Requiem for a Planet: Pluto

Artist conception of Pluto and its moon Charon. (Copyright Lee Krystek, 2012)

For almost three-quarters of a century schoolchildren learned that our solar system had 9 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Then in 2006 this changed, Pluto got demoted and now there are only eight planets. What happened? Why did poor Pluto get kicked out of the planetary club?

Wandering Stars

To understand why Pluto isn't a planet, perhaps it helps if we define what the term planet means. The expression goes back to ancient times and comes from the Greek word for "wandering star." Because the planets are bodies that move in orbit around the sun, they do not stay in a fixed location in the sky in relation to the stars. Ancient peoples, including the Greeks, noticed that over a course of months these special stars seemed to move along a particular path across the sky. Usually it was in one direction, but occasionally they would move backwards, then again forwards. The constellations (or star groups) that they passed through were given special significance, and today we know these as the zodiac.

It was clear to these early observers that the wandering stars were different from the stationary ones, but they didn't know exactly why. Back then the Greeks didn't have telescopes and they were limited to using their naked eyes. This meant they were only familiar with the most brilliant of these wanders which they named for their gods. Today we are aquainted with the Roman versions of these names: The brightest was Jupiter, king of the gods, Saturn was his father, Venus was the goddess of love, Mars, which appeared to be an angry red, was the god of war, and Mercury was the messenger god.

Uranus was the first planet discovered in modern times, though Herschel's telescope wasn't powerful enough for him to see that it had rings. (NASA)

So in ancient times there were only six planets. They weren't the only things that moved through the night sky, however. Comets also progressed against the background of the stars. Comets only appeared at very irregular intervals, though, and they often had a long tail (comet actually means "hairy star") so they were put into a different category than the planets.

New Planets

Additional planets were not discovered until telescopes were invented and the mechanics of how the solar system was arranged - with the Earth orbiting around the sun, and not the reverse - was understood. By the end of the 16th century Uranus had been observed and recorded several times, but people who saw it just thought it was a regular star or comet. In March of 1781 Sir William Herschel found it again and began to track its motion. His first thought was that it was a comet, but he and his fellow astronomers calculated that its orbit was nearly circular around the sun and realized it was a planet beyond Saturn. Herschel wanted to name it after the British King, George III, but foreign astronomers disagreed and eventually the name Uranus, god of the sky, was approved by consensus.

Surprisingly, Pluto was not the first member to be kicked out of the planetary club. As far back as 1596, Johannes Kepler had come to the conclusion that there was a gap between Mars and Saturn large enough to accommodate another planet. The discovery of Uranus further confirmed that there was a pattern to how far planets were apart and that there was indeed an empty space. In 1800 a number of astronomers started on an intense search for such a planet in the gap. On January 1st, 1801, Italian scientist Giuseppe Piazzi found a small object that was in a circular orbit in the right location. It was declared a planet and named Ceres after the Roman god of agriculture.

It soon became apparent, however, that Ceres, with a diameter of 569 miles (960km), was much smaller than the other planets. Within a year astronomers also realized it was not alone. A large number of similar, but smaller, objects were found orbiting in that region of space. Sir William Herschel coined the term asteroids (which means "star-like") for such bodies. Though there was no official organization at the time to change the status of Ceres, within a few decades all references to it being a planet disappeared from the textbooks. Today we refer to the area in which Ceres and the other asteroids are found as the "asteroid belt."

Ceres: Got kicked out of the club too (NASA).

In 1846 the French scientist Urbain Le Verrier, looking at the orbit of Uranus, came to the conclusion that the way it moved suggested it was being pulled by the gravity of an unknown body, probably another planet beyond Uranus's orbit. He calculated where this planet could be found in the sky and asked German astronomer Johann Gottfried Galle to search for it with his telescope. Galle found it and it was dubbed Neptune after the Roman god of the sea. In England John Couch Adams had independently made the same calculations, but couldn't find anybody with a telescope who would take him seriously enough to do a search.

