If
you thought you learned everything about the solar system
in grade school, think again. Nine planets orbiting the Sun
in neat circles is a good model for learning the names, but
it misses most of what astronomers have discovered in the
last 50 years.
So
many unusual objects have been found, they challenge our ability
to classify and label them. This was the catalyst for the
recent change in Pluto’s status from “planet” to “dwarf planet.”
The reclassification has received criticism from both professional
and amateur astronomers.
IAU
Resolution 5a
On
August 24, the International Astronomical Union (IAU) voted
to redefine the term “planet” and exclude Pluto. Pluto had
been a planet for 76 years, since discovery by Clyde Tombaugh
at Lowell Observatory in Flagstaff.
The
IAU resolution defines “planet” by three criteria. A planet
must:‑orbit the Sun.
‑exert
enough gravity to form itself into a sphere.
‑have
cleared its orbit of debris and other small bodies.
Pluto
fails the third criterion. It orbits in the Kuiper Belt, a
region containing thousands of other small bodies. This belt
extends from Neptune to beyond Pluto. Major planets orbit
the Sun a neat, flat plane, but Kuiper Belt objects have orbits
that are tilted at weird angles. Pluto’s is tilted 17 degrees.
Why
labels are important
The
first science learned in school is “classification.” Is it
a plant or an animal, igneous or sedimentary, planet or moon?
Classification is important in science. It lets us group things
with similar properties–the first step toward understanding.
Here,
the IAU was recognizing the likely different origin of Kuiper
Belt bodies. Origins are also important to scientists. Henceforth,
unless their decision is reversed, Pluto is officially a “dwarf
planet.”
What
would Clyde say?
I
had the privilege of meeting Tombaugh in 1988 when he was
speaking to local astronomy clubs about his discovery. Pluto’s
large moon Charon had been discovered in 1978. The IAU was
considering reclassifying Pluto/ Charon as a “double planet,”
due to the size of its moon. I asked Clyde how he felt about
this. His prompt reply was, “It’s a planet ¼ has been for 50 years. Leave it alone.”
His
response may seem almost curmudgeonly in retrospect, but it’s
an appeal to history. The meaning of “planet” has changed
often. To the ancients, planets were celestial lights that
varied in brightness and moved through the constellations.
To Galileo, inventor of the telescope, they were worlds with
visible geography. To us, they’re a mix of geology and meteorology.
Some are made of rock, others of gas, one might be a ball
of water.
New
knowledge is the business of science, and classifications
will continue to multiply. But discovery only happens once.
Dan
Heim, is president of the Desert Foothills Astronomy Club.
