|Why Pluto *is* a planet
||[Jun. 24th, 2008|03:34 pm]
Well, the title is misleading. In fact, I don't care whether the Pluto is a planet. What I do care however is that the definition of the planet used is to be consistent and practical for scientific applications. Unfortunately, the current one accepted by IAU in 2006 is not. The "non-planet" Pluto status is just one of many side effects of the ugliness of that definition.
A lot has been said on this topic already, but to my surprise nobody has brought up a point that I'm about to make now. My view is certainly not a deeply scientific one, but hopefully has some value. Maybe it will help those who are tracking the topic.
Ask yourself: what does Planetary Science study? Well, planets, you would say. Right. It is interested in planets, not in eggs or diamonds or computer trojans. Largerly, this is because it studies the planets with purposes and techniqies suited exactly for planets, not for eggs... err, and other stuff.
There are bodies which undoubtely are planets. Pick Mars or Neptune for instance. If a Planetary Scientist is sending a robotic mission to either of them, he or she will put there an equipment to study attributes that are essentially associated with "real" planets, such as:
* Gravity: its' strenght, its' impact on the planet's shape and internal structure.
* Internal structure: does it have a core and other interesing layers? How does the matter flow inside? How does the chemical composition change with depth?
* Geology, volcanism, tectonics, either past or present.
* Surface: types of landscape, matter exchange with interior, atmosphere, space.
* Atmosphere, if any. Climate, ionosphere, etc.
* Magnetic field, if any.
* Origin, history and evolution of the body.
This may be not the complete list, as I'm not the Planetary Scientist. But it gives you the idea.
Every object in space that we for sure consider a planet possesses most of these traits to study. On the other hand, objects that clearly are not planets have little to none of them.
Eggs do not have geology, gravity, or atmospheres [unless you are talking about rotten eggs, but I'm sure astronomers won't support that view -- though for some reason they supported the smelly IAU's planet "definition" in 2006 ;))] Fist-sized rocks do not have and have never had a geology, or surface remodeled by their own volcanism, or an atmosphere, or any self-created internal stuctures, or self-propelled active magnetic field. Ergo: rocks or eggs are not planets. Even if they are of an interest to a Planetary Scientist, he/she would never study these ojects as planets.
The opposite is true, too. You can call an object a "dwarf planet", or a "Guaquadratum", or whatever else. But if it has all or most of the attributes listed above, such as an atmosphere or strong gravitational field, it would be studied as if it is a planet. Period. Sure, by forcing an artificial classification you can still categorize it as a "Guaquadratum", but that would result only in the burth of a new "Guaquadratumology" science, and that science would not differ much from the good old Planetary Science!
In short: a planet is an object that is to be studied as a planet. We know how we study "classical" planets like Mars or Neptune. Anything else that has similar set of attributes to study is effectively a planet. Even if you call it a "dwarf something" ;))
Now, let's take a look at Pluto. It has gravity strong enough to make Pluto spherical. It has or had tectonics and diverse surface structures, and it had a non-trivial history. It even has an atmosphere! Of the 7 key traits listed above, it misses only magnetic field... probably. From practical standpoint, it is a planet. So I consider it a planet.
Now back to the definition. I feel, some are tempted to say: OK, your "7 attributes" approach is nice, but is it practical either? Figuring out an atmosphere or a volcanism story can take a lot of time. It took over 50 years for Pluto. This does not look like a practical definition, huh?
They are right. It is not. But here is a nice and beautiful twist. Watch my hands.
It is possible to show that any body with a mass over certain threshold will have all or most of the "7 attributes" listed above. Specifically, in the area where Pluto resides that mass limit corresponds to the body's size of roughly 2000 kilometers. Any body of that size, located there, will likely have from 3 to 6 out of 7 "planetary" properties listed above.
Object that are massive enough can not be non-spherical. Their own gravity would nicely force them to spheres if there were cubes. Any body that is large enough will generate enough heat during accretion time to warrant at least some rudimentary geological activity early in its' history. And wherever there is the tectonics, most likely there is a rich surface re-forming, history, and non-trivial internal structure. Finally, any body that is heavy enough can hold around gases dense enough to qualify for an atmosphere.
size mass does matter. In fact, mass is about the only critical factor here. It is somewhat arguable what exactly the "minimal mass" is [I would leave these debates to Planetary Scientists] - but it is clear and undoubtful that any body that is large enough will look, smell, taste and sense to a planetary scientist like a jucy planet to launch a new mission to it. That's it.
Mass is the only parameter that defines whether the body is or is not a planet from a "practical" standpoint. What it revolves around, of whether it has cleared it's surroundings is negligible compared to the mass parameter.
Therefore I'm a strong supporter of the old, classical, broadly used geophisical definition of the panet. That definition says: any body that is large (massive) enough for gravity to make it roughly spherical, but not too massive to initiate internal nuclear fusion, is a planet. It may need some slight modifications to include the notion of geology/atmosphere, but at the end mass is still the primary defining factor.
And in addition, I believe that Pluto fits that definition and thus is a planet.
Thanks a lot to those who have read this. Yes, you have my permission to refer or to copy... in full please and with crediting me ;)).