Pluto's Honor

Natural History Magazine

by Neil deGrasse Tyson

From Natural History Magazine, February 1999

According to the latest orbital data, on Thursday, February 11, 1999 between 11 A.M. and 12 noon eastern standard time, having been closer than Neptune to the Sun for the last 20 years of its 248-year orbit, the planet Pluto will regain its distinction as the most far-out planet in the solar system.

In an informal poll of ten thousand junior-high-school children, Pluto was the overwhelming favorite among the nine planets. The poll was simply a measure of how much noise the children made during a tour of the solar system in a planetarium show I presented live to groups of five-hundred children at a time. They consistently cheered the loudest for Pluto, especially when I recited the planets in sequence, aided by the time-honored mnemonic My Very Educated Mother Just Served Us Nine Pizzas.

But Pluto has peculiar written all over it. Found by Lowell Observatory astronomer Clyde W. Tombaugh in 1930, Pluto was discovered the same year that Walt Disney created the lovable, slow-witted bloodhound that shares its name. Among all planet names, Pluto sounds the most like a punch line to a hilarious joke: ...he thought he was on Pluto! And while the names of all other planets are traceable to mythical gods whose talents or powers one might envy, Pluto is named for the god of Hades—a dark and dank residence for the dead. Curious about moon names? Those of Uranus are named for Shakespearean characters, while the lone moon of Pluto, Charon, is named for the boatsman who ferries your unfortunate soul across the river Styx into the underworld.

Peculiar enough for you? I'm not finished.

Pluto's orbit is tilted seventeen degrees out of the plane of the solar system, two and a half times that of Mercury, which has the next most tipped orbit among the nine planets. Pluto moves in the most eccentric ellipse among all planets. To picture the orbit, just take a perfect circle (such as a hula hoop) and gently sit on it: the shape formed under your butt greatly resembles an ellipse. Pluto is the only planet whose orbit crosses that of another planet. Pluto has tidally locked the rotation of its moon Charon, forcing it to forever show the same face to Plutonians. Pluto is in good company here. Earth has tidally locked the rotation of its moon (the Moon) so that it always shows the same face to Earthlings. The embarrassing part is that Charon is so large compared with Pluto that its tidal forces have tidally locked Pluto's rotation where both moon and planet show the same side to each other as they waltz forever in space. In fact Pluto and Charon's barycenter, the point around which planet and moon revolve, falls outside the body of Pluto. (The Earth-Moon barycenter lies 1000 miles beneath Earth's surface.) With a diameter of 1,400 miles, Pluto is, by far, the smallest planet. Seven moons in the solar system are larger: Jupiter's Io, Europa, Ganymede, Callisto; Saturn's Titan; Neptune's Triton; and of course, Earth's Moon (although Mercury is smaller than both Ganymede and Titan). Finally, neither rocky, nor gaseous, Pluto is the only planet made primarily of ices.

Maybe Pluto isn't really a planet.

Dare I have made such a suggestion when Clyde Tombaugh's body is barely cold? Tombaugh died in 1997, at the age of ninety, seemingly secure in his status as the third person ever to discover a planet in our solar system. But there is no question that if Pluto were discovered today, it would not be classified as a planet.

Is no knowledge sacred? What's the definition of a planet, anyway? The ancient Greeks called anything that wandered against the background stars a planet (the Greek word for wanderer transliterates to planet). There were seven of these planets: the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. All seven names would ultimately become assigned to days of the week (in the sequence given) among the various western languages. When the Copernican sun-centered universe was installed and confirmed, Earth was demoted to the rank of planet, the Moon became a satellite, and the Sun joined the ranks of the stars.

Perhaps a planet is simply anything other than a comet that orbits the Sun. William Herschel discovered Uranus in 1781, and Neptune was discovered by Johann Galle of the Berlin Observatory in 1846. Few people know, however, that Giuseppe Piazzi discovered the planet Ceres in 1801, orbiting the Sun between Mars and Jupiter. The suspiciously large gap between them had finally been filled. But astronomers rapidly determined that Ceres was much, much smaller than any other planet: at six hundred miles in diameter, it was dwarfed by Mercury, the reigning smallest planet. Maybe size does matter. Maybe an object can be too small to be defined as a planet. Shortly after 1801, other small objects were found in orbits similar to that of Ceres. A new class of object had been identified: the rocky asteroids. And a new swath of real estate in the solar system had been settled: the asteroid belt.

Ceres was discovered first because it is the brightest and largest. At twice the mass of all the other asteroids combined, of which there are thousands known and millions that await discovery, Ceres swiftly went from being the smallest in the class of planet to being the largest in the class of asteroid.

How about Pluto? When enough time had elapsed to confirm its funky orbital parameters and tiny mass, some skeptical eyebrows were raised. The more we learned about Pluto, the more it did not fit any reasonable classification scheme that applied to the other planets. It was in a class by itself. But can you have a class of one? Should you have a class of one? It wasn't rocky like Mercury, Venus, Earth, and Mars. And it wasn't gaseous like Jupiter, Saturn, Uranus, and Neptune. Pluto supporters breathed a sigh of relief in 1978 when James Christy of the United States Naval Observatory discovered Pluto's lone moon, Charon. If Pluto has a moon, it must be a planet because only a planet can have moons.

