Okay folks, the vote is in: Pluto is no longer a planet. Sort of. It’s in a new category called “Dwarf Planet” and is also the prototype of a category of objects in the solar system called “Trans-Neptunian Objects,” or TNOs for short. We’ve known about TNOs for a while, but today’s vote by the General Assembly of the International Astronomical Union made that category of objects official, along with the dwarf planets.

In addition, all the other small bodies of the solar system, comets, asteroids, etc., are now part of a class of objects called “Small Solar-system Bodies.”

So, I’ve noted already on CNN.com and other so-called “news sites” that the main story is that Pluto has been demoted. A little bit is being said about the fact that, with these newly ratified definitions, astronomers are codifying the exciting discoveries that have expanded and enriched our understanding of the solar system. It’s no longer just some planets, comets, and asteroids. There’s a whole frontier out there, populated with worlds we’re only just starting to explore. I wish somebody somewhere would make THAT the main news story, instead of the “bleeding lead” that Pluto is no longer a planet. It’s a dwarf planet, and that is a big story, too. One of the most valuable lessons to learn about science is that it grows as new data comes in. And so does our understanding of the cosmos.
For those of you who have been following the IAU news from home, here’s an excerpt from the press release about the whole vote. It is far more nuanced than any news reports you’re going to see.
IAU 2006 General Assembly: Result of the IAU Resolution votes 24-August-2006, Prague: The first half of the Closing Ceremony of the 2006 International Astronomical Union (IAU) General Assembly has just concluded. The results of the Resolution votes are outlined here.

It is official: The 26th General Assembly for the International Astronomical Union was an astounding success! More than 2500 astronomers participated in six Symposia, 17 Joint Discussions, seven Special Sessions and four Special Sessions. New science results were vigorously discussed, new international collaborations were initiated, plans for future facilities put forward and much more.

In addition to all the exciting astronomy discussed at the General Assembly, six IAU Resolutions were also passed at the Closing Ceremony of the General Assembly:

1. Resolution 1 for GA-XXVI : “Precession Theory and Definition of the Ecliptic”

2. Resolution 2 for GA-XXVI: “Supplement to the IAU 2000 Resolutions on reference systems”

3. Resolution 3 for GA-XXVI: “Re-definition of Barycentric Dynamical Time, TDB”

4. Resolution 4 for GA-XXVI: “Endorsement of the Washington Charter for Communicating Astronomy with the Public”

5. Resolution 5A: “Definition of ‘planet’ ”

6. Resolution 6A: “Definition of Pluto-class objects”

The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

This means that the Solar System consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called “dwarf planets” was also decided. It was agreed that “planets” and “dwarf planets” are two distinct classes of objects. The first members of the “dwarf planet” category are Ceres, Pluto and 2003 UB313 (temporary name). More “dwarf planets” are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate “dwarf planets” are listed on IAU’s “dwarf planet” watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The “dwarf planet” Pluto is recognised as an important proto-type of a new class of trans- Neptunian objects. The IAU will set up a process to name these objects.

Below are the planet definition Resolutions that were passed.

RESOLUTIONS

Resolution 5A is the principal definition for the IAU usage of “planet” and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype.

The IAU will set up a process to name these objects.

IAU Resolution: Definition of a “Planet” in the Solar System Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”.
The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A

The IAU therefore resolves that “planets” and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A “planet”^1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape^2 , (c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

(3) All other objects^3 except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

IAU Resolution: Pluto

RESOLUTION 6A

The IAU further resolves:

Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.1

1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

So, you’re standing there on the golf course at the last hole, waiting to sink your final putt of the game. The other members of your foursome have finished and it’s up to you to go for par. You bend over and take a practice putt, concentrating on the lie, the break, the distance to the hole, the wind speed, the time of day—all the factors that influence your putts.

Suddenly from out of nowhere comes a burned-out little piece of slag. It whooshes past your head and lands directly in the cup for a hole-in-one, shattering into pieces as it does so.

I logged into a discussion board earlier today and found the members all chattering up such a scenario, based on a story on today’s CNN.com: Space Station Cosmonaut ‘go’ for golf stunt.

Of course, being the pragmatic scientist (and former golfer) that I am, I felt constrained to point out that such a cosmic hole in one is pretty unlikely to happen. The biggest reason is that the golf ball is going to burn up in the atmosphere on its way down to Earth’s surface. Anything that’s left is going to be ash, or less.

But, just for fun, let’s imagine that some piece of the golf ball survives the trip. While the odds are against this happening, think about the variables in the problem.

First there’s the composition of the golf ball, which we assume meets at least some of the standards of the Royal and Ancient Golf Club of St. Andrews and the U.S. Golfing Association (which appears to be a world-wide standard). (Those standards are laid out here, if you’re interested.) The golf ball to be hit from the station will weigh considerably less than most regulation balls, coming in at only 3 grams, while regulation balls are heavier, but cannot weigh more than 45.9 grams.

Then, there’s the whole question of the construction of the golf ball to be used. It’s not mentioned in the story. However, a typical golf ball has a hard rubber core, a wound secondary layer (usually some kind of polymer), and a hard outer covering made of some kind of plastic. You know what happens to plastics when they are heated, so imagine this 3-gram ball slicing through our atmosphere, and the kind of friction it will encounter. (And, keep in mind that an incoming meteor (a rock!) of the same size as a golf ball is likely to burn up (although maybe not completely, depending the variables of its flight and its composition) on its way in.)

Other variables? We know the speed of the space station from which the golf ball will be hit, the rotation speed of Earth, the size and weight of the golf ball, and the gravitational pull it will feel as it comes down. What we don’t know is the strength of the cosmonaut’s swing, the direction he’ll hit it in, or whether or not he’ll slice it and send the ball whacking off some piece of the space station, thus changing its trajectory entirely. (Which reminds me of the last golf scramble I played in. My younger brother was part of the foursome and he has a hell of a swing. He stepped up to the tee, sliced the ball, sending it underneath the golf cart, where it swirled around and came shooting out the other side and hit a tree. Not only were WE staggering around laugh so hard it hurt, but the foursome of doctors behind us was howling in laughter, too. But I digress.)

A fun thought problem, but in reality the”Whack a Golf Ball off the Space Station” thing is just what CNN says it is—a publicity stunt to commemorate Alan Shepard whacking golf balls across the surface of the Moon during Apollo 14. It’s pretty unlikely that anybody on Earth will do more than hear about it on the evening news late on Thanksgiving Day this year.

But, if you’re a science teacher or somebody who likes to pose thought problems to people, this story might be a good way to teach a few basics in physics or strike up a heated discussion at a party.

Travel Pages Posted

In other news, we finally got our web pages up describing our trip to Australia and the tour of astronomy installations we made. While this was primarily a business trip, we did manage to have some fun and see some great stuff along the way. So, in the grand tradition of all those “What I did on my trip” stories, here’s a link to our trip pages:
Australia Trip Pages.