Defining Planets

In 2005, Michael Brown of the California Institute of Technology and his team discovered a large body in the outer solar system. It was not the first distant object that had been found in the Kuiper Belt — that region encircling our solar system is composed of hundreds of icy objects. But it was the largest known Kuiper Belt object, just beating out Pluto in terms of size, and so their discovery was heralded as “the tenth planet.”

Scientists think eventually we will discover many planet-sized globes in that distant region of space, and that brings to the forefront questions about what defines a planet. Recent discoveries of many unusual extrasolar planets in other solar systems also have placed the definition of “planet” under scrutiny. This planetary debate was put to a vote at a meeting of the International Astronomical Union in 2006, and scientists there chose to reclassify Pluto and other large Kuiper Belt objects as “dwarf planets.”

Michael Brown, head of the California Institute of Technology Planetary Astronomy Group.

In a lecture given at NASA’s Jet Propulsion Laboratory, Brown talked about Pluto, the controversy about how to define a planet, and why he thinks such a definition ultimately has no scientific importance.

(This edited transcript is part one of a five-part series)

“Are objects in the outer solar system planets or not planets? There’s a reason for all of this debate that’s been going on for the past couple of years. The real blame rests squarely on that crazy little ice ball Pluto.

The problem with Pluto, of course, is that while it’s one of the largest objects that we currently know of in the Kuiper Belt, it’s not very big compared to the rest of the planets.

If you were an alien – some of you might be, we are at JPL and you have to be worried about those things – but if you were an alien, and you were coming into the solar system for the first time in your space ship, and let’s say you were born on the spaceship so you’ve never seen a planet before and your alien life form has no word for planet. What do you see? You’re headed for the star, so first you see the sun. Next you see this big extra-bright thing next to the star which is Jupiter. Then you see Saturn, Uranus, Neptune. So you realize, “Wow, this is not just a star, there are four things going around the star.” You’d be very excited about these things, because aliens get excited by stuff like that. You keep on going along and you realize there’s more! Mercury, Venus, Earth, and Mars – there are four more things going around the star.

You might take the first four bright things and put them in one category, and the next four things and put them there. And then you keep on driving in and getting closer, and you see that between Mars and Jupiter there’s a swarm of material, a bunch of objects that we call asteroids. So you put those in another category. Then finally, as you get in closer, you realize that outside Neptune there’s another swarm of objects, the Kuiper Belt objects, and you put those in another category. This is a very rational way to categorize the objects in the solar system. There are eight large objects in the solar system, and then two populations of objects in the solar system.

The green circles represent the orbits of some of the planets in the solar system. The outer blue and purple dots represent the location of Kuiper Belt objects. Click image for larger view.
Credit: NASA/JPL-Caltech.

And in your alien mind, you might be forgiven if you only wanted to call the four big ones planets and the four smaller ones something else. I try to teach this to my classes at Caltech, but they don’t buy it – everybody wants to keep Earth as a planet. Sentimentalists!

So we’ll keep it as eight planets. That’s essentially the definition the International Astronomical Union accepted. There still are arguments about whether it’s a sensible definition or not. Astronomers are not above being politicians at the same time, so it’s hard to separate the politics from the science.

Some people are arguing that there is an alternative way to classify things in the solar system; that if something is big enough to be round and have important geologic things happening on the surface, let’s call it a planet. That’s also a sensible way to categorize things in the solar system, and it would give you a couple hundred things that we could call planets.

Which definition is right? It’s a stupid question. In astronomy and in most sciences, we don’t tend to focus on a precise definition for things. We focus on the concepts. The word “planet” now incorporates scientific concepts that are relatively new, but it was never meant to be a scientific definition. Given that there are many alternative ways to classify, not one of which means the word “planet,” it’s clear that the word “planet” is not a scientific word and need not be given a scientific definition.

Look at the word “continent” that geologists have – if you haven’t figured it out by now, the word continent actually has no scientific definition. There is no reason that Europe is a continent. Australia – just a big island. But why are they called that? Because culturally, Europe has always been a continent. I think they came up with the word so they grandfathered themselves in. India is a subcontinent — does anybody know what a subcontinent really is? India — that’s the answer. Indonesia is sometimes defined as a microcontinent. But geologists don’t sit around and argue, “Yes, Europe should be a continent.” “No it shouldn’t!” “Australia must go!”

The familiar continents of Earth are embedded in tectonic plates on the planet’s surface that slowly collide with each over time. Click image for larger view.
Credit: USGS

You could talk about continents all day long and geologists won’t argue with you. They’ll talk about continental crust, collisions, continental plates and dynamics, but they won’t debate the word “continent.” If they did, I would insist that Madagascar should be a continent. It’s the smallest thing that’s on its own plate in the continental crust.

You could say, “Who cares?” People who have spent a lot of money and effort to visit all seven continents probably care. Culturally, we care about these things. It’s a way that we wrap our minds around the geography around us. And planets are the way we wrap our minds around the solar system. So I understand why people are upset about Pluto being demoted, because it’s telling you that Australia is not a continent –- it’s exactly the same thing. Of course, there are no people from Pluto that are big and drink a lot of beer and would beat you up if you said that.

I would have been happy if we’d tried to come up with some new words to describe the eight large objects that orbit the sun, but the word “planet” is cultural, so we’re not touching it. But the IAU felt the pressure to come up with a scientific definition. As scientific definitions go, I think saying there are only eight planets works better, but it’s an aesthetic choice. It’s a religious choice. There’s no science there.

I also like it better because it makes more sense historically. The same thing happened to Ceres the asteroid back in the early 1800s. It was originally thought to be a planet, and when people realized it was just part of a population they said, “Nope, we’re going to call you a minor planet or an asteroid.” So the eight planet definition is a closer match to the historical definition.

Cutaway of view of Ceres shows the differentiated layers of the asteroid. Credit: NASA, ESA, and A. Feild (STScI)

And aesthetically, I like the word “planet” to be special. That’s the most honest definition I can give you –- I don’t like having 200 planets. If you like 200 planets, more power to you. I have to warn you though, most religious arguments like this get settled by slaughter, and I’ve got a big stick here (brandishes blackboard pointer), so don’t start with me.

If we ignore the people who complained that the IAU decision was made by a small group of astronomers, and took a random sample of all astronomers, most of them would support the eight planet definition for purely aesthetic reasons. Some would prefer the 200 planet view, for either aesthetic or political reasons. The debate should not continue, because at this point it’s not interesting, but it will.

So Pluto is not a planet now, it’s a dwarf planet. A dwarf planet, for those of you who were wondering, is something that looks like a planet because it’s round, but is not a planet. We currently know of about 35 objects that fit the bill of dwarf planet, but we’ll find hundreds.”

Read part 2 of this lecture: "Hunting Distant Dwarfs"
Part 3: "The Discovery of Eris"
Part 4: "Studying Dwarf Planets"
Part 5: "Questioning Dwarfs"

Related Web Pages

Speeding by Jupiter
Questioning Pluto
The Ice Dwarf Cometh
Moons Over the Kuiper Belt
A Glimmer of Pluto on the Horizon
Tenth Planet
To Pluto and Beyond