Frequently Asked Questions

Update: One of the IAU requirements for the "planet" classification is that a planet have "cleared its orbital neighborhood." Wouldn't this disqualify planets with moons and asteroids in their orbits, like Earth, Jupiter, and most of all Neptune, which itself hasn't cleared Pluto from its orbital neighborhood?

The problem is not so much Neptune and Pluto as it is the poor phrasing of "clearing the neighborhood." As Michael Brown explains, it's not a matter of a planet sweeping out other orbital bodies; rather, what makes a planet is its gravitational ability to totally dominate those bodies in its orbit. Earth totally dominates the Moon by keeping it in its orbit. Jupiter totally dominates the Trojan asteroids by keeping them at its LaGrange Points. And Neptune totally dominates Pluto by locking Pluto into a 3:2 orbital resonance along with other Kuiper Belt Objects of a special class called "Plutinos." Pluto, on the other hand, exercises little gravitational influence over the rest of the Kuiper Belt, and is therefore unqualified to be classed as a major planet.

Update: Isn't it true that only a tiny minority of IAU astronomers voted on the demotion?

Not all of the members of the IAU are planetary scientists. Others study galaxies, cosmology, astrophysics, and other fields of astronomy which do not touch upon classification and study of solar system bodies. It was not required that every single IAU member vote on the status of Pluto; simply a reasonable percentage of those who deal with matters related to planetary categorization.

Are you serious?

Do I look like I'm not serious? This is the internet, and the internet is serious business.

Why do you hate Pluto so much?

Pluto killed my master, and Charon kidnapped my family.

We've always regarded Pluto as a planet; I grew up learning the nine planets. Don't you have any respect for tradition?

Nostalgia cannot be the deciding factor as to whether Pluto is a planet or not. Do we subscribe to Ptolemy's geocentric model of the universe just because Copernicus' heliocentric solar system lacks the rich sepia tones of Ptolemy's papyrus? We appreciate Clyde Tombaugh's discovery of this distant planetoid, but it is more proper that the period of history when we called it a "planet" be treated as a first step towards greater knowledge of the primordial non-planetary objects in the far reaches of our solar system.

In the same way people adapted to knowing that the world is not flat, that the universe was not made in six literal 24 hour days, or that there's really no such thing as a brontosaurus -- we will adapt to knowing that there are eight planets, and a huge number of other objects, among which Pluto ranks first of many discovered in the outer solar system.

Doesn't Pluto having an atmosphere make it a planet?

Titan has an atmosphere, but we still refer to it as a moon of Saturn. Ceres may have once had a tenuous atmosphere of xenon gas which has since escaped into space. Does that mean it was once a planet but now isn't? And after a nice big super bean and cheese burrito with lots of guacamole and hot peppers, I tend to develop extra mass and a thick atmosphere of carbon dioxide, methane, and sulfuric compounds; do I then become a planet?

Pluto has moons. Wouldn't that qualify it for planet status?

The "moon" Charon is half the size of Pluto, and the center of gravity between the two bodies rests somewhere between the surfaces of both, such that they orbit around each other rather than one orbiting around a relatively stationary object. That makes them more of a "binary planetoid" system than a "planet-moon" combination. Besides, asteroid 243 Ida has a small, rocky moon called Dactyl. That doesn't make Ida a planet.

Update: Michael Brown points out that as many as 80% of KBOs have satellites orbiting them. Pluto is looking less and less unique as we discover more and more bodies like it.

The word "Pluton" is already used in geology. Wouldn't using it for astronomy be a redundant mistake?

The word "nucleus" is used in physics and biology for atoms and cells respectively. That hasn't caused too much confusion, has it?

Who… are you?

A friend.