Ask the Astronomer: The Elephant in the Solar System

ASK THE ASTRONOMER

Hi Joseph,
Why isn’t Pluto a planet?

–Helena, Pasadena

By Joseph Masiero

Helena, I have been asked this question probably hundreds of times since 2006 when Pluto’s classification was changed from “planet” to “dwarf planet”.

Before we can discuss Pluto’s status (the elephant in the Solar system, as it were), let’s first take a quick trip back to ancient Greece. The term “planet” comes from the Greek word for “wanderer”. It originally was used thousands of years ago to refer to the lights in the night sky that stubbornly refused to stay in one place, instead wandering among the stars. There were seven of these “planets”: Mercury, Venus, Mars, Jupiter, and Saturn, as well as the Moon and the Sun. Notably, Earth was not a planet originally.

When the Copernican revolution  put the Sun at the center of the Solar system, Earth joined the ranks of the planets, and the Sun and Moon were removed. As astronomers scanned the sky, new planets were discovered including Uranus and Neptune beyond  Saturn, and Ceres in between the orbits of Mars and Jupiter. But as we kept searching, first Pallas, Juno and Vesta and then many more objects were discovered between Mars and Jupiter. Astronomers realized that not one or a few objects were in this region but rather there was a belt of millions of asteroids, and so Ceres was changed from planet to asteroid with little fanfare.

In much the same way that the discovery of this “Main Belt” of asteroids resulted in Ceres’ designation being changed, our realization that Pluto was just one of a  vast sea of objects out beyond Neptune in the Kuiper Belt also resulted in a change in our understanding of the Solar system. Pluto was not alone, but rather was the first of a wave of discoveries stretching to the far reaches of the Solar system. The progress of science demands that we be able  to take new information and adapt our understanding of the universe to incorporate that new knowledge, and so Pluto’s designation was changed from “planet” to a new class of body: “dwarf planet”.

Pluto hasn’t gone anywhere, and is still as fascinating as ever, despite being reclassified. NASA’s New Horizons mission will fly by Pluto next summer, and will surely wow us with our first up-close view of a Kuiper Belt object. And though it may not be a planet any longer, it still will forever be known as the first of a new kind of object never before seen.

Joe Masiero is a Scientist at NASA’s Jet Propulsion Laboratory and holds a Ph.D. in Astronomy from the University of Hawaii’s Institute for Astronomy.  His research primarily focuses on the asteroids and comets of our Solar system, and he is a member of the science team for the NEOWISE space telescope. You can send your questions by placing a comment below.