Should We Call Pluto a Planet?

Astronomers once called Pluto the ninth planet. In 2006, the International Astronomical Union, IAU, downgraded Pluto to the status of dwarf planet. After the New Horizons craft flew by Pluto in 2015, it showed us a very dynamic world. Many astronomers and a large percentage of the public now believe we should reconsider Pluto’s demotion.

Pluto as seen by the New Horizons spacecraft. 

Since before written history, humans have known of seven regularly observed heavenly bodies that didn’t behave like the vast majority of stars. They called them planets, Greek for “wanderer.” In those ancient times, they considered anything that changed its position relative to the “fixed stars” to be a planet. Since both the sun and the Moon moved relative to the fixed stars, they were also considered planets until Copernicus proved that planets circled the sun and the Moon circled Earth.

Astronomers then recognized six planets, Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Comets also orbited the sun, but because they had weird orbits and grew a tail, they were considered different types of celestial objects. That changed in 1781 when William Herschel discovered Uranus and Italian astronomer Giuseppe Piazzi discovered Ceres in 1801 between Mars and Jupiter while searching for comets. Both orbited the sun.

Ceres, the largest member of the Asteroid Belt.

Initially, astronomers called Ceres the smallest planet until many more such objects were discovered in the same area of our solar system. They reclassified Ceres and all those other even smaller objects as “asteroids,” calling that region the Asteroid Belt.

In 1846, two astronomers independently discovered Neptune, adding an 8^th planet to the solar system. Clyde Tombaugh added Pluto in 1930, making nine planets, and there it stayed for years.

In the 1990s, astronomers began finding many more objects beyond Neptune. They were small like Pluto and in the area of our solar system that was relatively crowded, unlike the inner parts. Some astronomers feared that it might be time to get a more scientifically based definition of “planet.”  

At the 2006 IAU meeting, astronomers agreed to redefine what constitutes a planet. There were two camps: geophysicists and dynamists. They all agreed that it had to orbit a star. Geophysicists said that any object big enough that its gravity pulled it into a spherical or nearly spherical shape should be a planet. Dynamists argued that a planet must also be large enough to “clear its orbital area of debris.” Ceres couldn’t be a planet since, even though it is round, there were many asteroids in the same region. Likewise, round Pluto shared its region with many thousands of objects.

The dynamists won. Pluto, Ceres, and other round solar system bodies became dwarf planets. Along with those two, astronomers now recognize three others, Haumea, Makemake, and Eris, all beyond Neptune, as dwarf planets. There may be many more that we just don’t have enough data on yet.

Count me in the camp of the geophysicists. If it’s big enough to pull itself into a nearly circular shape, technically called hydrostatic equilibrium, then I believe it should be a planet. I’m convinced that the dynamists just didn’t want to remember that many planet names.

  

    Each month, I write an astronomy-related column piece for the Oklahoman newspaper. After publishing it there, I post that same column to my blog page.

   This is reprinted with permission from the Oklahoman and www.Oklahoman.com.