Pluto - The Planet That Was

Mathias Pedersen

I’ve recently had two encounters with Pluto, the former ninth planet, which got me thinking.  The first encounter was during trivia night at a local bar when the host asked “What planet has a highly elliptical orbit, is mostly rock and ice, and is not a gas giant?”  According to the host, the correct answer was Pluto.  My team got it wrong because we knew that Pluto wasn’t a planet.  The other encounter was a Target commercial in which a bubbly, young kindergarten teacher unceremoniously snips Pluto from the classroom mobile.  Poor Pluto.  In 2006, the International Astronomical Union (IAU) demoted Pluto to the rank of Dwarf Planet.  What did a respectable planet like Pluto do to deserve this?

Clyde Tombaugh

In the late 19^th century, aberrations in the orbit of Neptune had astronomers hypothesizing a “Planet X” beyond Neptune.  In 1909, Percival Lowell’s Lowell Observatory in Flagstaff, Arizona began an extensive search for a possible ninth planet but did not find it before Lowell’s death in 1916.  It wasn’t until 1929 that the observatory’s search resumed.  The job went to a 22-year-old farm boy/self-educated astronomer named Clyde Tombaugh who, after more than a year of comparing photographs of the skies taken a week apart, found “Planet X”.  News of the discovery was transmitted to Harvard College Observatory on March 13, 1930.  The name ‘Pluto’ was suggested by an 11-year-old girl named Venetia Burney, whose grandfather was a former Oxford librarian who knew an Oxford astronomer who sent the suggestion to Lowell Observatory (once again proving it’s not what you know, but who you know!)  The new name was announced May 1, 1930, Venetia received five pounds from her grandfather as a reward, and Mickey Mouse got a pet dog named Pluto soon thereafter.

Pluto's eccentric orbit

As planets go, Pluto was always sort of an oddball.  It is made up of ice and rock like objects in the far-flung Kuiper belt. It’s puny - a third of the earth’s moon’s volume and a fifth of the moon’s mass.  While the other planets have circular orbits on the same plane, Pluto’s has an elliptical orbit which is 17 degrees out of alignment with the other planets.  Pluto actually comes closer to the sun than Neptune on the inside of its 248 earth-day orbit.  Pluto’s troubles really began in the 1970’s when objects of similar mass were discovered far beyond Pluto, casting doubt on Pluto’s planet status.  In 2005, when Eris was discovered with a mass 27% greater than that of Pluto, the IAU had to do something; either Eris was a planet or Pluto was not.  On August 24, 2006, the IAU issued a new definition of a planet which proclaimed that 1) the object must rotate around the sun, 2) the object must be massive enough to be a sphere by its own gravitational force, and 3) the object must have cleared the neighborhood around its orbit. Pluto fails the last condition because there are other objects of similar mass within its orbit.  A few weeks later, the IAU assigned Pluto, Eris, and Eridian moon Dysmonia to its ‘dwarf planet’ classification.  And then there were eight!

Worth1000.com

The public reaction was loud and furious.  Internet petitions arose demanding that Pluto’s planet status be reinstated immediately.  Several state legislatures actually passed legislation protesting the downgrade.  California legislators, in a whimsical mood, introduced a bill that stated “the deletion of Pluto as a planet is a hasty, ill-considered scientific heresy similar to questioning the Copernican theory, drawing maps of a round world, and proving the existence of the time and space continuum.”  Bumper stickers reading ‘Honk if you think Pluto is still a planet’ appeared.  Protesters for both sides of the controversy took to the streets with tongues firmly in cheeks.  Time Magazine’s Person of the Year article listed Pluto among the ‘people who mattered’ in 2006.

The Pluto controversy highlights the problem with any classification scheme: as more members are found, the scheme may have to evolve.  We set up classifications, labels, and definitions to facilitate our very human need organize things.  When there are a fixed number of members of the group, the classifications usually work out just fine.  However, when new members are found that don’t fit the old buckets, we can either assign the new members to their categories even if the fit isn’t perfect, or change the classification scheme.  For example, Barack Obama is the 44^th President of the United States and the 43^rd person to have the job.  That’s because Grover Cleveland is both the 22^nd and 24^th President.  Apparently, the 19^th century definition of President of the United States didn’t allow for non-consecutive terms.  Another example: according to Webster’s, a continent is defined as “one of six or seven great divisions of land on the globe” because Europe and Asia may or may not be separate continents depending on who you ask.  I once was involved in a research project that attempted to classify small-molecule compounds into various biological activity groups.  However, as more compounds were screened for activity, the clearer it became that our classification scheme sucked.  After months of trying to defend the obsolete scheme, the whole thing was thankfully scrapped.  That’s science.

So, as new objects were discovered in our solar system, the IAU made the bold decision to refine its definition of a planet, demoting Pluto in the process.  However, this decision strengthens their ability to identify new planets as they are found - no, not in our solar system.  I’m talking about the millions of planets that circle the stars…and have sufficient mass to form a sphere…and have cleared their orbits of other objects.  *sigh*

Question of the Day:  What were the other two names Lowell Observatory considered before selecting Pluto?  Why didn't they get a single vote?