The Planetary Status of Pluto

What makes a planet a planet? In 2006, the International Astronomical Union stripped Pluto of its planet status. Phillip Metzger, a planetary scientist at the University of Central Florida joins Hari Sreenivasan to discuss to the reasoning behind the reclassification of planets.


What makes a planet a planet?

In 2006, the International Astronomical Union stripped Pluto of its planet status.

Philip Metzger, a planetary scientist at the University of Central Florida, joins me now to discuss the reasoning behind the reclassification of planets, and I suspect you find fault with that.

You are a 'Pluto should be a planet' guy.

That's right.


Glad to be here today.

So tell me, what was wrong with their ruling, and what were the rules that they imposed when they kind of kicked Pluto out of the planetary club?

Okay, well, there were a number of things wrong with it.

First of all, we contend that taxonomy, the classification of objects in nature, is actually a vital part of the scientific process, and therefore it's not subject to voting.

It should be only handled by scientists publishing papers and arguing in conferences, and it gets established by consensus, so it was a major violation of the scientific process to begin with.

Second of all, it violated 500 years of progress in the tradition of Galileo in refining what we mean by a planet, and it disrupted our entire literature by changing the terminology, attempting to change it, but the planetary scientists have largely ignored that decree in 2006, and we continue to use the word planet in different ways instead.

But what they said in 2006 was that to be a planet, a body has to orbit the Sun.

It has to be large enough to be rounded, and it has to clear its orbit, or more charitably, we would reinterpret it to say it has to gravitationally dominate its orbit, and we take issue with the part about orbiting the Sun and the part about clearing an orbit.

We don't think a planet has to orbit the Sun or clear an orbit.

Okay, so by your definitions, Pluto would be a planet, but then so would lots of other things, right?

That's true, yeah, and in fact, scientists have always called lots of other things planets besides the eight gravitationally dominant Sun-orbiting bodies.

Galileo called our Moon a planet, and so have almost all lunar scientists ever since then, and we call the large moons around Jupiter and Saturn, for example, planets, and we call the large geologically complex bodies like Ceres and Pluto planets even though they don't dominate their orbits gravitationally, and there are good scientific reasons for this.

Okay, so I was going to say, what are the reasons for calling all of these other objects planets when people are, I don't know...depending on when they were educated, they might be calling them moons, or they might be calling them planets?

Yeah, well, so there's a long tension between cultural concepts about nature and scientific progress about nature, and we see these tensions happening in other fields as well such as in biology centered around questions of evolution.

Despite this struggle between planetary scientists, how does this impact what we see in a textbook, in a classroom, how young people are educated about what a planet is and is not?

Well, I think one of the problems with the IAU's definition is that it really took away the excitement of discovering planets.

When they told us that there are exactly and only eight planets in our solar system, and these are the names, they took all of the air out of the room because up until then, we were very excited.

The public was very excited because we were starting to discover new planets.

Our telescopes had gotten better, and now we know that there are at least 150 planets in our solar system, possibly as many as 450, and yet nobody knows this, and the reason nobody knows it is because they've been told wrongly that these are not planets.

They're just the leftover junk from planet formation similar to asteroids.

Well, we've now had a spacecraft fly by one of these bodies, Pluto, and we found out it's not anything like an asteroid.

It's in fact, in my opinion, the second-most interesting planet in our solar system after the Earth.

It's the most complex, most amazing body in space other than planet Earth.

If there are all of these other planets.

We're not talking about habitable life-forms per se, but if we say we have a solar system with 100 more planets in it, how does that change how we perceive the universe?

Well, so first of all, there might actually be life on Pluto.

There could be.

The conditions are right.

We believe that it has an underground ocean, that surprisingly Pluto must have enough radioactive decay occurring in it to keep its interior warm so that the ocean has stayed liquid even until now, and we know looking at the surface of Pluto, there's organic material, and so the right molecular chemistry is there, so it is possible that Pluto is a second location in our solar system that can have life, but it's important I think first of all because it gives the correct message about the central truth of progress in planetary science.

We've discovered that the solar system is not this simplistic, clockwork order of just seven or eight bodies in sequence moving outward from the Sun all perfectly ruling their orbits forever.

Instead, we've learned that it's actively changing.

It's dynamic.

We think Uranus and Neptune may have actually swapped places at one time because the action of the Kuiper Belt dragging Saturn slowed it down and caused resonances, so it's a very dynamic, changing place, and so the central message is not static order.

The central message is all this amazing dynamism that has even resulted in us being able to be here today.

All right.

Philip Metzger from the University of Central Florida, thanks so much for joining us.

It was my pleasure.