Does anybody else feel that this current controversy over the status of Pluto is a bit silly? Official planets? Give me a break. That's not science.
This is a perfect time for us all to revisit the structure of solar systems. There are of course many types of objects that orbit stars. What we have traditionally called "planets" are just one type.
On the flight home yesterday from a conference, I quickly sketched out my ideas for rethinking the structure of this and other solar systems. We'll get to the status of Pluto in just a bit.
A solar system is simply a collection of objects that orbit one or more stars (yes, there are binary star systems, and I'm holding open the possibility of triple-star quadruple-star, and even more star-packed systems). That being said, the basic entities and relationships in solar systems are as follows:
- Star(s): the orbital hub(s) of all objects in the system; example, our sun
- Orb(s): the objects that orbit the star(s) of the system; may be massive or minute; examples, our "planets," comets, asteroids, microscopic particles floating in the void between other orbs
- Ring(s): the discrete paths of grouped, like orbs in the system; examples, the inner "ring of rock" around our sun (primarily including Mercury, Venus, Earth, Mars, and the asteroids); the "ring of gas" (including Jupiter, Saturn, Uranus, and Neptune); the "ring of ice" (including Pluto, Charon, Xena, the Kuiper Belt, and whatever else lies out beyond)
- Satellite(s): orbs that orbit other orbs in the system; examples, the moons the various planets; the rocks, dust, and other objects in the rings of various planets
Notice that I've defined "orbs" to include all objects, from microscopic to Jupiter, that directly orbit the sun, or that orbit other orbs. The crux of the debate over Pluto is whether it's too small to qualify as a "planet" in the traditional sense of that word. Well, maybe we shouldn't using that word any more--it's become an arbitrary, non-scientific term that obscures and distorts the actual structure of the solar system.
But before we do that, let's ask why we have historically latched onto this term. What exactly is a planet, in the traditional sense of the word? My sense is that it's an orb that is massive enough to a) appear in telescopes, b) has great enough gravitation to pull its shape into a sphere, and c) has essentially cleared out its own orbital path, through centripetal attraction and collisional deflection, of all other nearby orbs.
"Appearing," "sphering" and "clearing" are the three core criteria for "planets," in the traditional sense of the word. Orbs smaller than a given threshold don't appear, sphere, and clear; rather, they simply jostle with other dark, irregular orbs into their aggregated rings; those aggregated rings and some of their constituent objects (e.g., asteroid belt) may occasionally appear, but they may occasionally clear out their own tiny neighborhoods (after all, these are huge empty neighborhoods, for the most part), but they don't sphere.
So never fear. To sum up: Pluto is the first-discovered orb in the ring of ice that satisfies all three planetary conditions: appear, sphere, and clear. Comets appear and clear, but aren't massive enough to sphere on their own (yes, they occasionally get whittled through solar wind into roughly spherical shapes, but that's a transient condition of the ever-changing ever-changing shape of an ever-diminishing object).