Space Matter: Pluto and the (re)Definition of a Planet

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Space Matter: Pluto and the (re)Definition of a Planet

Space Matter is a weekly column that delves into space science and the mechanics of spaceflight. From the latest discoveries in the universe around us to the fits and starts of rocket test flights, you’ll find analysis, discussion and an eternal optimism about space and launching ourselves into the cosmos.


Pluto. The little brother of the solar system, who got kicked out of the planets club. It may be a small planet, but it’s got a pretty large fan club, and now, we’re once again talking about restoring the dwarf planet’s full planet status.

You may have read or seen in the news that scientists are debating a new definition of “planet” that would make Pluto a planet again. These reports are a bit premature, but it’s true that a new definition has been proposed. Before we talk about it, let’s discuss the history of Pluto’s demotion and why, exactly, it lost its status as a full-fledged planet.

The proceedings and votes of the International Astronomical Union (IAU) don’t usually make the front pages of the major newspapers around the world, but in 2006, one such vote did. Because that year, the IAU officially kicked Pluto out of the planet club by agreeing on a definition of planet that did not include it.

1. pluto-charon.jpg Pluto (lower right)-Charon, as taken by the New Horizons spacecraft. Photo courtesy of NASA/JHUAPL/SwRI

Specifically, the IAU decided that three conditions had to be in place for a celestial body to be considered a planet:

(1) The body has to be in orbit around the Sun.

Check. Pluto has a strange inclined orbit around the Sun that actually crosses Neptune’s orbit, but it does orbit our star. Its orbital period is 248 years.

(2) It has to have sufficient mass to be nearly round.

Yep, Pluto is round. We’re good here.

(3) The object must clear the neighborhood around its orbit.

... Not so much, Pluto. This third, crucial criterion is where Pluto failed the planet test. “Clearing the neighborhood” means that the celestial body must be the dominant gravitational force in its area. There must be no similarly sized bodies—other than its own satellites, or moons—in the area. It has to be gravitationally dominant enough such that smaller objects within its orbital path are either repelled by its gravity or drawn into the planet.

Pluto has five known satellites, the largest of which, Charon, doesn’t actually orbit it. Any object with a satellite is affected by the gravity of that satellite; for example, the Earth is affected by the Moon. The Moon doesn’t technically orbit the Earth. The Earth and Moon both orbit a point that is the center of their mass, called the barycenter; the Earth just happens to be so much more massive than the moon that that point is located within the Earth. It’s the same for Pluto and Charon, except Charon has 1/8 the mass of the object it orbits. As a result, that gravitational point is actually not inside Pluto. The duo actually orbit one another around a fixed point in space.

Pluto also shares its orbit with Kuiper Belt objects. The Kuiper Belt is an area outside the orbit of Neptune that surrounds our solar system, filled with small, icy bodies. Scientists speculate that these might be leftovers from the solar system’s formation. A specific type of Kuiper Belt object, Plutinos, lie along Pluto’s orbit—meaning it doesn’t have sufficient gravitational pull to clear its orbit. Thus, Pluto was demoted to a “dwarf planet”—and quite a few Kuiper Belt objects have joined it in that definition.

2. kuiper belt.png The Kuiper belt, with Pluto’s elliptical orbit traveling through it, is shown in this diagram. The yellow line is the path of NASA’s New Horizons spacecraft. Photo courtesy of NASA/JHUAPL/SwRI/Alex Parker

This is an orbital dynamics definition of a planet, and we’ve lived with it for the last decade. But now a new definition is being proposed by a different group, one that will drastically increase the number of planets in our solar system. It’s based on mass.

Kirby Runyon and Alan Stern are two scientists interested in the geology of celestial bodies, rather than their gravitational influence. They propose to broaden the definition of planet to anything that has enough mass to pull itself into a sphere, but not so massive it becomes a star.

This would restore Pluto to planet status, but also add dozens more to its ranks. Moons, Kuiper Belt objects, dwarf planets: anything within our solar system massive enough to be round would be included. They argue that it would be easy to subcategorize from here: icy planets (currently dwarf planets), gas giants (outer solar system planets), moon planets (moons), and rocky planets (inner solar system planets).

3. jupiter.jpg This image of crescent Jupiter was taken with JunoCam. Photo courtesy of NASA/JPL-Caltech/SwRI/MSSS/Roman Tkachenko

The idea is that the word “planet” would become a catch-all term. That’s okay for colloquial speaking, but science values precision. Right now, when a scientist says “planet,” people understand that they’re referring to a certain type of celestial body, as opposed to an asteroid or a Kuiper Belt object or a moon. If the only way to really make the new definition of the term useful is creating category descriptors to subdivide the types of things that the term encompasses—does it make sense to change it? These categories already exist. Does calling a moon a “moon planet” do anything beyond adding a word to the term and making an already clunky definition even worse?

The IAU’s definition isn’t perfect by any means, but it makes sense. This feels like adding a layer of complication onto something that is already a bit confusing. I’m not a scientist, and while I do work with these terms on a daily basis as a science writer, precision and proper labeling isn’t nearly as crucial to my work as it is to a scientist’s. I’ll defer to their expert opinions here, but my general opinion is that the less complicated, the better.

Top photo courtesy of NASA/JHUAPL/SwRI

Swapna Krishna is a freelance writer, editor and giant space/sci-fi geek.