Expert Voices

Why Pluto Is a Planet, and Eris Is Too (Op-Ed)

This artist's view shows Pluto center, and its large moon Charon, as well as two of its four smaller moons (at top left and right). Pluto's four tiny moons are Nix, Hydra, Kerberos and Styx.
This artist's view shows Pluto center, and its large moon Charon, as well as two of its four smaller moons (at top left and right). Pluto's four tiny moons are Nix, Hydra, Kerberos and Styx. (Image credit: David A. Aguilar (CfA))

Tim DeBenedictis is the lead developer of the SkySafari line of iOS and Android apps at Simulation Curriculum, the makers of Starry Night, SkySafari and the free Pluto Safari app. DeBenedictis has been writing astronomy software since high school, and graduated from MIT in 1993 with a degree in earth, atmospheric and planetary science. Passionate about space, DeBenedictis is self-taught in mechanical and electrical engineering, and has launched his own private microsatellite into space. He contributed this article to's Expert Voices: Op-Ed & Insights.

The International Astronomical Union (IAU) got it wrong. Our solar system has 10 planets.

As NASA's New Horizons spacecraft glides its way to the cold outer reaches of our solar system to take the first-ever up-close look at Pluto, the time is right to revise the International Astronomical Union (IAU)'s 2006 definition of a planet, which resulted in Pluto's "demotion" from planet to ambiguous dwarf-planet status.

Pluto's place

For those unfamiliar with the issues that led to that highly controversial decision, here's a quick recap: It started with Pluto itself, discovered on Feb. 18, 1930, by Clyde Tombaugh, a young American astronomer working at Lowell Observatory in Flagstaff, Arizona. Pluto turned out to be rather unlike the other eight large objects orbiting the sun. Pluto is much smaller than Mercury, and only two-thirds the size of Earth's moon. Its orbit is tilted and eccentric, crossing Neptune's. No other planet acted like this. In 2000, astronomers found other objects orbiting the sun in the deep outer solar system, with qualities very much like Pluto's. They were given names like Sedna , Quaoar, Ixion, Varuna, Makemake and Haumea . Many were close (but not quite equal) to Pluto in size. All of them had tilted, eccentric orbits; quite a few of those orbits crossed Neptune's.

The tipping point came in 2005. California Institute of Technology astronomer Mike Brown, along with Chad Trujillo of Gemini Observatory and David Rabinowitz of Yale University, discovered a new massive body in the solar system. This new body, which astronomers latter dubbed Eris, was particularly noteworthy: Not only did it possess a moon, but at the time, it was estimated to be larger than Pluto. Subsequent observations revealed that Eris and Pluto are nearly identical in size, though Pluto is likely a few kilometers larger. Initially, Brown had named the newly discovered body Xena (after the protagonist of the eponymous TV show, with a sneaky Planet X reference). Although the name Xena didn't stick, the IAU later officially — and aptly — christened it Eris after the Greek goddess of chaos and discord. 

So, it seemed quite clear that if Pluto was our solar system's ninth planet, then Eris should be its 10th. And if Eris and Pluto were planets, why shouldn't Makemake and Haumea be considered planets as well? And what if there were even bigger objects out there to be discovered? Why shouldn't the solar system have 15 planets, or 40? (Can you imagine the mnemonic device that would be required to remember 40 planets in the solar system?!) 

For all who were in support of granting planet status to these objects, an equally adamant camp insisted that none of these objects, including Pluto, deserved to be called planets, and that our solar system contained only eight objects worthy of planet status. Neptune would be the last and final.

A worsening problem 

With the intention of solving the debate once and for all, members of the IAU met in 2006. They spent days debating how to establish unambiguous definitions for the objects in our solar system. In the end, Resolution 5A was born:


The IAU therefore resolves that planets and other bodies in our solar system, except satellites, be defined into three distinct categories in the following way:

(1) A planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.

These are, to put it bluntly, terrible definitions. Despite its goal of providing unambiguous definitions, Resolution 5A actually contains the kind of ambiguity that most scientific organizations would protest. It adds confusion, resolves little and makes nobody happy. 

So how did that become the official definition of a planet? A very strange vote. If you think low voter turnout is limited to politics, consider this: Only about 4 percent of IAU members were present for the vote on Resolution 5A. But it was travel schedules, not apathy, that caused this abysmal turnout. You see, the vote took place on the last day of the IAU meeting, when many people had to leave to catch flights back home — 424 astronomers were present, even though IAU membership in 2006 was just more than 10,000. As a result, Pluto lost the status it had enjoyed for more than 80 years and became a dwarf planet overnight. [Pluto Demoted: No Longer a Planet in Highly Controversial Definition]

The term "dwarf planet" itself causes confusion. You often hear people say Pluto is still a planet but that it just happens to be a dwarf planet now. But despite the name, the IAU does not consider a dwarf planet a planet — unlike a dwarf star, which is still a star, or a dwarf galaxy, which is still a galaxy. So much for eliminating ambiguity! But just the conflicting use of the word planet isn't the most unclear part of the resolution. 

