PLUTO AND THE PLANETARY BLUES
Hello everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences; welcome to The Night Sky.
Remember when our solar system had nine planets? Our little corner of the universe seemed so familiar and well organized back then: we had the Sun, four rocky inner planets, an asteroid belt, four outer gas giant planets, a number of moons, the occasional comet, and diminutive little Pluto hanging out all alone in the outskirts. But in 2006 we lost Pluto as a member of the planet club. What changed? How can you demote a planet? And who has the authority to even do such a thing?
Our story begins in1905 with the hunt for a theoretical Planet X thought to exist just beyond the orbit of Neptune. Astronomers noted that there were some slight irregularities in Neptune’s orbit and they suspected that an undiscovered planet’s gravitational tug might be the culprit. In 1929 the Lowell Observatory in Flagstaff, Arizona hired a 24 year old farm boy and self taught astronomer named Clyde Tombaugh to hunt down Planet X.
The work was tedious and involved Tombaugh taking two photographs on glass plates of the same section of sky several nights apart. He would then put the plates on a machine that allowed him to flip the two images back and forth so quickly that they would appear as one. Each plate contained thousands of stars and Tombaugh was looking for one tiny dot in a sea of dots to move ever so slightly between frames. After a year of doing this day in and day out the tedium finally paid off and Tombaugh found what he was looking for. The story made front-page headlines around the world and since the object appeared to be round and everyone expected to find a planet, a planet it was deemed to be.
A young schoolgirl in England read the story and thought that a world in such a far-flung corner of the solar system should be named for the Greek god of the underworld, Pluto. Her grandfather even wrote a letter to the Lowell Observatory’s director to make the suggestion and the name stuck. Later that year animator Walt Disney, who always had an interest in space and space exploration, debuted a loveable dog character with the same name in one of his cartoons. If anything this helped to cement the popularity of planet Pluto with the public.
The technology available at the time couldn’t really tell us much about an object so small, faint, and distant as Pluto. For many years the only thing we knew with any certainty is that Pluto orbited the Sun in a very odd manner. All of the other planets move around the Sun in nearly circular orbits but not Pluto. Its orbit is highly elliptical; what’s more is that during part of its 248 Earth-year long orbit around the Sun Pluto actually crosses over Neptune’s orbit. This means that for 8% of Pluto’s orbit it’s closer to the Sun than is Neptune. Odder still is that Pluto doesn’t orbit within the same plane as all the other planets, its orbit is tilted by 17 degrees relative to the solar system’s plane. It’s almost as if Pluto were trying to behave differently from all the other planets!
Pluto remained both an oddity and a mystery well into the 20th century.
After Pluto’s initial discovery people began to puzzle over the fact that this remote little world appeared so all alone, it seemed to exist out of context with the rest of the solar system. Many astronomers, Clyde Tombaugh included, began a search for any neighbors that Pluto might have. Nothing turned up but, given what we knew about the solar system’s formation, astronomers were certain there must be a realm just beyond Pluto where the frozen relics that went into its making were sure to be found. Most of these objects, the reasoning went, would be the source for our short period comets while others would most likely be small worlds akin to Pluto
Yet Pluto remained a lonely anomaly until 1978 when astronomers discovered that it had a moon. This companion to Pluto was named Charon and the two are locked together in synchronous rotation. By watching the two orbit around one another astronomers were able to work out their mass and size. Pluto is some 1,433 miles in diameter while Charon is a little more than 750 miles across; both worlds could easily fit across the continental United States. Some astronomers prefer to think that the two are in fact a double planet system rather than a planet and moon. Today there are a total of five known moons, all tightly packed together, and it’s thought that the system may be the debris left behind after something big collided with Pluto long ago.
Beginning in the 1990’s astronomers began to find even more objects within Pluto’s neighborhood; in fact, there are some 1,300 known icy objects in this outer zone of the solar system we now call the Kuiper Belt (in honor of astronomer Gerard Kuiper, one of the big proponents of its existence in the 1950’s). Some of these objects are icy boulders while others are nearly as big as Pluto and we suspect that there could be billions more out there.
All of these discoveries begged the question as to whether or not Pluto and other Kuiper Belt objects could rightfully be called planets or whether they should belong to a special category all their own. Much to the embarrassment of the astronomical community came the realization that there wasn’t a formal definition of what a planet actually is. The International Astronomical Union, the organization responsible for officially naming celestial objects, decided to rectify that. The IAU came up with three criteria an object must meet to defined as a planet:
1) The object must orbit the Sun. No problem there, Pluto certainly does that.
2) The object must be massive enough to have pulled itself into a spherical shape. Pluto meets this requirement too.
2) The object must be able to have cleared the neighborhood of its orbit. This
means that a planet must be the dominant gravitational presence in its orbit. Like all the other classical planets it must have consumed smaller bodies or hurled them out of its way. Clearly, Pluto was not massive enough to have done that and, so, it was deemed not a planet. Instead, it belongs to the new category known as a dwarf planet. Rather than Pluto being the solar system’s ugly stepchild it had now found its rightful place as one of the largest members of its kind in a newly discovered realm of the solar system. Unfortunately the public and many in the astronomical community didn’t see it this way. There were organized protests and many an angry letter written to various scientists deemed as being “Pluto Killers”.
Perhaps in the end it doesn’t really matter what we call Pluto, the argument may be nothing more than a case of “you say tomato and I say tomato”. What really matters is that Pluto is a world worth knowing and this month, after a nine and half year journey, NASA’s New Horizons spacecraft will make a flyby to image, map, study, and reconnoiter this distant world and its busy moon system. Up until now all of our images of Pluto have been nothing more than fuzzy blobs of pixels but now we should be getting high definition images of this mysterious world. After its flyby of Pluto the spacecraft will head deeper into the Kuiper Belt to explore. New Horizons is literally sailing into unknown waters and who knows what wonders await us there. Exploration and discovery is what makes the human adventure worth being a part of and I for one eagerly await what we will learn about Pluto and the other denizens of this remote corner of our solar system.
Also, in a fitting side note, New Horizons is carrying some of Clyde Tombaugh’s ashes out to the little world he discovered all those years ago in 1930.
Visit our web site to see postings of New Horizon’s discoveries as they become available.
Until next time I encourage you to take a little time to step outside and look up in both awe and wonder.