The Search for the Ninth Planet

By the start of the 20th century scientists had noticed the same type of unexplained motions in Neptune's orbit that when seen in Uranus's orbit had led to Neptune's discovery. Many astronomers began to suspect that there might be a 9th planet out there somewhere. In 1906 Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory, started a search for it. When it still hadn't been found by his death in 1916, the observatory hired an eager, 23-year-old astronomer named Clyde Tombaugh to continue the search in 1929.

Tombaugh went at the tedious task tirelessly. Each night he would photograph a section of the sky where the planet was suspected to be. He then used a device called a blink comparator, which allowed him to quickly switch back and forth between pictures, to contrast that photo with one taken a few weeks before. If any of the stars seemed to jump between the two photos, he would know it was moving. This meant it was either a comet or a planet.

A young Clyde Tombaugh found Pluto (NASA)

On February 18, 1930, Tombaugh was looking at a pair of photographs taken the month before when he saw such a jump. Additional observations confirmed that the object appeared to be in a near circular orbit around the sun and it seemed that the 9th planet had finally been found.

The discovery was a world-wide sensation. Tombaugh urged that a name be quickly selected. Some of the suggestions included Zeus (the Greek name for the king of the gods) and Percival, after the Lowell founder. Strangely enough, the name Pluto came from an 11-year-old schoolgirl named Venetia Burney who lived in Oxford, England. She had been studying the Greek classics and thought that the name of the god of the underworld would be perfect for such a cold and lifeless orb at the edge of the solar system. Her grandfather, a retired Oxford librarian, passed the idea onto a professor, who cabled the idea to the Lowell. The name received a unanimous vote by the Lowell staff and was announced on May 1, 1930. Its selection may have been influenced by the first two letters being "PL" which were also the initials of Percival Lowell.

The Odd Planet

It wasn't long after Pluto's addition to the planetary club that people began to notice that "one of these things was not like the others." The first problem was its orbit. Further careful studies showed that it wasn't as circular as had been supposed. In fact, the orbit was so elliptical (like a squashed circle) that it actually was closer than Neptune to the sun for part of its year. Also, while the rest of the planets orbited along a flat plane called the ecliptic, Pluto's orbit was tilted off that plane by over 17.

There was also the question of its size. Early estimates made shortly after Pluto was found suggested it was about the same diameter as Earth. However, as telescopes got better and better it appeared that Pluto was smaller and smaller. In 1978 astronomer James Christy found that Pluto had a moon (later named Charon after a minion of the god Pluto in mythology) and observation of its orbit allowed scientists to get a very accurate measurement of Pluto's size: only 0.2% of the mass of Earth.

The telescope Tombaugh used in his search for Pluto. (Copyright Lee Krystek, 2012)

Other discoveries appeared that caused some astronomers to question Pluto's status. In 1992 scientists found evidence that there was a large disc of scattered rocky and icy material out beyond the orbit of Neptune. They dubbed this the "Kuiper Belt" in honor of Gerard Kuiper who had speculated about such a region back in the 1950's. Some scientists began to suggest that rather than being a planet, Pluto was just the largest member of this belt.

Planet Eris

The matter came to a head in 2005 when astronomer Mike Brown, working at the Palomar Observatory, discovered an object far out beyond Neptune. It was eventually named Eris (after the Greek goddess of strife - an apt name given all the controversy it would cause). What was alarming about Eris was that it appeared, with a diameter of 2,397km (1400 miles), that it was just slightly bigger than Pluto. Should it be the tenth planet?

The International Astronomical Union (IAU), which was the group that had taken over the official naming of celestial objects, was forced to confront the question the next year. The IAU had three ways they could resolve the problem. Each solution required them to develop an official definition of the term planet, something that had never actually been done before.

First, they could simply define a planet as an object bigger than Pluto that orbited the sun in a roughly circular orbit. This would make Eris the tenth planet. The downside of this was that there were indications that Eris was not out there alone. This meant that the ranks of the planets might rapidly swell with these small, far-away objects.

