What is a Planet? Upcoming New Horizons Pluto Flyby Resurrects Hot Debate, Old Passions

In just two days, NASA's New Horizons space probe will fly past Pluto, which was the 9th and final planet in the solar system at the mission's launch. Now, 9 years and over 3 billion miles later, Pluto has long since been demoted to the status of 'dwarf planet.' That aside, scientists and the public are buzzing over the mission, which has, to the surprise of none, re-ignited the debate over Pluto's planetary status.

It was August 24, 2006 when our solar system lost one of its own. It was on this date that the International Astronomical Union (IAU) voted on a new definition of the word 'planet.' By changing what makes a planet a planet, the IAU instantly stripped the outermost planet of the solar system, Pluto, of its planet status,demoting it to a 'dwarf planet' instead. For Pluto, the road to demotion was a long time coming.

Pluto was discovered in 1930 by Clyde Tombaugh, who had yet to earn a college degree at this point. In the decades since the discovery of Uranus, astronomers noticed that its orbit seemed to exhibit some unexpected eccentricities. This observation led to the discovery of Neptune in 1846 and, following this logic, the implication was that there was an even more distant planet tugging on Neptune after it, too, seemed to exhibit orbital oddities.

So, the search was on for the mysterious 9th planet, which was found by Tombaugh on February 18, 1930.

Initially, astronomers had a very hard time determining the size of Pluto as it was so far away and the tools available at the time of discovery were primitive by today's standards when it came to their planet-measuring capabilities. However, as time progressed, Pluto only seemed to get smaller and smaller, eventually reaching the point where it was estimated to be smaller than many of the moons in the solar system, including our own. By 1978 and the discovery of its first known moon, Charon, Pluto was known to be only about 1/500^th the mass of Earth, far from the 1 Earth mass first suggested shortly after discovery. By virtue of its size alone, some scientists started to question whether Pluto deserved to be called a planet at all.

The next blow for Pluto came with the advent of digital imaging technology. For astronomers, digital CCD chips, which came into mass use in the 1990s, were far more sensitive than film and could reveal much greater details. With the advances in imaging technology, many objects at Pluto's distance from the Sun were found. Now, with the fact known that Pluto was not unique at all, scientists were faced with a dilemma: start adding more planets to the solar system (and thus overwhelm the mind of schoolchildren the world over) or reconsider the definition of a planet.

As history shows, the astronomers took the latter option.

The final blow to Pluto's status as a planet came on July 29, 2005, when the existence of Eris, a body 3 times more distant but nearly 30% more massive than Pluto was confirmed. Eris was discovered by astronomer Mike Brown, who, in a TV interview, recalled calling his wife immediately after the discovery to announce that he had discovered the 10^th planet. Unfortunately for Brown, he would not enter the Pantheon of astronomers occupied by the other planet finders: Tombaugh, Gallee, and Herschel. Instead, Brown's 'planet' was merely the largest in a series of Trans-Neptunian objects discovered since the advent of digital imaging, which is far more adept at low-light photography than film. As a result, being larger than Pluto but now known as nothing unique, Brown's planet, temporarily named Xena (but since named Eris), would never gain planetary status and, despite being larger than Pluto, would retain its status as a newly-dubbed 'dwarf planet' while Pluto would continue to be defined as the 9^th planet.


It was because of this problem, how could the larger of two distant bodies be considered not a planet while the smaller one was a planet, that the scientific community began to reassess the definition of the word 'planet.'


Result: on August 24, 2006, the International Astronomical Anion (IAU) came up with the following definition of the word 'planet,' which reads: “a body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood of other orbiting bodies.” Obviously, Pluto met the first 2 conditions (it orbits the Sun and nothing else, it is round), but not the third, as it has failed to clear its neighborhood thanks to the fact that its moon, Charon, is not a true moon in that Charon does not orbit Pluto, but both bodies orbit a point in space between them where their gravitational fields meet, making for more of a double planet system than a planet-moon one.

For many people, both scientists and especially members of the public, the demotion of Pluto was a tough pill to swallow as most everyone alive (save the people 77+ years of age) grew up on the notion of 9, not 8, planets. In fact, there was even a massive 'save Pluto' petition being circulated online, but to no avail as the IAU refused to budge on this question of what defines a planet.


So, regardless of what you call it, Pluto is due for a visitor in two days, so stay tuned!

Humble Requests:

If you found this informative (or at least entertaining), help me pay my bills and check out my Examiner pages for space news, cleveland photography, national photography, and astronomy for more great stuff.

If you think this was cool, why not tell a friend?

For something even better, follow this blog.