365DaysDate: February 18, 2009


Title: Pluto – The Planet that isn’t a Planet Anymore

Podcaster: Susan Murph

Organization: How To Grow Your Geek

Description: Beginning with a convoluted discovery in 1930, then being named by an 11-year-old girl, Pluto got off to a rather non-traditional start as a planet. It continued to be largely uninvestigated until only the last few decades. It became the subject of much debate when it was demoted to “dwarf planet” status in 2006, which caused much hullabaloo in the scientific world as well as with schoolchildren. Pluto continues to be unusual within our solar system, as it is now classified as a dwarf binary planet with Charon. I will discuss this history briefly and get insight and commentary from my daughter Amanda, 8 yrs old, and my son Kevin, 6 yrs. old, on this captivating little planetary body and its trials and tribulations.

Bio: Susan is a life-long sci-fi, fantasy and science geek, and loves to incorporate her favorite hobbies into her current career of raising her two kids as a stay-at-home mom. She earned a Master of Science degree in Clinical Psychology, with specialties in Health Education and Behavioral Medicine. She stopped working when her first child was born, and has happily been at home ever since. She believes that including her kids in her hobbies not only strengthens her relationship with them, but also benefits their development of useful skills such as critical thinking, logic, creativity and reading comprehension, just to name a few. Susan currently hosts and produces the “How to Grow Your Geek” podcast.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by, your online guide to space and astronomy news plus advice on how to choose and use a telescope. Find out more at


Susan: Hello Everyone! My name is Susan Murph, also known as Susie the Southern Geek on the How to Grow Your Geek Podcast. My show is about being a geeky parent and doing geeky things with your kids. My daughter Amanda is often my co-host. Can you say “Hello,” Amanda?

Amanda: Hi everyone!

Susan: One of the great geeky things we decided to do together is help out with the 365 Days of Astronomy Podcast. So, let’s get started. Amanda, can you name the planets in our Solar System?

Amanda: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune

Susan: What about Pluto?

Amanda: Mom, Pluto isn’t a planet any more.

Susan: Oh, yeah – do you want to know what happened and why Pluto isn’t a planet any more?

Amanda: yeah

Susan: Well, Pluto was discovered in 1930, but astronomers had suspected that there was another planet beyond Neptune since the late 19th century, due to disturbances in the orbits of Neptune and Uranus. In 1906, Percival Lowell, founder of the Lowell Observatory, began a search for this elusive “planet X”, and that search continued until his death in 1916. Little did he know that the search had actually taken pictures of the planet on March 19, 1915, but they just weren’t recognized at the time.

After Lowell’s death, the search didn’t resume until 1929, when the job of looking for “planet X” fell to Clyde Tombaugh, a newly hired employee of the observatory. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. Another blurrier photograph taken on January 21 helped confirm the movement. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.

The right to name the new object belonged to the Lowell Observatory. Tombaugh was urged to name  the new object quickly before someone else did. Name suggestions poured in from all over the world. One suggestion was from an 11 year-old-girl in England, Venetia Burney. She liked to study classical mythology, and thought that Pluto, one of the alternate names for the Greek God of the underworld Hades, would be a fitting name for such a cold, desolate planet.

On March 24, each member of the Lowell Observatory was allowed to vote on a short-list of three: “Minerva” (which was already the name for an asteroid), “Cronus” (which had garnered a bad reputation after being suggested by an unpopular astronomer), and Pluto. Pluto received every vote. The name was announced on May 1, 1930. Pluto became the 9th planet in our solar system.

Amanda: So what happened after that to change things?

Susan: Good question. Things continued along unchanged for Pluto until 1978, when its moon Charon was discovered by James Christy. Then, in 1992, astronomers began discovering a large population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This belt of objects is known as the Kuiper belt (Ki-per), and is believed to be the source of many comets.

The discovery of the Kuiper belt and Pluto’s relation to it led many to question whether Pluto could be considered separately from others in its population. In 2002, the KBO Quaoar was discovered, with a diameter of roughly 1280 kilometers, about half that of Pluto. In 2004, the discoverers of Sedna placed an upper limit of 1800 kilometers on its diameter, near Pluto’s diameter of 2320 kilometers. Debates arguing whether Pluto should be reclassified as one of the Kuiper belt objects raged.

On July 29, 2005, the discovery of a new Trans-Neptunian object was announced. Named Eris, it is now known to be slightly larger than Pluto. This was the largest object discovered in the solar system since Neptune’s moon Triton in 1846. Its discoverers and the press initially called it the “tenth planet”, although there was no official consensus at the time on whether to call it a planet. Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.

The last remaining distinguishing features of Pluto were now its large moon, Charon, and its atmosphere. These characteristics are probably not unique to Pluto: several other Trans-Neptunian objects have satellites, and Eris‘s spectrum suggests that its surface has a composition similar to Pluto’s. It also possesses a moon, Dysnomia, discovered in September 2005.

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the solar system. Some omissions were intentional; the Hayden Planetarium reopened after renovation in 2000 with a model of only eight planets.

The debate came to a head in 2006 with an IAU (International Astronomical Union) resolution that created an official definition for the term “planet”. According to this resolution, there are three main conditions for an object to be considered a ‘planet’:

The object must be in orbit around the Sun.

The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.

It must have cleared the neighbourhood around its orbit.

Pluto fails to meet the third condition, since its mass was only 0.07 times that of the mass of the other objects in its orbit (Earth’s mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.  This category of Pluto-like objects was then applied to dwarf planets that met the conditions of being trans-Neptunian and “like Pluto” in terms of period, inclination, and eccentricity. On June  11, 2008,  the term was announced, along with a greatly-simplified definition: all trans-Neptunian dwarf planets are plutoids.

So now, Pluto is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Pluto and its largest moon, Charon, are often treated together as a binary system because the center of their orbits does not lie within either body, but halfway between. The International Astronomical Union (IAU) has yet to formalize a definition for binary dwarf planets, and until it passes such a ruling, Charon is classified as a natural satellite moon of Pluto. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005. So, as you can tell, we are far from finished discovering things about Pluto, as well as the other planets in our solar system.

Amanda:  If you would like to hear more of our show, please visit us at

Susan: We will now leave you with a part of Jonathan Coulton’s song, “I’m your Moon,” which is about Charon still loving Pluto even though it lost its planet status. You can find more songs from him at Until we talk to you again, have a great Year of Astronomy!

Jonathan Coulton’s song “I’m Your Moon” used by permission.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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