August 2008

• The big fuss over Pluto!
  
  
• The Story of Pluto
  
  
• The Details of the Discussion
  
• Some facts and figures about Pluto and other objects
  

The big fuss over Pluto!

My, oh, my, Pluto has certainly made a big splash in the news over the past week or so. I often receive questions about some Astronomical events -- A recent one has been: Will Mars really be as big as the full moon right around now? (Answer: Unfortunately for viewers, but fortunately for the fate of Planet Earth, no.)

But no other topic has grabbed the attention of so many folks like the news that Pluto will no longer be considered a planet!

"But we were ALWAYS told Pluto was a planet!"

"If it's not a planet, what is it?"

"We will need to come up with a new sentence to remember the planets!" (My favorite: "My Very Educated Mother Just Showed Us Nine Planets." There are MANY others...)

"We'll need to change all the textbooks!" (This one cracks me up -- trust me, you've needed to change the textbooks for a LONG TIME... most of them still say Jupiter has 16 moons. The count is up to 63!)

The interesting part of this whole thing is: the question of whether or not Pluto is a planet is NOT the issue AT ALL. The issue is really: WE DO NOT HAVE A DEFINITION OF THE WORD PLANET. It doesn't seem to bother many people, but it's really scientifically awkward.

It's going to be hard for a little while. I know it. We've had Pluto as a planet for over 75 years. But it will be OK. In the rest of this newsletter, I will try to help you through all this -- there's a story to help simplify the discussion, and then a bunch of details for the scientifically curious.

The Story of Pluto

Many years ago, an astronomer named Clyde, who lived on a Planet called Earth, was wandering around at the edge of Solar System. He found a little 'puppy' who appeared to be all by himself. He called him "Pluto" and ran back to the rest of the Planets and said, "Hey! Look what I found! I will call him 'Pluto'! Can we keep him?"

Pluto was very cute, and small, and alone, so we decided to keep him. We introduced him to Mercury Planet, Mars Planet, Saturn Planet, and all the rest of the Planet family. We officially adopted him and called him "Pluto Planet".

Pluto got along great with the other planets. For almost 75 years, Pluto was a fine member of the Planet family.

But then, as some other astronomers were wandering around the edge of the Solar System, some other little fellows started showing up, out near where we found Pluto!

We found Quaoar Object. And then Sedna Object. As well as Xena Object! They looked like Pluto. They lived in the area where we had found Pluto! As we found more and more members of The Object Family, we realized that these were obviously Pluto's actual relatives -- Pluto's REAL family!

The Planet Family loved Pluto, but realized that he belonged with his real family. They had no right to adopt him from his proper home. They realized that he would be happier back with the other Objects where he was from.

So they decided to let Pluto go back to his actual home.

As Pluto claimed his rightful place as an Object, he reminded the Planets not to be sad. "Whenever you want to see me, I'll be right there, at the edge of the Solar System where I've always been! I will be the same size, the same shape, and be as playful as I've ever been. Hey! If you feel like it, build a little space ship to come visit me and my family and all the other great stuff there is to see out here! There are whole new horizons to explore! And I am at the edge of them. Thank you, Planets, I've had a great time. But, I'm no Planet. I'm an Object."

The Details of the Discussion

The story above gives you a decent overview of what's been happening. For those who want the details, read on!

A little about Pluto, its discover, and history...

In 1930, Clyde Tombaugh found a little speck out at the edge of the Solar System. At the time, astronomers were intrigued by this distant object. Early estimates put its size at at least that of Mercury, maybe even close to the size of Mars, which easily allowed us to refer to it as a new planet.

As we studied it further through the years, Pluto had a bumpy history. In the 1970's, we discovered that our early estimates of the size of Pluto were grossly exaggerated. It's far less than half the size of Mercury, about 2/3 the size of Earth's Moon! As we realized how tiny it was, its status as a planet first became questioned. In 1978, astronomers began to re-reverse their thinking as it was discovered that Pluto had a large moon! (In 2005, we found 2 more moons around Pluto.) If it has a moon, it must be a planet! It seemed the case was closed.

But in 1993, the Galileo spacecraft found an asteroid named Ida, which has a natural satellite (i.e., a 'moon') called Dactyl. Since then we've found about two dozen small asteroids with even smaller moons. We would never consider an asteroid a planet -- if it lives in the asteroid belt, then it's part of the asteroid family. So, Pluto's status was again in question.

Throughout all of this, scientists were also realizing the main problem -- if we had a good definition of the word 'planet' there would be no discussion! Astronomy is a science! A term as basic as 'planet' should be well defined. When we refer to something as a 'plant' or 'animal', we have a well-defined meaning. There's no question as to how it should be classified. 'Planets' were fuzzy. They were some kind of biggish object whose orbit centered around a star. Well, there's lots of stuff that orbit stars -- asteroids, comets, who knows what else -- we needed to fix this problem.

So, last week, the International Astronomical Union decided to. A small committee came up with a definition, proposed it, and voted on it. Here's what it looked like:

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

• A planet is 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 hydrostatic equilibrium (nearly round) shape, and
  • has cleared the neighborhood around its orbit.
• A dwarf planet is 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 hydrostatic equilibrium (nearly round) shape,
  • has not cleared the neighborhood around its orbit, and
  • is not a satellite.
• All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". (These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.)

What does all that mean?? Basically, we define a planet to be something that is "big enough to be round, and all by itself." Most asteroids are potato-shaped... they don't have enough mass for gravity to pull in equally from all directions and make it a sphere. And, things like asteroids are in a collection. Pluto appears to be part of a collection of objects.

This definition has already caused problems. The phrase "has cleared the neighborhood around its orbit", which was supposed to clear things up, has already become problematic to some. The argument: Well, if Pluto (and other objects) cross the orbit of Neptune, then Neptune has not cleared all the stuff out of its orbit. So, according to this definition, NEPTUNE is not a planet. Also, Jupiter has a collection of objects called "Trojans", which lead and follow Jupiter through its orbit. So JUPITER does not have a clear orbit -- it mustn't be a planet either. And that's simply ridiculous.

So, be prepared for more discussion on this.

For those of you who think Pluto is being undeservedly kicked out of the club, remember that it's not a big deal. We still all like Pluto. Pluto was the window to a whole host of new and interesting objects. We still have a spacecraft launched earlier this year on the way to Pluto, and beyond. The New Horizons spacecraft is going to be a window into the objects which are part of Pluto's family. It wasn't like we said, "Oh, Pluto is NOT considered a planet?? Well, turn that ship around!" No -- we will continue to study Pluto. We will still call it Pluto. Just because we have given it a new classification doesn't diminish it at all. In fact, it used to be the runt of the planets. NOW, it's one of the LARGEST of its kind. It is still as fascinating as ever. There's no need to cry for Pluto.

The video blog Rocketboom has a nice little video showing highlights from the IAU Convention. It's nice to hear some comments direct from the astronomers who are in attendance.

Some facts and figures about Pluto and other objects

If you would like to receive this newsletter in your own e-mail box at the beginning of every month by simply send a note requesting subscription to "The Digital Skyline".

Your e-mail address remains private and secure - we shall never provide your e-mail address to any other party. You will simply receive the Digital Skyline at the beginning of every month, and occasional reminders of some significant sky event (such as a meteor shower or eclipse).

The Digital Skyline may be freely distributed.

The Digital Skyline content and banner logo: ©2002-2006, Chris Fenwick, Astronomer, County College of Morris