The New Horizons spacecraft will fly by Pluto on July 14, 2015. (Image credit: Made with the Pluto Safari app for iOS and Android.)

Picking apart Resolution 5A

Let's dissect resolution 5A "Definition of 'planet'", one of six IAU Resolutions that were passed at the Closing Ceremony of the General Assembly in 2006.

*Nearly round shape. There is an element of something good here. We all intuitively feel that a planet should be round, or nearly so. But what is "nearly"? How lumpy and bumpy must an object be to no longer qualify as a planet or a dwarf planet? How smooth must the "ball" be? The Earth, which we all agree is a planet, is nearly round on some scales, but on others, it's not. If you're standing in the bottom of the Grand Canyon, the Earth isn't even close to nearly round. 

*Cleared the neighborhood. I've tried to wrap my head around this phrase for years, tried to convince myself that it makes sense — but I just can't swallow it. The IAU is trying to express that, in addition to being round, a planet should be the dominant gravitational force in its local region of the solar system. That's not an unreasonable position. Certainly the Earth and Jupiter are the dominant objects in their local regions. Neptune surely is, too. Even though Pluto's orbit crosses Neptune's, Neptune forces Pluto into something called a 3:2 resonance (for every three times Neptune goes around the sun, Pluto goes around twice), preventing collision. But have any of these planets actually "cleared the neighborhood" around their orbits? No. Pluto is still clearly in Neptune's "neighborhood." For that matter, Jupiter has two well-known groups of asteroids, the Trojans, which lead and follow Jupiter along in its orbit. For that matter, the Earth hasn't quite "cleared the neighborhood" around its orbit, either, to which anyone who saw the near-Earth asteroids that entered Earth's atmosphere near Chelyabinsk, Russia, on Feb. 15, 2013, or Tunguska, Siberia, on June 30, 1908, can attest. So are Earth, Jupiter and Neptune the dominant gravitational objects in their local neighborhoods? Yes, clearly. Have they cleared their neighborhoods? No. Not by a long shot. 

Other scientists have weighed in on the matter. Alan Stern, principal investigator for the New Horizons mission to Pluto, made it clear he disagrees with the IAU resolution. "Any definition that allows a planet in one location but not another is unworkable. Take Earth. Move it to Pluto's orbit, and it will be instantly disqualified as a planet," Stern said.

The biggest problem with the IAU's planet definition is that it replaces an already-ambiguous concept ("What is a planet?") with three more ambiguous concepts, ("nearly round," "cleared" and "neighborhood"). Indeed, the only definitive part of the IAU resolution on which everyone can agree is the first part: (1) A planet is in orbit around the sun. It's why the moon is not a planet. My 6-year-old niece intuitively understands this. It's the only part of the IAU definition I would keep. 

The way out

Let's look at some other kinds of definitions that are clear and unambiguous.

*International boundaries. It's well understood that the portion of North America north of 49 degrees, between the Canadian provinces of British Columbia, Alberta, Saskatchewan and Manitoba, and the U.S. states of Washington, Idaho, Montana, North Dakota and Minnesota, is called Canada, and the portion below that latitude is called the United States. There's no physical demarcation — no river, no mountain range — along the 49th parallel. There's no subtle change in vegetation or geological structure. But there is a hard, sharp, clearly defined, well-understood boundary that unambiguously answers the question, "What is Canada?" It's the country north of the 49th parallel. It passes the 6-year-old-niece test.

*Constellation boundaries. Back in 1888, the IAU defined an intricate set of boundary lines in the sky, precisely outlining the groups of stars that were commonly referred to as constellations. It also declared that there would be 88 of these constellations. Lines were chosen carefully, to respect traditional choices about which star might lie in which constellation, and in the end, the definitions were clear. There is no ambiguity about which particular point in the sky falls within which constellation. And, you can tell precisely when a moving celestial object (like Pluto) might cross from one constellation to another. It's an arbitrarily agreed upon but well-defined system of definitions that has served the astronomical community well for more than 100 years.

