Across the Universe: Planetary Prejudice

This column first ran in The Tablet in July 2015

Credit: New Horizons Image of Pluto - NASA

The wonderful excitement about Pluto, visited by the New Horizons spacecraft [July 2015], has resurrected the old issue of defining a “planet”.

Many Very Erudite Mathematicians Just Say the Usual Numbers, Probably. Here, Michael Ryschkewitsch of APL and New Horizons Educator Fellow Sally Jensen, who insist that Pluto is still a planet, are shown here as they model spacecraft hats while giving the nine-finger "Pluto salute."

But why? Most people approaching this question have one clear goal: they want Pluto to be a planet. Once you realize that, you can make your definition clear and simple: “A planet is one of the bodies that I was taught was a planet when I was a child.” Of course, such a definition is useless for any other purposes.

The IAU, which defined Pluto and similar bodies as “dwarf planets” back in 2006, needed a definition so it could name such objects and the features on them, to know whose committee and what set of rules will apply. But there’s another aspect to this issue.

Fifteen years ago I was involved in a research program studying the Trans-Neptunian Objects (TNO) that orbit alongside Pluto, comparing their shapes and colors with where they orbit. But I kept running into the inconvenient fact that Pluto, which certainly needed to be included in our study, wasn’t listed in any of the tables of KBO orbits. Why? Because back then it was a planet, following a definition maintained mostly to make children happy.

Looking up one extra set of orbits wasn’t so hard, back then. But today we know of easily a dozen TNOs big enough to be “planets” like Pluto, with more to come, no doubt. I can’t be sure I have them all, if I have to go looking in separate tables.

Many definitions suggest that bodies with “interesting geology” (beyond mere craters) should be called planets. A similar suggestion says a planet is any object big enough for its gravity to pull itself into a sphere. That would include our Moon and many other planets’ moons, some much larger than Pluto.

A ball of ice that’s 500 kilometers in diameter can pull itself into a sphere. A ball of iron the same radius would be strong enough to hold itself up against its own gravity. But given their relative densities, the iron body could have eight times the mass as the icy guy. Would we really want to say the massive iron body is not a planet, but the icy body is?

Asteroid Vesta is covered with lavas, suggesting that it has had interesting geological processes in its past. But today you only see craters on its surface. And given its extensive battering by other asteroids in its early history, Vesta is visibly non-spherical today. But back when it was melted, it would have been quite spherical. Is planethood something that changes with time? (In fact, my colleagues and I have proposed that Vesta’s history is even more complicated; it could be a re-accreted collection of lavas from a different, or once larger, asteroid.)

Furthermore, geologists aren’t the only scientists playing in this sandbox. Before we thought about planets as places, we knew them as bodies with interesting orbits. And there is a clean divide between the eight bodies in our solar system whose orbits are stable against perturbations, and bodies (like Pluto, sorry) whose orbits are controlled and moved about by their larger neighbors.

Indeed, the “bones” of a solar system — the complex gravity fields that define what is stable and what isn’t — reflect how the solar system was formed. These orbits are not just curiosities. They are fundamental to what our solar system is, and how it got to be that way. By contrast, there is nothing fundamental about Pluto’s orbit. If Pluto weren’t there, the other planets would never notice.

Besides… Pluto doesn’t need to be a “planet” like us, to be worthy of our affection, or our study. That’s a form of cultural prejudice. Planetary scientists aren’t the only ones guilty of that.

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: Planetary Prejudice — 2 Comments

  1. Br. Guy:

    This is one time that I do have to disagree with you! The question about how we classify Pluto and other objects in the Solar System and beyond is more than just wanting Pluto to be a planet. It is more about how we do science and how scientists and humans classify the things around us. A scientist does not classify things based on just one characteristic. He or she creates a list of characteristics and then determines if a given thing has some or all of those characteristics.

    The IAU definition of a planet is completely mathematical (it comes down to having enough gravity to “clear your space,” “roundness” does not count for much) and is dependent on the object’s distance from the Sun (the farther from the Sun, the larger the area it must “clear” and the slower it moves in its orbit). It completely leaves out the geology of the object. If we assume that a planet has to orbit a star (the IAU definition REQUIRES that it must orbit the Sun so forget about 3,000 or 4,000 exo-planets), is Pluto any less of a planet than Mercury? Pluto has a more dynamic surface, Pluto has a thicker atmosphere and seasons, Pluto has more moons, etc. You mention that an object such as Vesta was big enough to have been defined as a planet (along with Pluto and the asteroid Ceres), but the IAU definition of a planet has evolved, too. I am far from an expert in this area, but the original definition would have meant that an Earth-sized body at the distance of Pluto would not cut it as a planet! With the “revised” definition, an Earth-sized body at 100 AU would still qualify, hence the recent discussions about the “discovery” of a ninth planet.

    I personally do not have a problem with there being more than one class of planets: classical planets (8), dwarf planets (at least 5), and planet-like moons. It should be noted that we orbit a dwarf star and the last time I taught Intro Astronomy, the Sun was still a star.
    If one does an extensive search on the definition of a planet (I Googled it), one comes up with a nice, self-consistent planet classification scheme that takes into account for a range of possible physical properties:

    Larry Lebofsky

  2. A few years back Steven Dick, who was chief historian for NASA, wrote a whole book on the question of classification”

    Discovery and Classification in Astronomy: Controversy and Consensus

    “Astronomical discovery involves more than detecting something previously unseen. The reclassification of Pluto as a dwarf planet in 2006, and the controversy it generated, shows that discovery is a complex and ongoing process – one comprising various stages of research, interpretation, and understanding….”

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