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Pluto: A Guide for Earth-Bound Explorers

Yesterday, NASA’s New Horizon mission made its closest approach of Pluto, providing the first direct views of the “dwarf planet,” or to many die-hards the solar system’s ninth planet, some 85 years after its February 1930 discovery by astronomer Clyde Tombaugh at Arizona’s Lowell Observatory. While viewing these history-making images here’s a handy guide to some interesting facts about Pluto’s discovery and about how our knowledge of the solar system has evolved.


1) Pluto wasn’t named for the Disney character

Walt Disney’s cartoon dog first appeared nameless in a September 1930 animated film, “The Chain Gang,” but was given the name Pluto the following year in “The Moose Hunt.” It was an 11-year-old English school girl, Venetia Burney Phair, who deserves the naming honor. As Venetia told me in a 2006 NASA interview, during a March 14, 1930 breakfast, her grandfather Falconer Madan, former head of Oxford’s Bodleian Library, opened the Times of London, “and read out the great news (of the confirmation of Pluto’s discovery) and said he wondered what it would be called. And for some reason, I, after a short pause, said, ‘Why not call it Pluto?’(The Roman God of the Underworld who was able to make himself invisible). I was fairly familiar with Greek and Roman legends from various children’s books that I had read, and of course I did know about the solar system and the names the other planets have. And so I suppose I just thought that this was a name that hadn’t been used.” Madan contacted Oxford astronomer Herbert Hall Turner, who sent this telegram to Lowell: “Naming new planet, please consider PLUTO, suggested by small girl Venetia Burney for dark and gloomy planet.” The name was approved by a unanimous vote.

Venetia, who passed away in 2009, never met Tombaugh and viewed Pluto through a large telescope for the first time in 2007. This humble school teacher (she taught economics and math at a girls’ school) who told me she only reluctantly talked about her distinction, was in fact the only woman to name a planetary body. Naming parts of the solar system was in her family genes as in 1877 her great Uncle Henry Madan suggested the names Phobos and Deimos for the moons of Mars.


2)  Clyde Tombaugh was more than a one-shot wonder.

If the naming of NASA’s great observatories for such 20th century luminaries of astronomy and physics as Edwin Hubble, Lyman Spitzer, Subrahmanyan Chandrasekhar and Arthur Holly Compton is any indication, than Clyde Tombaugh may be seen as a lesser light; a junior astronomer at Lowell who lucked out by taking on the drudge work of painstakingly comparing photographic plates containing at least 40,000 star images to find evidence of a feint body moving against a background of stationary stars. In fact, Tombaugh, the self-taught Kansas farm boy who built his own telescopes, was a distinguished scientist deserving of lasting recognition.

During his 14 years at the Lowell Observatory, Tombaugh discovered hundreds of new variable stars, hundreds of new asteroids and two comets. He’s credited with observing nearly 30,000 galaxies and finding that rather than being distributed evenly, as most astronomers, including Hubble had predicted, they clustered into clearly defined groups. He later taught navigation to U.S. Navy officers during World War II and designed a classified optical telescope capable of tracking ballistic missiles for the White Sands Missile Range. He was a faculty member at New Mexico State University from 1955 to 1973, helping to teach a new generation of astronomers.

Pluto and Charon

A portrait from the final approach. Pluto and Charon display striking color and brightness contrast in this composite image from July 11, showing high-resolution black-and-white LORRI images colorized with Ralph data collected from the last rotation of Pluto. Color data being returned by the spacecraft now will update these images, bringing color contrast into sharper focus.

Tombaugh’s role in Pluto’s discovery is reason enough for his lasting fame. New Horizon’s principal investigator Alan Stern told Astronomy Magazine in 2006 that he was amazed not only “that a boy off the farm did what the professional astronomers couldn’t accomplish in almost 30 years of searching,” but also that Tombaugh “never once came to his boss with a false alarm – he came to his boss precisely once.”


3) Don’t blame Pluto’s demotion on Neil deGrasse Tyson.

Yes, Tyson caused a stir in 2000 when New York City’s Hayden Planetarium at the Rose Center for the Earth and Space that he directs decided not to include Pluto in its depiction/categorization of the terrestrial and Jovian planets, leading to hate mail from third graders.  But an active debate about Pluto’s status was already underway in astronomical circles leading to the International Astronomical Union’s decision in 2006 to demote Pluto to dwarf planet status. Its small size (our moon is larger), elliptical orbit overlapping with Neptune’s, and icy nature which had more in common with comets than other planets had always raised eyebrows about whether Pluto shared the essential characteristics of other bodies labeled as planets. Indeed, in 1956 Time Magazine’s Science section questioned Pluto’s status under the headline, “Demoted Planet.” By the 1990’s astronomers began finding many other large objects in the ring of icy bodies beyond Neptune known as the Kuiper Belt, raising the question why is Pluto special? In 1999 the International Astronomical Union issued a press release to deny it intended to reclassify Pluto but noted “ways to classify planets by physical characteristics are also under consideration. These discussions are continuing and will take some time.”


4) Our understanding on how many planets there are is contingent on how we define them. 

In the 2nd millennium BC Babylonian sky watchers had identified the wandering planets from Mercury to Saturn, making for a solar system of six planets. The discovery by telescope of Uranus in 1781 and Neptune in 1846 brought us to eight, right? Not so fast. “If you lived in the 1850s and you pulled out a library book on the solar system it told you about 23 planets, because they were first discovering objects in the asteroid belt that were initially labeled as planets,” notes James Green, NASA’s Planetary Science program director. Later, astronomers recognized that many of these bodies were too small to be called planets, leaving the solar system with the planets we know. Alan Stern observes that if you add the planet-sized large asteroids between Mars and Jupiter and the recently discovered planet-sized Kuiper Belt objects, and the predicted number of planet-sized objects in the still more distant Oort Cloud, the number of bodies one could conceivably classify as “planets’ may total more like 900.


5)  We’ve only just begun to explore our Solar System

Although U.S. and other nation’s spacecraft have basically completed the initial reconnaissance of the inner and outer solar system, we’ve basically scratched the surface of learning about our local celestial neighborhood.  In addition to the intriguing targets that are talked about often—clearly Mars and Jupiter’s moon Europa, which has a potentially life harboring ocean, there’s also our often overlooked sister planet Venus, which may have been much more Earth-like in its early history, before a runaway greenhouse effect took hold, Saturn’s moon Enceladus, which has been observed to spew out plumes of methane and Neptune’s large moon Triton, home to a unique ice volcano.

We may also want future missions to take a more detailed look at far away Kuiper Belt objects, which could be the key to the existence of life on Earth. James Green points out that some scientists believe and some computer models suggest that in Earth’s infancy, our home planet was bombarded by material from the Kuiper Belt, composed of frozen volatiles such as methane, ammonia and water, which may have “rained in our upper atmosphere, bringing a significant amount of water, maybe enough to populate the oceans. These objects may have been fundamental to bringing water to Earth at a stage where life could evolve.”

Edward Goldstein, NASA’s former lead writer, currently is a Senior Writer/Editor at the Aerospace Industries Association and teaches a National Science Policy course at Georgetown University’s McCourt School of Public Policy.


Edward Goldstein has more than 20 years' experience in the U.S. space community. From...