12 Planets (and Counting)

Time to update your solar system?

Friends, our solar system has 12 planets, not nine as you've been told all your life. Or so says a committee of experts appointed by the International Astronomical Union (IAU), the folks who officially keep track of celestial bodies.

The IAU asked its committee to come up with an answer to the question "what is a planet?" Surprisingly, there's never been an official scientific definition. As one astronomer has lamented, "It's something of an embarrassment. . . . We live on a planet; it would be nice to know what that was."

Now, we will--assuming the IAU's members vote to approve the committee's recommended definition at a meeting in Prague next week. So, what does the new definition say? And what might a new map of the solar system look like?

Round and Unbound

According to the new definition, a planet is any celestial body that meets three criteria:

• It orbits a star.
• It's neither a star nor "a satellite of a planet" (a moon).
• It's round. More technically, it "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."

With this definition, the debate over whether Pluto's planetary license should be revoked ends. Tiny Pluto gets to be a planet after all. But so does Ceres, the largest asteroid in the asteroid belt between Mars and Jupiter. It's promoted from "largest asteroid" to "smallest planet" (or, more accurately, restored, since astronomers counted Ceres as a planet when they found it in 1801). Just 590 miles (950 km) wide, Ceres is less than half Pluto's width.

Two New "Plutons"

Along with Ceres, two other known celestial bodies would immediately become planets. One is the frigid, faraway object officially known as 2003 UB313, nicknamed Xena. At least as big as Pluto--and three times more distant--Xena's discovery helped push astronomers to define "planet."

The other is Charon, currently known as Pluto's main moon (Pluto has two other tiny moons, just discovered). Why should Charon count as a planet when our moon doesn't? Because--at more than half Pluto's size--Charon isn't really a satellite of Pluto. Instead, Pluto and Charon continually orbit each other. Under the new proposal, Pluto and Charon would count as a "double planet."

Pluto, Charon, and 2003 UB313 would also be part of a new class of planets called "plutons," which the proposal differentiates from the eight "classical planets." Like Pluto, the plutons take centuries to circle the sun, and their orbits are elongated and tilted compared with those of the classical planets. Scientists expect to find a lot more plutons in the years to come--perhaps dozens more.

Solar System Summary

It all sounds like radical change. But bear in mind that it's mainly a matter of nomenclature. On a journey out from the sun, you'll still find the following in our little solar system:

• Four "terrestrial" planets: Mercury, Venus, Earth, and Mars. These planets are made mostly of rock, they have solid surfaces, and they don't have rings.
• An asteroid belt beyond Mars and before Jupiter--though on your way through it, you probably won't see a single asteroid. It's far less crowded than you may think. By far the biggest body is Ceres. It accounts for about a third of the entire mass of the asteroid belt.
• Four gas giants or "Jovian" planets: Jupiter, Saturn, Uranus, and Neptune. These are big, they're made mostly of gas, and they have rings.
• Pluto, Charon, and a growing number of other "trans-Neptunian objects." These bodies are distant, cold, and to blame for our changing conceptions of "planet."

Astronomers didn't discover Pluto until 1930. They didn't discover Charon until 1978. And they didn't discover another "trans-Neptunian object" until 1992. Since then, they've discovered hundreds of them--some as big, or bigger, than Pluto. Whether we wind up calling these objects "planets," "plutons," or something else entirely, they're clearly part of a new astronomical frontier--and we're witnessing its birth.

Steve Sampson
August 18, 2006

Want to learn more?
Read the IAU's answers to your new planet questions
Take a virtual tour of the 12-planet solar system