are considered similar enough to fall under the same category, it's not crazy or unscientific to think that Pluto should also be included, especially since Pluto shares the same properties with Jupiter and Mercury that give these two objects their commonality. And Pluto is in no danger of exploding or being devoured. Sure, call Pluto a "dwarf planet," but it should still be considered a "planet" with no officially mandated qualifiers. From Jupiter's perspective, Mercury and Earth are dwarf planets as well.

The astronomers who voted at the IAU to boot out Pluto were basing their decision on legitimate scientific principles that have considerable merit. But those on the other side have equally strong arguments. That’s why this issue is so contentious. By creating objects with a continuum of sizes and other properties, Mother Nature didn't make it easy for astronomers, and both sides can summon reasonable and valid arguments. So let's all take a deep breath and appreciate Mother Nature's wondrous diversity, and take time to understand and respect other peoples' points of view.

Update on September 7: Pluto has been assigned the number 134340 from the Minor Planet Center. The trans-Neptunian object announced last year that is even larger than Pluto, 2003 UB313 (informally known as Xena), has been assigned the number 136199.