Redefinition of Planet Includes Any Ball in Space That’s Not a Star
published during a waxing crescent moon.

Kirby Runyon cuts the figure of an astronaut, and you just know that he would be helpful in a bar fight, but you also get the impression that he would defuse things for you before it got that far. He is a young man, two months shy of a Ph.D. and with an abstract submitted to the Lunar and Planetary Science Conference last week in The Woodlands, Texas, Runyon has just stepped into a white-hot spotlight with an international audience. He and one of his co-authors, Alan Stern, the principal investigator on the New Horizons mission to Pluto, have taken a swing at the question of planethood.

Runyon’s definition of a planet is a single sentence in length: “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has enough gravitation to be round due to hydrostatic equilibrium regardless of its orbital parameters.” In other words, a planet is a ball in space that’s not a star. That means, yes, Pluto is a planet. It also means that the Moon is a planet. Europa is a planet. Ganymede is a planet.

In other words, a planet is a ball in space that’s not a star.

In comparison, the newly-established definition of a planet by the International Astronomical Union states: “A planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighborhood around its orbit.” In this regime, were Ganymede knocked from orbit around Jupiter and into orbit around the Sun, it would still be Ganymede, but might suddenly, according to the IAU, be a planet. At an instinctive level, this feels wrong, like saying that if a dog climbed onto a bookshelf, it would then be a cat. The astronomy view categorizes a planet based on what it orbits. Runyon’s assertion is that a planet should be defined by what it is.


Kirby Runyon presents his research on Martian sand sheets. Credit: Lunar and Planetary Institute

What imbues Runyon’s definition with resilience is that it doesn’t seek to somehow overturn that of the IAU, and he has no intention of submitting it to the IAU for consideration. “If certain types of astronomers want to have an orbital dynamic definition of a planet,” he says, “and that’s useful to them, fine. But most scientist who study planets are more aligned now with the geosciences than they are with astronomical scientists. And that definition of ‘planet’ just isn’t useful to us. It doesn’t help us communicate our ideas.” Informally, planetary scientists have always called all sorts of bodies in space “planets.” But formally, too, in peer-reviewed literature, technical moons are called planets. Runyon lists scores of such references made both before and after the IAU redefinition.

This is in part about the coming of age of planetary science. It is a young field, a single generation old, the plucky upstart once the exclusive domain of physics, then of astronomy, but whose constituent sciences now include geology, chemistry, and biology. Mars was once something you look up at, a dot in the sky. Celestial. Now it’s something you look down on from orbit, or across from the surface. It’s terrestrial. Geophysical.

“Carl Sagan said, ‘In science there are no authorities; at most, there are experts,’” Runyon tells me as we talk outside the convention center where the conference is being held. This isn’t a heated argument, and he counts Mike Brown, the famed “Pluto Killer,” as a friendly correspondent. Let the astronomers do what they want, he explains brightly, but leave us—i.e., geologists—out of it. The concept isn’t even unique. “To astronomers studying the composition of stars and nebulae, especially stars, they call anything heavier than element number two―anything heavier than helium―a metal. That’s just a convenient word for them. No one’s fighting about this,” says Runyon. “They know what they mean when they say metal, and it’s different from the common definition. You take the spectra of stars, and you see there’s oxygen and nitrogen and argon in stars’ atmospheres, you call that a high metallicity star. And that’s fine. Metallurgists aren’t fighting them over the word metal.”


Eight planets and new solar system designations. Credit: International Astronomical Union

This matters beyond the arcane world of scientific abstracts and poster sessions. Very rarely does a scientific debate spill into the public sphere and draw not only keen interest, but steely opinions. Evolution, certainly. The age of the Earth in some religious circles. But you don’t often see finger-pointed assertions over scientific nomenclature. The Washington Post doesn’t give a thousand words to disagreements over the precise definition of “suevites” (a type of rock formed during impact events) though scientists do debate its usage. This matter of Pluto, however, is both consequential and easily understood. Everyone has their own take on whether it is a planet.

Under Runyon’s definition, there are at least 110 planets in the solar system. This seems at once absurd, but resolves into something very interesting. He explains that the idea of planets being something you must memorize is a pointless exercise. Memorizing the periodic table of elements doesn’t make one a chemist. But just as the table itself is elegant and informative, plot all the planets on a table and you get something equally elegant. Terrestrial planets, gas giant planets, ice giant planets, dwarf planets, exoplanets, each arranged and subgrouped with common characteristics. Europa so plotted might be categorized as an icy dwarf satellite planet.

And suddenly, rather than rote memorization, Mercury, Venus, Earth, and so on, you have an ambitious and quite possibly tectonic effect on education. Rather than eliminating things to learn―Pluto itself has been rendered ontologically unsound since the IAU announcement, disappearing not only from textbooks but also consumer goods and media―you have the introduction of worlds that rarely appear in the classroom, Makemake, Mimas, Miranda and more. The conversation about Pluto has arguably been a net positive for the public, whose idea of the solar system is too often limited to plastic beads on wires circling a light bulb.


This composite of enhanced color images of Pluto (lower right) and Charon (upper left), taken by NASA’s New Horizons spacecraft on July 14, 2015, highlights the wide range of surface features on the small worlds. Working with the New Horizons mission team, the International Astronomical Union (IAU) has approved the themes to be used for naming the surface features on Pluto and its moons. Credit: NASA/JHUAPL/SwRI

An invested public with a robust science education is as important as ever before. Even with our pedestrian terrestrial political problems, it’s a good time to be a human being. We stand on a precipice of sorts, in which asteroids, planets―even stars―are accessible not only by scientists with instrument-laden spacecraft, but soon by the working man and woman. One easily imagines a real future in the lifetimes of our children in which “planet” relates not to hydrostatic equilibria or accretion disc formation, but by something inherently more utilitarian. Planet will be a shorthand for something vaguely accessible by humans and our tools for long durations. Somewhere useful. Somewhere with a horizon commensurate to that which the human mind has evolved to expect. Can I drive a shovel into the body and pull up raw materials? In plain science fiction terms, can I land my spaceship on it? Is it round and can I fly my gas mining barge through it?

In both the very short term and very long, what is or isn’t a planet is not particularly important. In the middle, however, in our present-day future in which dot-com billionaires want to put people in space, and not in capsules of three, but transports of hundreds, and they say this with authority and are investing the capital to make it a reality, suddenly “planet” is a word due to be handed back to the people. Given a better spacesuit, if I lived on Titan or Mars, is there a measurable difference in bodies? This land is your land.