Not so fast.

In the 1980s, periodic fluctuations seen in the brightness of asteroids led to the suggestion that some might have orbiting companions that create variations in light via eclipses. Asteroids with moons? Yes. Best studied among these is the asteroid Ida, which was found to have a tiny, gravitationally tethered moon called Dactyl. No longer could the moon argument be invoked to preserve Pluto's planet status.

And so Pluto teetered, until 1992 when David Jewitt of the University of Hawaii and Jane Luu of Harvard began to discover icy bodies just beyond the orbit of Neptune. In the 1990s, dozens of such objects have been discovered with similar properties: They are small (compared with planets), they are icy, they all orbit just beyond Neptune, they have somewhat eccentric paths, and their orbits are tipped out of the plane of the solar system. A new class of objects has been identified and duly named the Kuiper belt, in honor of the Dutch-born American astronomer Gerard Kuiper, who in the 1950s advanced the idea that such a belt of comets might exist. The belt has a sharp inner edge at the orbit of Neptune and extends outward for perhaps ten billion miles. It marks the inner boundary of the outer solar system, where no massive planet has formed and where the crumbs of solar system formation remain uneaten.

Lest we think our solar system is unique, new and improved techniques for observing the dim surroundings of bright stars revealed that nearby sun like systems have rings of debris that resemble our Kuiper belt in shape and size. Among them is 55 Cancri (the fifty-fifth brightest star in the constellation Cancer), which has already been shown to have at least one Jupiter-sized planet, and which reveals a remarkably well-vacuumed region between the host star and the inner edge of its Kuiper belt.

Alas, Pluto, which is small and icy and orbits just beyond Neptune and has an eccentric orbit that is tipped out of the plane of the solar system, is none other than a Kuiper belt object—a leftover comet from the solar system's formation. If Pluto's orbit were ever altered so that it journeyed as close to the Sun as Earth, Pluto would grow a tail and look like a jumbo comet. No other planet can make this (possibly embarrassing) claim.

Just as Ceres went from being the smallest planet to the biggest asteroid, so too must Pluto be reassigned from its smallest planet status to the biggest object of a new class. But all is not lost. There are at least three entities in the world that will remain unaffected by Pluto's problems: (1) Chicago's Adler Planetarium, the nation's first, opened to the public on May 12, 1930 with its sparkling entryway containing bronze plaques representing all eight planets. They missed the official naming of Tombaugh's discovery by two weeks; (2) On the 5th Avenue side of New York City's Rockefeller Center, the sculptor Lee Lawrie has a large bronze of a well-muscled Atlas holding the universe upon his shoulders. Designed in the late 1920s, this Art Deco masterpiece also barely predates Tombaugh's discovery, and identifies all eight planets along a celestial arc that his shoulders support; (3) The orchestral masterpiece The Planets by the English composer Gustav Holst, is a sequence of movements that capture the mythological theme for each planet in the sky—except Pluto. The music was composed between 1914 and 1916 and premiered in 1919, eleven years before Pluto's discovery.

Although the decade of the 1990s will carry the stigma of having demoted Pluto, it is the same decade in which the first batch of planets around other stars were discovered. We reliably know of more planets outside the solar system than within it, and the tally continues to rise. No, we didn't lose a planet. We gained more branches in what is now a richer family tree.

As citizen Tyson, I feel compelled to defend Pluto's honor. It lives deeply in our twentieth-century culture and consciousness and somehow rounds out the diversity of our family of planets like the troubled sibling of a large family. Nearly every school child thinks of Pluto as an old friend. And there was always something poetic about being number nine.

As professor Tyson, however, I must vote—with a heavy heart—for demotion. Pluto was always an enigma to teach. But I'd bet Pluto is happy now. It went from being the runt of the planets to the undisputed King of the Kuiper belt. Pluto is now the big man on a celestial campus that occupies a larger tract of the (outer) solar system than that spanned by the eight planets. But before we become complacent about all this, we may still need to root for Pluto because not since Tombaugh's extensive survey (in the late 1920s and 1930s) has a complete and systematic search of the outer solar system been conducted. The physical extent of the Kuiper belt means that cometary objects even larger than Pluto may await discovery in more distant orbits. Several ongoing telescope projects, including the Sloan Digital Sky Survey, are well-positioned to undertake this search.

With the demotion of Pluto, no longer do I need to look askance at its nonplanetary properties. No longer do I need to make excuses for its odd behavior. But be forewarned. Earth itself may not be entirely immune from the further downsizing of the solar system. Astronomers on Jupiter could argue that the solar system contains four nice-sized, gaseous planets (Jupiter, Saturn, Uranus, and Neptune), while everything else—Earth included—amounts to wayward debris. To these Jovian scientists, size would surely matter: Jupiter is 1,400 times larger in volume than Earth, the largest of the four rocky planets. Yet Earth is only 180 times the volume of Pluto—a planet that the Earthlings just kicked out of the club. If and when the time comes, I will reluctantly write the sequel: Pluto's Revenge.

Neil deGrasse Tyson, an astrophysicist, is the Frederick P. Rose Director of New York City's Hayden Planetarium and is a research scientist at Princeton.