Thursday, February 18, 2016

Tonight's Sky for February 18: Pluto Discovered (1930)


It was on this date in 1930 that Clyde Tombaugh, yet to earn a college degree, discovered Pluto, the former 9
th planet in the solar system.

In the decades since the discovery of Uranus in 1781, astronomers noticed that its orbit seemed to exhibit 
some unexpected eccentricities. This observation led to the discovery of Neptune in 1846. When Neptune, too, seemed to exhibit similar orbital disturbances and following the logic that led to its discovery, the implication was that there was an even more distant planet tugging on Neptune, too.

So, the search was on for the mysterious 9th planet, which was apparently 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 1/500th 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, astronomers were able to kick the proverbial can down the road thanks to the fact that none of these new bodies were anywhere near as large as Pluto, realizing that, while Pluto was no longer alone in the outer, it was an anomaly in that it was a relative giant among dwarfs.

However, the road on which to kick the can was not endless.

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 (Neptune), and Herschel (Uranus). Instead, Brown's 'planet' was merely classified as 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. 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 at the time) 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.

Still, regardless of what one chooses to call it, Pluto was revealed in stunning detail by the
New Horizons mission last July, the data from which is expected to continue trickling back to Earth for the remainder of 2016. 

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