Across the Universe: Tradition… and Pluto

The imminent flyby of Pluto by New Horizons has brought that Dwarf Planet back into everyone's mind. The following column was written in 2006, just after the IAU had voted to name Pluto a Dwarf. It ran in The Tablet in August, 2006.

Explaining the voting procedure on Pluto in 2006 were Rick Binzel, Fr. Chris Corbally SJ (of the Vatican Observatory) and Jocelyn Bell Burnell

Explaining the voting procedure on Pluto in 2006 were Rick Binzel, Fr. Chris Corbally SJ (of the Vatican Observatory) and Jocelyn Bell Burnell

Never mind the 2,500 astronomers attending dozens of seminars and joint discussions about stars and galaxies in Prague; the news this week [2006] at the triennial General Assembly of the International Astronomical Union (IAU) is the changing status of Pluto.

General Assemblies are different from typical scientific congresses. Rather than being focused exclusively on presenting scientific results, the main point here is to decide on all the arbitrary but necessary definitions that let us talk to each other and understand each others’ data. For example, we’re worrying about tweaking definition of latitude and longitude on the Moon to match the the expected precision of the coming generation of lunar spacecraft from India, China, Japan, and the US.

Likewise, defining “what is a planet” is both arbitrary and necessary. How do we name the newly discovered objects out beyond Pluto, that rival Pluto in size? (The rules, and the committees, for planets are different than those for comets and asteroids.) Which committee keeps track of their orbits, and assigns names to their surface features? What definition works for planets around other stars?

I’d sat on earlier planet-defining committees, but always at the last moment our agreements fell apart. The proposal by the most recent committee (this time much smaller, and including historians and journalists as well as scientists) may already have failed by the time you read these words; the final vote is slated for Thursday afternoon. But following a meeting of IAU’s Division III, the planetary scientists’ corner of the IAU, a general consensus appears to be forming around a slight modification of the original proposal.

To be a planet, an object would have to be smaller than a star, in orbit around a star, but large enough to pull itself into a rounded shape. (The actual definition speaks of an approach to hydrostatic equilibrium, and other such technicalities.) One then divides the planets in our solar system into the “classical” eight largest planets, whose gravity dominates their regions of space, and a new class of less dominant, Pluto-like planets.

The name for the latter is still to be settled on. “Pluton” was suggested, but that word describes a lava mass in geology, and is already the word for Pluto itself in many languages. Other fanciful possibilities include “planetoid” or “plutenoid” or “plunet,” but I suspect in practice we’ll just call them “dwarf planets,” to go along with dwarf stars and dwarf galaxies in the astronomical bestiary.

More controversial aspects reported in the news coming from the original definition, worrying about double planets and the like, will likely be dropped in the final definition. They were originally just footnotes to the original proposal, in any event. And all the really tough issues, deciding which committees handle dwarf planets, including naming the new candidates in the outer solar system, will be left to us in Division III.

To me, the definition makes scientific sense: my own research shows a distinct difference between small but compact objects like Pluto and the loose rubble piles of asteroids. And one advantage of this definition is its creative ambiguity. In reply to the question, “Is Pluto a planet?” it will be equally true to say, “yes, it’s a dwarf planet” and “no, it’s a dwarf planet.” That reflects the ambiguity of nature itself.

Still, it all does have an aura of counting angels on a pin. Indeed, the entrenched positions of many astronomers (and the public) resisting change, and the feeling that somehow “the rest of us” were left out of the decision, reminds me a lot of what the Church went through during Vatican II. Like the post-Vatican II church, astronomy will survive… until these changes become such second nature to us, that we begin to think of them as tradition.

The full story behind the Pluto decision can be found in chapter two of our book, Would You Baptize an Extraterrestrial? Images and data from the New Horizons flyby will be sent back over the next few months, if all goes well; stay tuned here for the latest updates!

Br. Guy Consolmagno

About Br. Guy Consolmagno

Brother Guy Consolmagno SJ is Director of the the Vatican Observatory and President of the Vatican Observatory Foundation. A native of Detroit, Michigan, he earned undergraduate and masters' degrees from MIT, and a Ph. D. in Planetary Science from the University of Arizona; he was a postdoctoral research fellow at Harvard and MIT, served in the US Peace Corps (Kenya), and taught university physics at Lafayette College before entering the Jesuits in 1989.

At the Vatican Observatory since 1993, his research explores connections between meteorites, asteroids, and the evolution of small solar system bodies, observing Kuiper Belt comets with the Vatican's 1.8 meter telescope in Arizona, and applying his measure of meteorite physical properties to understanding asteroid origins and structure. Along with more than 200 scientific publications, he is the author of a number of popular books including Turn Left at Orion (with Dan Davis), and most recently Would You Baptize an Extraterrestial? (with Father Paul Mueller, SJ). He also has hosted science programs for BBC Radio 4, been interviewed in numerous documentary films, appeared on The Colbert Report, and for more than ten years he has written a monthly science column for the British Catholic magazine, The Tablet.

Dr. Consolmagno's work has taken him to every continent on Earth; for example, in 1996 he spent six weeks collecting meteorites with a NASA team on the blue ice regions of East Antarctica. He has served on the governing boards of the Meteoritical Society; the American Astronomical Society Division for Planetary Sciences (of which he was chair in 2006-2007); and IAU Commission 16 (Planets and Satellites). In 2000, the small bodies nomenclature committee of the IAU named an asteroid, 4597 Consolmagno, in recognition of his work. In 2014 he received the Carl Sagan Medal from the American Astronomical Society Division for Planetary Sciences for excellence in public communication in planetary sciences.

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Across the Universe: Tradition… and Pluto — 2 Comments

  1. On the July 10 episode of Science Friday on NPR, Alan Stern, the Principal Investigator for NASA’s New Horizon mission kept referring to Pluto as a planet. When challenged about this reference by the radio program’s host, Ira Flato, Stern said of course Pluto is a planet. He basically said Pluto was reclassified by bunch of astronomers who study stars and not planetary scientist who know about planets. I believe he also said that if one asked any planetary scientist they would tell you Pluto is a planet.

    You point out the IAU’s Division III of planetary scientists did have input.

    The book Would You Baptize an Extraterrestrial? states that after the vote was made to accept the designations of classical planet, dwarf planet (into which Pluto fit), and asteroid some American planetary scientists who do not participate in the IAU and/or did not attend its General Assembly felt Pluto was demoted.

    With the great excitement of being able to explore a world so far from us, the emotional residua from reclassifying Pluto remains.

  2. Pingback: Across the Universe: Planetary Counsels - The Catholic Astronomer

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