Date: February 18, 2010

Title: The Discovery of Pluto


Podcaster: Ted Haulley


Description: This podcast will look at the discovery of Pluto by Clyde Tombaugh, eighty years ago today. Tombaugh and the Lowell Observatory was searching for the mysterious ‘Planet X,’ a planet though to be gravitationally influencing the orbits of Uranus and Neptune. Percival Lowell spent the last years of his life looking for this planet, and even predicted the size, orbit, and distance from the Sun for this 9th planet. What Tombaugh found has gone from Planet X to Planet Pluto, to dwarf planet.

Bio: Ted is pleased to provide his sixth podcast to the 365 Days of Astronomy. A native of California, he currently lives in Maryland with his wife and two daughters. A member of the United States Navy Reserve, he is currently preparing for a deployment to the Middle East. Ted would like to thank the everyone for the feedback he has received, and hopes to continue with astronomy outreach and education upon his return

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Don Hoverson, not because I think our species will one day reach those distant stars, but because I hope we will.


Hello again! This is Ted Haulley speaking to you from Waldorf, Maryland. It is great to have 365 Days of Astronomy continue for another year. For one thing, it allows me to share another anniversary. 80 years ago, on February 18, 1930, Clyde Tombaugh of the Lowell Observatory was comparing pictures of a region of Gemini taken six days apart and saw that one of the dots of light had moved. He had discovered a planet, but not the planet he was looking for. Today his discovery is known as the dwarf planet Pluto.

After Uranus was discovered in 1781, astronomers noticed that it was not behaving quite like it should. It was if something unknown was gravitationally influencing Uranus’ orbit. Mathematical computations of this anomaly led to the discovery of Neptune in 1846. However, this just added to the problem. Uranus still had some anomalies that Neptune alone could not explain. In addition, Neptune also showed some anomalies in its orbit that could not be explained, either.

Many astronomers, notably American Percival Lowell, believed the answer was yet another planet beyond Neptune. Lowell is probably best known for his observations of what he called canals on the planet Mars, but he spent the last decade of his life searching for this planet beyond Neptune that he coined ‘Planet X.’

In the cartoons, Duck Dodgers of the 24 ½ Century simply had to follow Planet A to Planet B, to Planet C and so on to Planet X. Lowell had to do a meticulous sky survey for this mysterious planet. Lowell concluded he was looking for a planet 7 times the size of Earth, about 43 astronomical units from the Sun.

Lowell searched from 1906 to 1916 from his Observatory in Flagstaff, Arizona with no success. This lack of success led many astronomers to doubt the existence of Planet X, but Lowell’s search was only halted by his sudden death in 1916. Lowell Observatory temporarily discontinued the search for many years, but in the 1920s it acquired a 13-inch photographic telescope that it planned to use to restart the search.

Enter Clyde Tombaugh. Tombaugh was a self-taught amateur astronomer from Kansas. In January, 1929 he was hired by the Lowell Observatory to analyze photographs taken by this telescope in a device called a blink microscope or a blink comparator. This device allows someone to immediately switch back and forth between pictures that were taken a few days apart. The stars would remain fixed, but objects like planets, comets, or asteroids would change location over a few days, and the moving objects would ‘blink’ as the photographic plates were switched back and forth.

Prior to his death, Lowell had predicted the orbital path of Planet X, and Tombaugh was anxious to photograph the area where Planet X was predicted to be, in constellation of Gemini. However, by the time his equipment had been tested, Gemini was just disappearing behind the Sun. He quickly discovered that it was best to photograph the area of the sky that was in opposition to the Sun. Realizing the suspected planet could actually be anywhere in the zodiac, he began his full-scale study in September of 1929 with the constellation that was in opposition, Aquarius.

As the months passed, the search moved to Pisces, Aries, and on to Taurus. Tombaugh had developed a routine of taking three plates of an area of sky over the span of a week. His plates often had 50,000 or more stars on them, which was daunting enough, but his search was approaching the Milky Way, where each plate could have as many 300,000 stars.

The large number of stars on each plate made them harder to examine, and he soon fell behind on his examinations. Plates taken of Gemini on January 21st, 23rd, and 29th were not examined until mid-February.

On the afternoon of February 18 Tombaugh found an object on the plates that had moved. He quickly realized that is was not an asteroid or a comet, and it had moved a distance that an object about 40 AUs from the Sun would move in a week. He had found his planet, remarkably close to where Percival Lowell predicted it would be found.

Tombaugh went out that night to photograph the area again, but suffered the frustration of all visual astronomers, clouds. The next night the observatory finally took photos of the area, and the planet moved exactly the distance it should have in three weeks. A few more weeks of observations confirmed Tombaugh had found a planet beyond Neptune, and the discovery was announced to the world on March 13, 1930.

The name Pluto was first suggested by Venetia Burney, an 11-year old English girl. It was an appropriate name for many reasons. It fit with an unofficial convention of planets having a name of a Roman God, Pluto being the Roman God of the underworld, and the first two letters were PL, the initials of the Planet X crusader, Percival Lowell. It was officially named Pluto on May 1, 1930.

At first it was thought that Pluto had a mass slightly more than Earth. Over the decades the estimate was constantly reduced, and now it is thought that Pluto has a mass about two-tenths of one percent of Earth. It has a diameter of about 2,400 km, making it smaller than Mercury and many moons of other planets.

Pluto’s small size cast doubt on it being Planet X. Tombaugh defended it as Planet X by pointing out it was discovered near where Lowell predicted it would be, and the actual orbit of the planet is close to the predicted orbit of Planet X.

Pluto is almost certainly not Planet X. It is far to small to have the effects on Uranus and Neptune that Planet X was claimed to have. It has a highly elliptical orbit that is well off the orbital plane of all other planets. It is just a pure coincidence that Pluto was discovered near the predicted location of Lowell’s Planet X, at a time in Pluto’s orbit brought it close to the orbital plane of the other planets.

As I’m sure most of you listening know, not only is Pluto not Planet X, it is not considered planet anything anymore. It was downgraded to the newly created category of dwarf planet in 2006. This caused a lot of popular uproar, at least in the United States, but not totally unexpected. Once the small size of Pluto had been established, there was a lot of discussion about the idea that Pluto should not be called a planet. There was a theory that maintained that Pluto was once a moon of Neptune that had somehow escaped from orbit. Discovery of other Kuiper Belt objects starting in the 1990s made this theory less likely, but put even bigger doubt on Pluto’s classification as a planet.

So what about those anomalies that were noticed in the orbits of Uranus and Neptune? There is still a small controversy over this. One explanation is that the anomalies were not really there and that they were just observational and calculation errors. Another theory says that a recalculation of the mass of Neptune explains the anomaly. Whatever the case, it removes the need for a Planet X, and most observations of the solar system past Neptune are focused on smaller Kuiper Belt objects, not the mysterious Planet X, though the search for it has not completely stopped. Recent doomsday talk concerning Planet X and the year 2012 is complete nonsense, unrelated to anything that even resembles reality, and is not taken seriously by any legitimate scientist.

As always, I welcome all feedback. I can be found at, or on Facebook. Thanks for listening.

End of podcast:

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