Eris as seen by the Hubble Telescope. (NASA)

The second thing they could do was to declare objects like Eris and Pluto to be something other than planets. This new class would include anything like them that was found after this point in time.

There was a third possibility which was to keep Pluto in the planetary club for historical reasons and place Eris and other newly-found objects into a new class. Supporters of this approach cited our commonly-held belief that the earth, for reasons of history, has seven continents, even though it is clear that under any logical definition Europe and Asia are a single landmass.

The IAU decided on the second approach and defined a planet as a body going around the sun, large enough to take on a spherical shape and "clear the neighborhood around its orbit" of other objects. Since Pluto shared its space with other Kuiper Belt bodies, it failed this last test. A new class of objects called dwarf planets was created and Pluto and Eris were placed into this category.

Controversy

Not everybody was happy with the IAU's decision. Astronomer Alan Stern, leader of NASA's New Horizon's mission to Pluto, complained that the new definition was too vague and inaccurate as many of the traditional planets have asteroid-type objects orbiting near them so they have not "cleared their neighborhoods." He also noted that only about 5% of professional astronomers had been available to vote on the issue. However, Mike Brown, who might have been credited with the discovery of a new planet if the decision had gone another way, said that he thought that the IAU had done the right thing. "There will be hundreds of dwarf planets," Brown predicted.

The decision was probably made easier by Tombaugh's death in 1997. Few members of the IAU would have wanted to strip the eminent astronomer of his greatest success. His widow remarked, however, that he would have accepted the change. She said Clyde "was a scientist. He would understand they had a real problem when they started finding several of these things flying around the place."

A picture of Pluto and Charon taken by the Hubble Telescope (NASA)

As least a few political entities have come to Pluto's defense because of their connection with Tombaugh. In Illinois, where the astronomer was born, the state senate passed a resolution saying that Pluto was "unfairly downgraded to a 'dwarf' planet." Similarly in New Mexico, where Tombaugh had been a long-time resident, the House of Representatives passed a resolution declaring that "Pluto will always be considered a planet while in New Mexican skies."

For those that are sad about Pluto's fate, perhaps it is best not to think of the planet getting as demotion, but of having the honor of being a founding member of a whole new class. The dwarf planets of our solar system now include Pluto and Eris along with Haumea, Makemake, Sedna and Quaoar. Even Ceres, which got kicked out of the planetary club back in the 19th century, has been able to join this new group (though it also retains its membership in the asteroid belt).

Will the definition of a planet change again? Perhaps. Astronomers have spent the last 20 years searching for planets circling stars other than the sun. As they find more it's likely that we will find objects that don't quite meet the current definition. Some change seems inevitable.

Is there still a 9th planet out there in our solar system that might fit the IAU's new definition? In 1992 when the Voyager 2 space probe flew by Neptune it got a chance to get a better gauge on the planet's mass. This new information showed that it was slightly smaller than originally thought. When the planet's new size was used to recalculate its orbit, the movements that suggested that there was a 9th planet tugging on Neptune vanished. So it seems likely that the eight local planets we are familar with are the only ones we have.

Meep reacts to rumors that Pluto will be demoted in our comic strip LGM.


A Partial Bibliography

Pluto Demoted: No Longer a Planet in Highly Controversial Definition, by Robert Roy Britt, Space.com, http://www.space.com/2791-pluto-demoted-longer-planet-highly-controversial-definition.html

The girl who named a planet by Paul Rincon, BBC News, http://news.bbc.co.uk/2/hi/science/nature/4596246.stm

Planet Quest: The Epic Discovery of Alien Solar Systems, by Ken Croswell, The Free Press, 1997.

Discovering Pluto, Lowell Observatory, http://www.lowell.edu/about_history_pluto.php

Copyright 2012 Lee Krystek. All Rights Reserved.