Well, it’s official: Pluto is no longer classified as a sovereign planet. Lots of people who don’t get out much are arguing and foaming over the mouth right about now over an issue that really boils down to one thing: semantics.
The whole question is really a matter of opinion rather than science.
Folks who call Pluto a planet do so because
- it orbits the Sun independently,
- it has an atmosphere, albeit a tenuous one,
- it has a moon (actually, three), and of course,
- it’d be a real pain to rewrite all those textbooks.
They’re not wrong. However, there are many cogent arguments against it:
- its orbit is off the main ecliptic (ie. slanted in relation to the plane on which the others orbit)
- its orbit carries it through that of Neptune, and if anything, said orbit resembles that of an object captured by the Sun rather than something formed out of the initial nova (compare it to a cometary orbit)
- its composition and nature more closely resemble that of asteroids and Kuiper Belt objects than a planet formed independently. Theoretically, if it were formed along the lines of the other eight planets, it should be a very small Jovian (gas giant) type rather than terrestrial.
There’s no right or wrong answer. Personally, I’ve always come down on the “not a planet” side of the argument, but I won’t begrudge those who insist on planethood for the poor little guy.
The plain fact is that the whole definition of “planet” is a little vague anyhow. It comes from the Greek and means “wandering star,” which, if you want to get technical, doesn’t match any of the objects we call planets.
So there, you stupid eggheads. Go back to debating Star Trek or whatever it is you do.
Anyhow, here’s a diagram of the “reclassified” Solar System.” Pluto, along with Quaoar and the asteroid Ceres (which the French used to insist was a planet until they realized nobody listens to the French), is now called a “dwarf planet.” Of course, you’ll notice that the Solar System still looks the same, except the labels have changed. Surely, all those smart folks in Prague have more pressing matters to attend to?
I’ve always thought of Pluto as the last signpost before leaving the solar system…kind of like those signs that say, “last gas for 120 miles” or “now leaving Hawkingville.”
Between the Sun and Neptune, you’ve got a clearly determined group of planets that were formed as a family, each having its own distinct set of features. Past that, it gets a little sparse and lonely, and most of it is a mystery.
Outside Neptune, you’ve got the Kuiper Belt, which is a cluster of light rocks that were debris from the Sun’s initial explosion. It extends another 20AU out into the inky black. Past that (50-100AU), you’ve got the Oort cloud, which is the nursery of comets.
Pluto’s kind of like the line of demarcation between the Solar System proper and all the stuff that lays beyond. Whether or not it’s a “planet” per se, it’s still got an important place. At the very least, it’s the largest and most interesting of the Kuiper Belt objects.
So, don’t look at it as a demotion, but rather as a change in job description. No matter what we call it, Pluto’s still out there holding its cold and dark vigil at the edge of known space.
A few postnotes: Pluto has never been visited by a space probe. The only craft to observe Uranus and Neptune was Voyager 2, and Pluto wasn’t situated in a position for the craft to visit, even though it was closer to the Sun than Neptune at the time.
The New Horizons craft is en route to Pluto and should arrive in 2015. It should shed a great deal of light on the nature of the “second solar system.” Plain fact is, even the Hubble has trouble resolving objects that small and distant, so it’s all a big unknown out there.
There’s a great article on Clyde Tombaugh, the discoverer of Pluto here.
And of course, a Disney spokesman had this to say: “Pluto is taking this news in stride, and we have no reason to believe he might bite an astronomer.”