What Is a Planet?

Though astronomer Mike Brown looks through powerful, modern telescopes to search for new objects in the solar system, he uses essentially the same technique that an astronomer used 75 years ago to discover the planet Pluto: the flip book method. He looks at three pictures of the night sky, each taken about an hour apart. To the casual observer, they look like three identical photos — a field of stars on a black sky. However, when he examines them in quick succession, one of the stars appears to move. By comparing its movement and brightness to other celestial objects, Brown, a professor of planetary astronomy at the California Institute of Technology, could tell that he’d discovered an object in our own solar system. Brown was reluctant to call the object, temporarily named 2003 UB313, a planet, but he said, right away “I knew it was bigger than Pluto.”

On July 29th, Brown’s team, including Chad Trujillo and David Rabinowitz, announced its discovery of the solar system object, whose brightness indicated that it was significantly larger than Pluto, practically begging that astronomers classify it as a planet. Though the media and the public enthusiastically embraced 2003 UB313 as Pluto’s long-lost big brother, scientists refused to be so hasty in labeling it the tenth planet. The reason the scientific community hesitated: they think they got it wrong last time around. Depending on your definition of the term “planet,” Pluto, the runt of the litter, barely qualifies.

The problem with classifying objects such as Pluto and 2003 UB313 is that scientists cannot agree on what exactly a planet is. With each new discovery of planet-like objects within our solar system, scientists have had to clarify and redefine their criteria for classification. And, as new findings cause the once distinct lines to blur, scientists struggle to please a confused public without compromising their systematic standards. New discoveries of solar system objects that could be classified as planets do more than cause academic squabbles among planetary scientists. They bring to light the delicate balance between the science that astronomers work by and the science that the public wants to understand.

Astronomers consider an object’s size as the major distinction between planet contenders and non-planets. Size not only contributes to an object’s visible importance in the night sky, but it also determines how round it is, the strength of its gravitational pull, and whether it has an atmosphere or moons, both prominent characteristics of most legitimate planets. While size plays a significant role in the modern understanding of what a planet is, this was not always the case.

In the history of astronomy, each new discovery in our solar system has led to modifications in the definition of the term “planet.” Prehistoric astronomers grouped the five planets that are visible to the naked eye, Mercury, Venus, Mars, Jupiter and Saturn, in the same category as the sun and moon, calling them “wanderers” because their positions traveled in the sky relative to other points of light. By the sixteenth century, when most astronomers finally accepted that the sun was at the center of the solar system, Earth joined the ranks as a sibling to the other planets. With the introduction of the telescope in the early 1600s came the ability to observe smaller and more distant objects. Soon after, Galileo’s discovery of four of Jupiter’s moons, which orbit a planet and do not have their own path around the sun, caused astronomers to reclassify Earth’s own moon as a non-planet. The discoveries of Uranus and Neptune in the eighteenth and nineteenth centuries did not sway astronomer’s conviction that a planet was an object that orbited the sun. In the early 1800s, with the discovery of Ceres 1 and other asteroids, which were considered to be the remains of a large planet that had broken apart or that had never formed, astronomers arrived at size as an important criterion for what defines an object as a planet. Clyde Tombaugh’s discovery of Pluto in 1930 seemed to fit the contemporary definition of a planet: a large object that orbits the sun and no other object.

However, Pluto, it turns out, has become a wrench in the machinery that scientists use to classify celestial objects, mostly because of its size. When Tombaugh first discovered Pluto, scientists grossly overestimated its size. Originally, they thought Pluto’s mass was ten times that of Earth and its diameter similar to Earth’s diameter. Decades later, astronomers have come to realize that Earth’s mass is about two thousand times that of Pluto. And, the tiny planet is smaller in diameter than Earth’s moon and only three or four times larger than the largest asteroids. Scientists were perhaps too quick to classify Pluto without knowing more about its character.

The problem of Pluto’s hasty classification has come up again and again, as astronomers discover members of a new class of objects that pepper the solar system just beyond Neptune’s orbit in a flat, donut-shaped region known as the Kuiper Belt. Dave Jewitt of Hawaii’s Institute for Astronomy discovered the first of these Kuiper Belt objects, QB1, in 1992. QB1’s small, though unconfirmed, size did not challenge Pluto’s reputation, but in the years that followed, as astronomers found hundreds more similar objects, they knew it was only a matter of time before someone would discover an object that did. Only thirteen years later, Brown’s 2003 UB313 became the first discovery that truly rocked Pluto’s standing based on size alone. However, with history books designating it as a planet for so long, many think it is already too late to revoke Pluto’s title.

Attempts to find other characteristics that might distinguish Pluto from Kuiper Belt objects show again and again that Pluto does not fit in with the other planets. For example, only eight of the existing planets (all but Pluto) have orbits that lie in the same plane, while very distant objects, such as Pluto, 2003 UB313 and other Kuiper Belt objects, have orbits that lie in planes that are at an angle to the orbital plane of the planets.

