What Happened to Pluto?

Sue Jagoda
March 03, 2007 | Science News

Top image: Pluto and Charon (Date: 02.21.1994)
NASA's Hubble Space Telescope (HST) provided this clearest view yet of Pluto and its moon, Charon. The European Space Agency's Faint Object Camera captured the image on February 21, 1994. Pluto was 4.4 billion kilometers (2.6 billion miles) from Earth—that's nearly 30 times the separation between Earth and the sun. That the Hubble Telescope was able to make out Pluto's disk at this distance is as if a human eye could see a baseball at a distance of 64 kilometers (40 miles).

The universe is what it is, not what we want it to be, and science must always be open to correcting its mistakes.

(From NASA—Pluto, Classification and Exploration)

A few weeks ago the topic of Pluto came up for discussion at our dinner table. My 14-year-old daughter mentioned that they had been talking about Pluto during band rehearsal. She related that one young man had considered that, "...When I'm older and have children, I'll be able to tell them I knew when there used to be nine planets in the Solar System."

So, what happened to Pluto? Why was it "demoted" from being a full-fledged planet to the category of dwarf planet? Why do we now name only eight planets in the Solar System? And, why does it matter?

Several astronomers had predicted the existence of Pluto after observing irregularities in the orbits of Uranus and Neptune. They suggested that a more distant, unseen planet was affecting their orbits. One of the astronomers was Percival Lowell, who is credited with the successful prediction of the planet's orbit. He started the search for the planet, which was ultimately discovered by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. The discovery happened on January 23, 1930, when Tombaugh compared the photographic plate he took through the observatory's 13-inch telescope that evening with two other plates taken earlier in January. Pluto showed up as only a point of light moving extremely slowly against the background stars. Even now, with the Hubble Telescope, astronomers have been able to capture only blurry images of Pluto and its satellite Charon (discovered in 1978).

In 1930 there was no way to determine the mass of Pluto, so without this information it was not recognized as a new class of obj-ect. For many years, the size, mass, and density of Pluto were thought to be similar to those of Mars. Then in 1978, James W. Christy discovered Pluto's satellite, Charon, using the 61-inch astrometric reflector of the U.S. Naval Observatory in Flagstaff. Pluto's mass could now be calculated using Kepler's laws of motion. Its mass was calculated as 1/400 of Earth's mass with a diameter of less than 2414 km—it was smaller than the Moon! The seeds of the debate about Pluto's planetary status were sown.

Until August 2006, there was no scientific definition for the word "planet." The word planet originally came from the Greek and meant "wanderer." Planets were ob0jects that appeared to move across the background of fixed stars. The historical use of the word identified nine planets in the Solar System: Mercury (My), Venus (Very), Earth (Educated), Mars (Mother), Jupiter (Just), Saturn (Sent), Uranus (Us), Neptune (Nine), Pluto (Pizzas). (Substitute whatever mnemonic you used to remember the order of the planets.) The historical definition went something like this: Any of the nine large celestial bodies in the Solar System that revolve around the sun and shine by reflected light (from WordNet).

The historical definition for planet workedjust fine for many years. Pluto fit the definition, but with the dawn of the space age and the development of new telescopes like the Hubble, other planet-like bodies were discovered orbiting the Sun, including Xena, Eris, and members of the Kuiper Belt, revolving around the Sun at a distance of 4.5 to 7.5 billion km (2.8 billion to 4.6 billion miles). The discussion about which obj-ects should be called planets came to the forefront at the International Astronomical Union (IAU) in Prague on August 24, 2006. At this meeting, the first-ever scientific definition came to a vote. The new definition changed Pluto from planet status to that of a dwarf planet.

New Definitions

In all scientific endeavors, when new evidence is produced, definitions and theories may have to be modified or created. This is what happened with the latest discoveries in the Solar System. Not only was the term "planet" provided with a scientific definition, new terms had to be selected and defined to help categorize other obj-ects.

So, what is the new scientific definition of a planet? A planet in our Solar System is:

  • A celestial body that orbits the Sun.
  • Has sufficient mass to become round.
  • Has cleared the neighborhood around its orbit.

Under this definition Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all qualify as planets. Neptune affects Pluto's orbit, so Pluto hasn't "cleared the neighborhood around its orbit." Pluto is now designated a "dwarf planet"—a descriptive term still under discussion. A dwarf planet is like a planet, except it does not clear the neighborhood around its orbit. Other dwarf planets include Ceres, the largest asteroid found in the Asteroid Belt, Pluto's satellite Charon, Xena, and Eris. At this time the IAU is evaluating other obj-ects in the Solar System, particularly those in the Kuiper Belt, to determine whether there are other dwarf planets.

With more observations of the eight planets, further categories have been established. Mercury, Venus, Earth, and Mars are the terrestrial planets—made mostly of rock. Jupiter, Saturn, Uranus, and Neptune are the gas giants—large planets composed mostly of hydrogen and helium and probably having a rocky or metallic core. These categories are again descriptive and are not officially defined by the IAU yet. A new category, pluton, is being reviewed and defined by the IAU. Plutons have enough mass to be round, but are distinguished from planets by their orbits. Their orbits around the Sun take longer than 200 years to complete (i.e., they orbit beyond Neptune), are highly tilted with respect to the classical planets, and are far from being perfectly circular. Pluto, Charon, Eris, and Xena would all fall into this category.

Why Does It Matter?

Person with Save Pluto shirt

Allyson models a t-shirt that may spark more discussion at band rehearsal.

The announcement that Pluto was being demoted to dwarf planet status caused a furor in some circles. Some members of the general public reacted strongly to the change in Pluto's status. The reaction is described in the web article, NASA: Pluto, Classification and Exploration.

Who would have thought so many people cared about Pluto? Some seem to be worried that their world view will now have to be changed from the one they acquired based on their textbooks. But surely the definition of that they change with new knowledge.

Even astronomers are not all in agreement over the new definition of planet. Over 300 scientists have signed a petition against the IAU resolution. Pluto's destiny may yet be undecided. The petition states:

We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed.

Such is the nature of science—new knowledge stimulates new ideas and possibly new classifications. Debate continues, and agreement may or may not happen. As exploration goes on and knowledge accumulates, old ideas may evolve or be scuttled. New questions will be asked, and answers will be proposed. That's how science is supposed to work.

Your students may think dropping Pluto from the classical planetary line-up is a good thing—one less planet to memorize. What a great opportunity to discuss the scientific process with your students! You might consider setting up a debate between groups of students concerning the new definitions, having them gather evidence to support their cases for accepting the new definition or retaining the old. Encourage them to study Pluto's place in astronomical history and keep track of the debate happening today. Then, when they mention to their children that they remember when there were nine planets in the Solar System, they'll have more to talk about than how an old memory device had to be changed.