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Pluto got a raw deal
November/December 2006
by David L. Rabinowitz '83

David L. Rabinowitz is a research scientist at the Yale Center for Astronomy and Astrophysics.

By astronomical tradition, anyone who discovers a celestial object has the privilege of proposing a name for it to the International Astronomical Union. But the members of the IAU cannot name something if they have not agreed on what it is. Therefore, although it was in July 2005 that my two colleagues and I announced we had discovered the most distant object known to orbit the sun, it was only this past September—after international debate had erupted over the definition of a planet and after Pluto had been purged from the planetary ranks—that the new object received its official name. (Up until then, we called it by the nickname Xena.) My colleagues and I chose “Eris,” after the Greek goddess of strife and discord. For Eris’s moon, we chose “Dysnomia”: Eris’s mythological daughter, the personification of lawlessness.

Eris figured heavily in the August IAU vote that demoted Pluto. Neither I nor Eris’s other co-discoverers—Mike Brown of Caltech and Chad Trujillo of Hawaii’s Gemini Observatory—were eligible to vote. But I believe the IAU made a mistake.

Eris, an icy ball slightly larger than Pluto, resides in the Kuiper Belt—a loose collection of sun-orbiting, relatively small bodies beyond Neptune. Pluto also resides in the Kuiper Belt, but until 2003, when Brown, Trujillo, and I began a comprehensive survey of the solar system, no known objects rivaled Pluto in size or brightness. Now that we have discovered Eris—as well as the slightly smaller Sedna and a few additional large bodies—it is clear that there are other objects of Pluto’s dimensions within and beyond the Kuiper Belt. This is why the IAU created a new category of solar system bodies, the “dwarf planets,” along with a new definition of “planets” that excludes Pluto.

In my opinion, the IAU was too narrow-minded. The assembly initially came up with a more open-minded definition whose main criterion was size. A planet, it proposed, must be big enough for gravity to force the body to be round. Pluto would have remained a planet, and several newcomers could have joined the club, including Eris; Ceres, the largest asteroid between Mars and Jupiter; Charon, Pluto’s moon; and many of the largest bodies beyond Neptune. There was a backlash against this proposal at the IAU general assembly. Astronomers who have long argued for the demotion of Pluto drafted new definitions. A vote by the 424 astronomers who stayed to the end of the meeting—out of the 2,400 present at the beginning—added the further criterion that a planet must orbit in a zone not shared by any other solar system bodies. The reasoning is that a newborn planet both accretes and expels all the gas, dust, and smaller bodies in the disk of material from which it originally forms.

This is physically reasonable, and it describes most of the planets in our own solar system. But it does not appear to apply to all of the large planets around other stars; some of these have wildly eccentric orbits that bring them to within scorchin g distance of their parent stars. Even in our own solar system, the dwarf planets we now see in the Kuiper Belt probably formed between Neptune and Uranus but were later driven out by near-collisions with these giant planets. It is not reasonable to expect a planet, even of large stature, to clear out its current neighborhood if it did not originate there.

Both definitions have some scientific justification. But I believe the one the IAU ultimately passed was popular simply because it kept the total number of planets low.

The new definition has no implications at all for the solar system research Brown, Trujillo, and I have undertaken. Whatever you call the distant, Pluto-sized bodies we are finding, they are intriguing because of what they tell us about the formation and evolution of the solar system. For instance, we now know that the sea of planetesimals from which the giant planets formed included many Pluto-sized bodies and was dense enough to provide multiple moons for these small planets. (Let me give some credit here where it is due: we were able to find these bodies because of the Palomar-QUEST camera, one of the world's largest digital cameras. It was built under the direction of Charles Baltay, the Eugene Higgins Professor of Physics at Yale.)

We still don’t know the size limit of these cast-out members of the early solar system. The farther we look, the larger the volume of space we explore and the greater the likelihood that we will find the extreme members of the population. Could there be a Mars-sized body at twice Eris’s distance, waiting for us to find it? We are looking for it. Will it be called a planet or a dwarf planet? The terminology will hardly make a difference.

Personally, I would have made an exception for Pluto. Many scientists have made a career studying the ex-planet. Ever since its discovery in 1930 by Clyde Tombaugh, it ha s been an enigmatic outsider whose frozen surface reveals the composition of the primeval solar system. Pluto easily outshines all other known objects beyond Neptune, owing to its currently closer proximity. It is the well-deserved destination of the New Horizons spacecraft, now on route to an encounter in 2015. It is easy to see why hundreds of planetary astronomers (including me) have signed a petition rejecting the new IAU definition. What Pluto lacks in size, it makes up for in significance. It remains the jewel of the Kuiper Belt.

Who said “Foolish consistency is the hobgoblin of little minds"? Probably not an astronomer.  

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