When does a hill become a mountain?

There been quite a bit of press lately (e.g., WaPo, La Times) of the International Astronomical Union (IAU's) deliberation on what, exactly qualifies as a planet. The IAU is a group of astronomers and planetary scientists who meet periodically to decide, among other things, what acceptable names are for various planetary features and what is or is not a planet. Of recent controversy has been the discovery of several large icy bodies discovered in distant orbits beyond Nepute, which is the outermost of the gas giants. Turns out there is probably lots of trans-Neputian icy objects out there, so the question is are these planets?

The IAU seems to be leaning toward a broad as definition of a planet as possible. Is it round? Objects larger than ~480 km in diameter are pulled into a round shape by gravitational forces. And does it orbit a star, rather than other planet? Several large moons of Saturn and Jupiter, for example, are larger than the planet Mercury but do not qualify as planets. The LA Times headline summed it up best: "Round and Orbity? Must be a planet."

To me, this debate is like arguing when a hill becomes a mountain. There's no exact definition because it depends on context. What would be called a large hill if located next to larger mountains might be deemed a mountain if it were located on an isolated plain. I think this issue strikes a nerve because we like to think of the heavens as constant. The stars appear night after night (clouds permitting), and so it is disquiting to have people say there are nine planets one day, eight the next (some want to nix Pluto), and twelve the day after that (several trans-Neptunian objects with sexy names like 2003 UB313 may qualify as planets under the new definition).

Personally, I think we should a) leave the number of planets at nine or b) demote Pluto and make it eight. It turns out that Ceres, the largest asteroid, was originally designated a planet when discovered in 1801, only to be later demoted as a multitude of other asteroids were discovered. Pluto was discovered in 1930 by American astronomer Clyde Tombaugh, so American astronomers may be a bit more hesitant to demote Pluto. Here's the best take on the subject by the now defunct "Brunching Shuttlecocks," presented as a conversation between Pluto (B) and Brunching Shuttlecocks (BS):

BS: There are some on Earth who think you ought to be classified as a Kuiper Belt Object.

P: Me, a damn KBO? Blow me! Your Mom's a KBO!

BS: Well, you would go from being the smallest planet to the largest KBO. Any joy in the big fish / small pond theory?

P: I'm a planet. Read me lips. Pla-net. You got a problem with that?

BS: I was just making a point.

P: I've got a point to make. Bite me.

BS: So you're content being known as the smallest planet in the solar system?

P: As a planet, I get my props. When was the last time a bunch of 3rd graders stood in a line and recited the names of the 10,000+ KBOs?

Mars: As big as the full moon? A: No.

You might have been the recent recipient of an email that breathlessly states that this August (2006) "...Mars will look as large as the full moon to the naked eye." I had several friends and family members send me this one. It's actually a) not true, and b) recycled falsehoods from 2003, when Mars did have a close approach to each (here close means 54 million km).

This email kills me for a number of reasons. What really hurts is that the people who have sent it to me are excited about space - they convey a bit of that child-like wonderment we all felt the first time we saw stars in the night sky unblemished by city light pollution. I love it when people are excited about space science and I hate to dim any of that enthusiasm, but I think it is important to bring people's expectations in line with reality.

Perhaps the best source to quickly check on emails such as these is Phil Plait's Bad Astronomy blog. Phil has attempted to slay this particular dragon several times, but like the hydra, new heads keep popping up.

There is some speculation that this email was written by a professional as a joke. The 2003 version of this email contains the following phrase:

"it [Mars] will attain a magnitude of -2.9 and will appear 25.11 arc seconds wide. At a modest 75-power magnification Mars will look as large as the full moon to the naked eye.

I think part of the reason this email seems so convincing is that is has some accurate-sounding facts included. It gives the apparent diameter of Mars in the sky to two decimal places! The 2003 email had a paragraph break between "At a modest 75-power magnification" and "Mars will look as large as the full moon to the naked eye." Thus, if you were quickly skimming the text, you may miss the first clause about looking through a telescope.

The emails I have received in 2006 had the wording altered:

"At a modest 75-power magnification.
To the naked eye Mars will look as large as the full moon!"

So someone has taken an misperception in a 2003 email and turned it into an outright lie in 2006. In 2003, Mars did appear pretty darn close, but close is a relative term. You'd still need binoculars to verify that is was a disc instead of a star. As the email(s) correctly state, Mars at closest approach is about 25 arcseconds across. The full Moon, however, is about 1800 arcseconds across. But what's a factor of 72 between friends and family members? By the way, 2006 is not going to be a good year for Mars-watching. Mars will be on the other side of the Sun. A good rule of thumb is that (relatively) good Mars observing times come around once every two years: 2003, 2005, 2007, etc. This two-year alignment cycle is the same reason why NASA has a launch window to Mars every two years.