*The Karman line that defines the edge of space. Where does the Earth's atmosphere end and outer space begin? Clearly, there is no physical boundary. There is no bubble holding the atmosphere that one must pierce on one's way to the International Space Station. The air just gets thinner and thinner until you can ignore it. But in reality, air molecules continue to exist, albeit in smaller numbers, out to an altitude of many thousands of kilometers and beyond — indeed, some of these air molecules may have made it as far as Pluto by now! However, since the early days of manned spaceflight, a near-universally accepted definition is that space begins at an altitude of 100 kilometers (62 miles). In fact, this definition is accepted by the Fédération Aéronautique Internationale (FAI), an international standard-setting body for aeronautics and astronautics. Pilots who've flown higher than 100 km have officially earned the title of astronaut. It's another arbitrary, but widely accepted convention that is clear, unambiguous and easily passes the 6-year-old comprehension test.

What the IAU should have done in 2006, and could easily do moving forward, is to crystallize the definition of the word "planet" as unambiguously as it defined the boundaries of the constellations in 1888. Yes, that definition would have been arbitrary, and yes, the actual physical objects themselves would gradually transition from larger to smaller, and don't care in the least what we choose to call them. But the IAU could have chosen a definition that resolves the debate in a far more satisfying manner than it actually did.

The orbits of some of the trans-Neptunian objects. (Image credit: Made with the Pluto Safari app for iOS and Android)

The 1,000-km rule

So, what would be a better definition for the objects in the solar system?

(1) A "planet" [1] is a celestial body that (a) is in orbit around the sun, and (b) has a maximum surface radius greater than 1,000 km (620 miles).

(2) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.

"But that's completely unscientific," you may say. "Why 1,000 km? Why not 1,200, or 750?" 

I submit that the precise definition of a planet as an object with a radius of at least 1,000 km is no less scientific than the definition of a kilometer as being a unit of distance equal to 1,000 m, or a degree being 1/360 of a circle.

And there are other reasons why the 1,000-km definition is more scientific than it might seem at first. But let's put that aside momentarily. Instead, let's see what would have happened if the IAU had adopted this definition. 

Here is a list of the largest known objects orbiting the sun, and their radii in kilometers:

Object Radii (km)

Jupiter: 69,911 km (43,441 miles)
Saturn: 58,232 km (36,184 miles)
Uranus: 25,362 (15,759 miles)
Neptune: 24,622 (15,299 miles)
Earth: 6,378 (3,963 miles)
Venus: 6,052 (3,761 miles)
Mars: 3,390 (2,106 miles)
Mercury: 2,440 (1,516 miles)
Pluto: 1,184 (736 miles)
Eris: 1,163 (723 miles)
Makemake: 715 (444 miles)
Haumea: 620 (385 miles)
Quaoar: 555 (345 miles)
Sedna: 498 (309 miles)
Ceres: 475 (295 miles)
Orcus: 458 (285 miles)

By the 1,000-km definition, all eight classical planets would remain planets. So would Pluto. And we'd add Eris. The solar system would have exactly 10 planets, a number that is deeply satisfying to two-handed, five-fingered humans who've been practicing base-10 mathematics for thousands of years. The "Plutophile" camp, fond of keeping Pluto's planetary status for historical reasons, would retain its dignity. And elevating Eris to a first-class planet would be an honorable nod to the cutting-edge astronomers whose work led to a need for this new definition in the first place.

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And finally, the 1,000-km rule, like any good arbitrary rule, actually does a pretty good job of respecting the underlying physical phenomena that it purports to define. Planets are made of physical materials like rock, metal, gas and ice. They may come in different proportions, but those materials all respect the same physical laws. When you put together a lump of rock, metal or ice, in any proportion, certain things start to happen as that lump approaches 1,000 km in radius. The materials will pull together under the force of their own gravity. Solid rock will start to deform. Ice, even frozen hard as granite at the edge of the solar system, will slowly flow. There is no known substance that can resist the force of its own gravity when made into a lump with a 1,000-km radius. Any object, made of any substance, of approximately that size, will eventually flow under action of its own gravity into a shape that is "nearly round" when viewed from far away. The 1,000-km radius just happens to describe something that naturally takes place for objects of a certain size and results in what we all intuitively want a planet to look like.

And for space enthusiasts, there's one more benefit to my proposed definition. While we're all looking forward to the New Horizons Pluto flyby, there's also a certain sadness to knowing that, after Pluto, there will be no more planets in our solar system to explore. If our solar system has 10 planets, that's no longer true. As far away and difficult as it was to reach Pluto, it will be even more difficult to reach Eris. It's another frontier, another first and another project to fund. That prospect alone should give the scientific community a reason to rethink and resolve Resolution 5A.

As the legendary astronomer Sir Patrick Moore said, "You can call it whatever you like. It's there!" Pluto is, and will always remain Pluto.

With the free Pluto Safari app for iOS and app for Android, users can simulate the July 14 flyby of Pluto, get regular mission news updates and learn the history of Pluto. Follow Simulation Curriculum on Twitter @SkySafariAstro, Facebook and Instagram. Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on

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