Because astronomers have found that Pluto has more in common with other Kuiper Belt objects than with the other eight planets, many are tempted to qualify its planetary status. “Pluto is unique because it was discovered before its time,” says Susan Kern, a doctoral candidate in MIT’s Department of Earth, Atmospheric and Planetary Sciences. Nearly seventy years after Pluto’s discovery, Kern’s group had discovered hundreds of Kuiper Belt objects that differ very little from Pluto. In fact, most astronomers now agree that though Pluto may be called a planet, it is also classified as a Kuiper Belt object.

Mike Brown’s new object, 2003 UB313, awaits the decision of a group impartial astronomers for a more user-friendly name and an official classification. “It’s in international committee limbo,” says Brown. The International Astronomical Union will eventually meet to decide the object’s status and, in the process, will no doubt have to clarify its definition of what a planet is. Though the public eagerly anticipates the final decision, the committee is not in a hurry to make the same mistake their predecessors made with Pluto.

If astronomers could “start from scratch,” says Brown, neither Pluto nor 2003 UB313 would likely be considered planets. “From a purely classification stance, it makes more sense to put them in one group and the planets in another,” says Brown. Most astronomers seem to agree that, as planets go, Pluto is an oddball, and most would not call it a planet.

However, scientists are reluctant to officially demote Pluto. They worry that classifying all objects that are the same size or larger as planets could render the term “planet” meaningless to the public. Announcing a revised list of dozens of planets could confuse and alienate young fans whose enthusiasm gives birth to new generations of astronomers.

It makes sense from a public relations standpoint to keep Pluto a planet, says Brown. If astronomers say that there are only eight planets and no more can be discovered, they are essentially disappointing their fans. The public gets excited when a new planet is discovered. They don’t get as excited when astronomers announce that they have found a new Kuiper Belt object. “If you tell people there is a tenth planet, they sit up and listen,” says Brown.

Plus, people just really like Pluto and like that it is given special status of a planet — the littlest one at that. Not only did astronomers discover Pluto at a time when it seemed that astronomy was through finding important Earth neighbors, but Pluto’s mysterious nature and small stature have made it a favorite among lay people. “It’s like the underdog,” says Chad Trujillo, a researcher at Hawaii’s Gemini Observatory and co-discoverer of 2003 UB313. People identify with the new guy, and because so little is known about Pluto, they feel that there is still uncharted territory of planetary importance left for young astronomers to explore.

While the public places such a huge importance on the classification Pluto and other objects, for scientific reasons, astronomers don’t actually care whether a newly discovered object is called a planet. That is, such a classification system does not actually matter to their work. “The whole subject of names,” says astronomer Dave Jewitt, “…is of the greatest interest to many members of the public but has no scientific dimension at all. It’s just a distraction from the real interest.”

The astronomical community deems a discovery important if it enlightens us as to how the solar system formed. Planets tend to be large. In general, the larger the object, the more likely it played a bigger role in the story of how the solar system came to be, and the more likely astronomers find it important to their work. Yet, even small objects, such as comets and meteorites, give insight into the physics and composition of the solar system. Size alone cannot distinguish planets from non-planets, or important objects from unimportant objects, because such an arbitrary criterion does not represent a scientist’s understanding of intellectual significance.

Whatever the official definition decided upon by the International Astronomical Union, to keep generations of budding astronomers interested, the public will likely need a more flexible definition. The discovery of Pluto inspired many young people who may have doubted that there was more out there to discover. Announcing the discovery of a new planet every month would lose its freshness, but announcing one every 75 years periodically inspires new waves of enthusiasm for the field of astronomy, without cheapening the discoveries. “There needs to be this right number,” says Brown, “maybe one or two per generation.” A flexible definition of the term can keep Pluto but allow for further discoveries. Most scientists don’t want to admit that their field could benefit from such a manipulation of scientific terminology. However, if the accepted definition must include non-scientific allowances for tradition and public relations, then many agree that it should at least be used to promote enthusiasm for their work.

Astronomers recognize the importance of publicity, as many of them found their calling because of it. In her office at MIT, Susan Kern proudly displays a poster depicting a photo-collage of Clyde Tombaugh and an artist’s rendering of Pluto and Charon, signed by the discoverer of Pluto himself when Kern was a sixth-grader, lucky enough to hear Tombaugh speak at the local planetarium. Though she herself has discovered many objects whose size and import, in the scientist’s view, vie with that of Pluto, Kern cannot deny the popular worth of owning the autograph of this astronomy great. Perhaps with a revised definition of planet, her signature too might thrill a star-struck nascent astronomer.

by Molly F. Wetterschneider (December 5, 2005)