“Plutonic” Relationships

8/20/06 – 8/26/06
by C. Zaitz

Well, it’s finally come out. The IAU’s definition of a planet! How exciting- we’ve all been on the edge of our seats waiting and wondering, “so what is a planet, really?” The International Astronomical Union is the world-wide collection of astronomers making up the body that has been the official arbiter of planetary and satellite nomenclature since 1919. And they don’t take bribes.

Historically and etymologically, the word planet referred to the “wanderers” or the “stars” that moved as the year progressed. As the planets orbit the sun, they appear to move in front of the much more distant stars. Mercury, Venus, Mars, Jupiter and Saturn have been known since people have looked up and noticed them. However, Uranus wasn’t discovered by telescope until 1781 by Sir William Herschel. Neptune was discovered in 1846 by Johann Galle, and Pluto is a mere baby, discovered in 1930 by Clyde Tombaugh.

Thanks to better ground-based and orbiting telescopes, we’ve recently discovered even more members of our solar system. Two more moons of Pluto have been discovered, as well as other small bodies like Sedna and Quaoar with eccentric orbits that can take well over 200 years to orbit the sun, beyond the orbit of Neptune. Pluto, by the way, is substantially smaller than our Moon. Controversy has raged about the definition of a planet, and much of it surrounded Pluto’s status. One of the more vocal opponents of Pluto’s planetary status was Dr. Neil DeGrasse Tyson of NYC’s Rose Center and Hayden Planetarium. His argument was that Pluto was too small to be a planet, and was most likely another Trans-Neptunian Body, such as those mentioned above. But who wants to deny that Pluto is a planet? It’s on all our placemats, mobiles and coloring books!

Here’s the quote: “The IAU therefore resolves that planets and other Solar System bodies be defined in the following way: 1. A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” Basically, if it’s round and goes ‘round the sun, it’s a planet. That means you, Mr. Pluto. However, small bodies like Ceres, traditionally called asteroids or minor planets, are technically now planets, but can be called “dwarf planets.” Pluto is now a double planet, joined at the gravitational hip with Charon, but is head of a new subclassification called “Plutons,” namely the Trans-Neptunian objects. 2003 UB313 would fall into the Pluton classification. That would make a grand total of 12 planets in the solar system, pending more discoveries.

As you can see, this is all getting a little more complex than it used to be, but a complex solution was called for. We ran out of names and ways to classify the new discoveries. Dr. Tyson wasn’t mad at Pluto, he was pushing for a clearer, more accurate classification scheme.

The vote on the new scheme takes place in Prague on the 24th of August. Stay tuned!

Meanwhile, nary a planet can be found in our August sky. Jupiter can’t escape the oncoming blast of light from the Sun much longer, and soon will be lost in the glare of sunset. Catch a quick view right at evening twilight in the west. Early morning birds may catch a glimpse of Venus in the east, though she is low and her light will be washed away as the sun rises.

Until next week, my friends, enjoy the view!

For more from the IAU, try: http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html