The three new planets would have been Ceres (in the asteroid belt), Charon (a moon of Pluto), and the recently discovered 2003 UB313.
However, the proposal was met with so much opposition and debate that the original wording was changed before the final vote came on August 24. Opponents attacked the many caveats and conditions and said it was too arbitrary.
“It [looked] to me like a definition that was written by a committee of lawyers, not scientists,” said Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington.
Early on August 24, the IAU voted on the revised proposal for a formal definition of “planethood.” Debates and discussions took place right up to the last minute.
As a result, for the first time in history, we can now officially say we are part of a solar system that has eight, not nine, planets. Pluto has been demoted.
Throughout this entire process, the phone calls and emails have been almost non-stop: What extra planets? Where did they come from? What are we losing? What’s going on?
I think it may be time for some rumor control.
Nothing in the solar system has changed. No worlds just magically appeared or disappeared, and everything will continue on as it has for nearly five billion years.
What has changed is the way that we classify, define and interpret the many diverse worlds that make up our solar system. Surprisingly, the word “planet” has never had a true proper definition.
This formality has been a long time coming. The historical idea of “planet” – a millennia-old notion of something moving against the background of stars – is antiquated and incapable of keeping up with recent discoveries. Modern technologies have allowed us to peer ever deeper into our solar system and better understand the nature of all our neighboring worlds, near and far.
In a nutshell, according to the new definition, a planet is a celestial body that:
Orbits the Sun;
Possesses sufficient gravity to overcome rigid-body forces and assume a nearly spherical shape;
Has cleared its orbital path around the Sun. That is, the orbit of a spherical body (and its moons) is solitary. Population objects (like asteroids and trans-Neptunian objects) that orbit the Sun with many other members are not planets.
So, the planets of our solar system, in order, are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. That’s it.
The definition scheme actually includes three classes of worlds:
1. Planets – Mercury through Neptune;
2. Dwarf Planets – spherical objects that have not cleared their orbital paths around the Sun;
3. Small Solar System Bodies – All other celestial bodies that orbit the Sun, such as comets and asteroids.
The first three members of this new class of dwarf planets are Ceres, Pluto, and 2003 UB313, although this figure will quickly grow. Currently, there are a dozen objects on the IAU’s watch list to be dwarf planets. The number will become larger and more defined as we strive to understand the many worlds that exist beyond Neptune.
Pluto, discovered by Clyde Tombaugh in 1930, has recently been recognized to be only one small icy world in a belt of objects residing in the outer reaches of the solar system. More and more, astronomers came to realize that this distant world should never have been called a “planet.”
So, after more than a decade of serious consideration and discussion among scientists, Pluto’s fate has finally been sealed. “Pluto is dead,” said Mike Brown, an astronomer with Caltech. “Pluto is not a planet. There are finally, officially, eight planets in the solar system.”
While this decision is closure for the scientific community, it may be disappointing to many people. Public support for Pluto has always been high since its status started being questioned in the 1990s. "The public is not going to be excited by the fact that Pluto has been kicked out," said Brown. "But it's the right thing to do."
As with all new ideas, however, there are critics. Some feel that the language of the definition is flawed and still too arbitrary. Indeed, a petition has already been started to attempt to overturn the IAU’s decision.
Some of the critics’ concerns:
1. Some “planets” do not truly have solitary orbits. Earth, Mars, Jupiter, and Saturn all share their orbits with some asteroids.
2. Only 424 astronomers were able to vote on the issue, out of more than 10,000 professional astronomers throughout the world (electronic voting was not allowed). "I'm embarrassed for astronomy," said Alan Stern, planetary scientist at the Southwest Research Institute. "Less than five percent of the world's astronomers voted."
3. The wording of the definition is contradictory. “I thought that it made a curious linguistic contradiction,” said Owen Gingerich, historian and astronomer emeritus at Harvard University. “A dwarf planet is not a planet. I thought that was very awkward.”
Overall, the new definition would seem to be a step in the right direction. One of the greatest strengths of science is that it allows for challenges, self-corrections, and refinements. As our bank of knowledge grows greater, further scrutiny will be required in the future.
So astronomy is just experiencing some simple growing pains.
Science is about curiosity, questioning, hypotheses, and discoveries. It is about changes and new horizons. It is about pushing the limits of our understanding to embrace the actual nature of the universe and ourselves.
And, most difficult of all, it is about recognizing when the time has come to abandon the old establishment and welcome the new. We have survived many such transitions before, from the Copernican revolution that removed Earth from the center of the universe to Albert Einstein’s fantastic new descriptions of space and time.
We are merely witnessing a single link in a long chain of events that will all be remembered by history.