FEATURE ARTICLES
January 2007 issue

What Is a Planet?

Most of us grew up with the conventional definition of a planet as a body that orbits a star, shines by reflecting the star's light and is larger than an asteroid. Although the definition may not have been very precise, it clearly categorized the bodies we knew at the time. In the 1990s, however, a remarkable series of discoveries made it untenable. Beyond the orbit of Neptune, astronomers found hundreds of icy worlds, some quite large, occupying a doughnut-shaped region called the Kuiper belt. Around scores of other stars, they found other planets, many of whose orbits look nothing like those in our solar system. They discovered brown dwarfs, which blur the distinction between planet and star. And they found planetlike objects drifting alone in the darkness of interstellar space.

These findings ignited a debate about what a planet really is and led to the decision last August by the International Astronomical Union (IAU), astronomers' main professional society, to define a planet as an object that orbits a star, is large enough to have settled into a round shape and, crucially, "has cleared the neighborhood around its orbit." Controversially, the new definition removes Pluto from the list of planets. Some astronomers said they would refuse to use it and organized a protest petition.

This is not just a debate about words. The question is an important one scientifically. The new definition of a planet reflects advances in our understanding of the architecture of our solar system and others. These systems formed by accretion within rotating disks: small grains clump together to form bigger ones, which draw themselves together to form still bigger ones, and so on. This process eventually produces a small number of massive bodies--the planets--and a large number of much smaller bodies--the asteroids and comets, which represent the debris left over from planet formation. In short, "planet" is not an arbitrary category but an objective class of celestial bodies.

When Earth Became a Planet
Astronomers' reevaluation of the nature of planets has deep historical roots. The ancient Greeks recognized seven lights in the sky that moved against the background pattern of stars: the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. They called them planetes, or wanderers. Note that Earth is not on this list. For most of human history, Earth was regarded not as a planet but as the center--or foundation--of the universe. After Nicolaus Copernicus persuaded astronomers that the sun rather than Earth lies at the center, they redefined planets as objects orbiting the sun, thereby putting Earth on the list and deleting the sun and moon. Telescope observers added Uranus in 1781 and Neptune in 1846.

MORE ON THIS ARTICLE OVERVIEW · Planet Definition The below infographics are no longer available. The full versions of this and other articles from the print edition--including all graphics and sidebars--are available for purchase at Scientific American Digital. · Historical Count of PLanets · The "New" Solar System · Frequently Asked Questions Click here to go to Scientific American Digital

Ceres, discovered in 1801, was initially welcomed as the missing planet that filled the gap between Mars and Jupiter. But astronomers began to have doubts when they found Pallas in a similar orbit the following year. Unlike the classical planets, which telescopes revealed as little disks, both these bodies appeared as mere pinpricks of light. English astronomer William Herschel proposed naming them "asteroids." By 1851 their number had increased to 15, and it was becoming unwieldy to consider them all planets. Astronomers then decided to list asteroids by their order of discovery rather than by distance from the sun, as for planets--the de facto acceptance of the asteroids as members of a distinct population. If we still counted asteroids as planets, schoolchildren studying the solar system would now have to cope with more than 135,000 planets.

Pluto has a similar story. When Clyde Tombaugh discovered it in 1930, astronomers welcomed Pluto as the long-sought "Planet X" whose gravity would account for unexplained peculiarities in the orbit of Neptune. Pluto turned out to be smaller not only than the other eight planets but also than seven of their satellites, including Earth's moon. Further analysis showed the peculiarities in Neptune's orbit to be illusory. For six decades, Pluto was a unique anomaly at the outer edge of the planetary system.

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The Future of Space Exploration

To the Moon and Beyond

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This is Your Brain on Food (extended version)