Cosmic Quiz Quetsion: Is Pluto a planet?

Seth Jarvis

It’s week two of our Clark Planetarium, KUTV-2 Cosmic Quiz and I’m enjoying reading through the many questions we have received. This week’s winner is Ramon Nieto and he wants to know, “Is Pluto a Planet?”

Short answer: Not anymore!

Long answer:

When Pluto was discovered by Clyde Tombaugh in 1930 (through an amazing combination of brainpower, hard work and patience) the only things astronomers knew that existed three billion miles from the Sun were planets and comets, so when Pluto was discovered and it didn’t seem to be a comet then, it must be a planet!

While the world celebrated Pluto’s discovery and the addition of a ninth planet to our solar system, other astronomers began worrying about Pluto’s planetary worthiness.

Pluto’s orbit was strange.  It was tilted significantly relative to the orbits of the other planets…


…and its orbit was very elliptical, actually sometimes getting closer to the Sun than Neptune…


…and what’s a tiny little ball of ice (much smaller than the planet Mercury) doing out in the region of the Gas Giant planets?

With better telescopes and observing techniques, astronomers began discovering other Pluto-sized icy worlds orbiting the Sun beyond Neptune’s orbit – LOTS of them.

These little worlds became collectively known as Trans-Neptunian Objects (TNOs).

Did I mention there were LOTS of them?


A new and somewhat troubling question arose:  Could Pluto merely be one of many other TNOs?

The question of what to do about Pluto’s planetary status became very important in 2003 with the discovery of a TNO that later was named “Eris.” This icy world was actually bigger than Pluto.


If Pluto is a planet, then shouldn’t Eris be one, too?  If both Pluto and Eris get to be called planets, then what to do with the thousand or so other TNOs discovered to date?

What of the tens of thousands of TNOs estimated to exist that haven’t yet been discovered? Are they all to be called “planets?”

If thousands of Pluto-sized ice balls in crazy orbits around the Sun get to be called “planets” then doesn’t that seriously diminish the meaning of the word “planet” as used to describe the eight biggest and best-known worlds orbiting the Sun in lovely flat and mostly circular orbits?

The International Astronomical Union, the only organization recognized by professional astronomers as the group authorized to definitively answer such questions, grappled with this difficult and contentious subject and after a lot of wrangling back and forth announced in August 2006 that Pluto had been reclassified as a “Dwarf Planet,” and this new Dwarf Planet designation would apply to the thousands of TNOs and the asteroid Ceres… and Pluto.

There are now officially eight “classical” planets in our Solar System:

Four “Rocky Midget” planets: Mercury, Venus, Earth and Mars


Four “Gas Giant” planets:  Jupiter, Saturn, Uranus and Neptune.

Poor Pluto – it’s been demoted.


But that doesn’t mean we no longer love Pluto – far from it!

As you read this, the New Horizons spacecraft is zipping through space at a speed of twelve miles per second for a rendezvous with distant little Pluto.


New Horizons was launched in January 2006 and will arrive at Pluto on July 14, 2015.

Mark your calendars.

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2 thoughts on “Cosmic Quiz Quetsion: Is Pluto a planet?

  1. Ceres Pallas Juno and Vesta were originally (in early 1800s) considered as planets (like Pluto later) but they were dropped from the list of planets when so many similar objects were discovered between Jupiter and Mars. After that they were commonly called “asteroids” or “minor planets”. In 2006 Ceres became a “dwarf planet”.

  2. A moon goes around a planet in orbit. Pluto still has its own orbit. Because it moves into another planet’s orbit, should not matter to whether it is a planet or not.

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