The IAU – International Astronomical Union – has finally come out with their long-awaited definition of what makes a Planet, and have determined that Pluto, the once coldest and most distant in our Sol system, is not a planet. What is more surprising, however, is that the same ruling also regulates Jupiter, Mars, Neptune and yes, even Earth as something other than planets.
The IAU definition of a planet is as follows:
“A celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit.”
The original definition that was proposed did not contain the last line, which would’ve rendered asteroid Ceres, the newly named Eris (formerly known as 2003 UB313 or Xena), and Pluto’s companion Charon all as planets, and leaving a dozen more distant objects waiting in the wings to earn their planetary status.
Well, we can’t have school children memorizing thirty planets! That’s preposterous! So, in all of their wisdom, about 435 of the 10,000 members of the IAU voted to approve the last line of new definition, which is, “and has cleared the neighborhood around its orbit.”
Why was this added?
The simple answer is that the IAU did not want to have dozens of objects scarcely 500 miles in diameter classified a “planets.” The general consensus was that a size limitation would be arbitrary, so to many the “neighborhood clearing” factor made sense. Not only did it eliminate many of the solar systems distant objects from the possibility of planet hood, it eliminated the largest planet, Jupiter, and also my favorite planet, Earth.
Pluto, according to the IAU, was automatically disqualified because its oblong orbit overlaps with that of Neptune. At the same time, all other Trans-Neptunian Objects (TNOs) were also eliminated because they orbit in a crowded neighborhood.
To avoid appearing arbitrary by using size as a measuring stick, the IAU decided to use the neighbors of celestial bodies instead. What could be more capricious? I do not think that it is fair to judge an individual by how their neighbor keeps their yard, nor does it make sense in any branch of science to define an object, a thing, a substance based on what lies next to it.
Pluto was largely demoted based on the fact that its orbit crosses millions of miles above and below that of Neptune, (the orbits to not actually intersect). However, by the same logic, Neptune, the third largest planet in our solar system, is not a planet either because it fails to clear its “neighborhood” of tiny Pluto.
Jupiter is automatically disqualified because it cannot manage to even clear out its own orbit. Jupiter, the largest planet in the family of the sun, is accompanied by thousands of Trojan asteroids. (A Trojan asteroid is one that shares the same orbit of another planet.) Both Neptune and Mars are also accompanied by similar Trojans during their trip around the sun.
Earth and Mars are instantly also revoked of their planetary status by this new definition because there are a several asteroids that orbit closer to the sun than those in the Main Belt, and cross the paths of both small rocky planets.
And then there were four…
Only Mercury, Venus, Saturn, and Uranus are officially planets, having successfully cleared their own neighborhoods of other celestial objects. However, Saturn and Uranus are only one discovery away from having their status revoked as well. As for Mercury and Venus, do they really get to be called planets? After all, they did not clear their own orbit, the sun likely did the work for them.
Surprisingly, the definition says nothing as to how a planet is formed. Is it really too much to ask that the line, “an object that formed in a protoplanetary disc around a star,” be included in the definition? This would prevent extrasolar brown dwarfs from being classified as planets. With new extrasolar planets being discovered each day, many of them 10-15 times the size of Jupiter, this is an important question, now more than ever.
One does not have to look far in order to instantly come to the rational conclusion that the IAU definition is preposterous.
What is to be done about this molestation of planetary science? A real definition is in order. One that both identifies the properties of what a planet is and also describes how a planet is formed, thus differentiating a planet from other large objects, such as brown dwarfs.
I propose the following to be adapted as the official definition:
A) It must have a differential interior. Meaning that the object is massive enough for its own gravity to have pulled more dense materials to its core. This also serves as a far better criteria than “roundness.”
B) It must orbit a star. (At least everyone can agree on this point.)
C) It must have been formed in a protoplanetary disc around a star. This eliminates large object that formed similar to the way that stars do in a gas nebula.
D) Must have a diameter of 1500 Km or greater. Thus designating other bodies that fit the criteria for A, B, and C as dwarf planets. This would not only define planets in the Sol system, but also all extrasolar planets. But there is that pesky arbitrary size requirement!
Why not size?
Size has been used as a measuring stick when defining all sorts of items in not only science, but in culture. When is a hill a mountain? When is sea an ocean? When is an island a continent? Size is the determining factor in all of the above.
Finally, the IAU planetary definition lacks foresight. It is inevitable that a TNO will be discovered that is greater than the size of Mercury. When this happens, and it will within the next decade, the definition will render its self obsolete.
Let us all disregard the IAU’s ill-advised and ill-conceived “definition” of a planet, which it in its self defines only absurdity. It is as clear that Pluto and Eris are planets as it is that Mercury, Mars, Jupiter and Earth are planets as well. In the interest of good science, let us write a definition that defines all planets in the universe, and not just in our own back yard. Let us adopt a definition that will stand up strong enough to endure use by hundreds of future generations who will inherit this planet.