JRehling makes an excellent point. While memorizing the nine planets has been a cornerstone in schools, the dirty little secret is that teaching about the planets this way has been unsuccessful - most people forget it after the quiz is over, and it does nothing to inspire interest. The emphasis on the number of moons probably dates back to pre-space age time when we didn't know much more about these moons than how many there were and their very approximate sizes. But it definitely is less than inspiring.

A vivid memory of mine on the "planet bias" is the National Geographic poster of the Moon that came out in the Apollo era. I had it hanging on my wall in the early 70s, and again in the late 90s. It had a series of circles showing all of the solar system's largest satellites to scale. Each and every one of them was a silvery disc, identical except in size. The culture shock ("science shock"?) of seeing the cover of Science with the montage of the Galileans was world-shaking. Jupiter looked about the same way it did in the Pioneer images, which were decent, but the Galileans -- all four of them -- were stunningly complex. Seeing any one of them like that would have been jawdropping, but to see four, and no two of them alike, was incredible. I don't think another moment could be as surprising. Even Huygens, because we hoped to see what it found. Nobody even hoped to see the Galileans like that.

There is a law of small numbers that applies. Kids don't learn the entire periodic table (in my experience, anyway). Too many items. Kids in the US may learn 50 state capitals.

I made my son a screensaver of the planets and the most interesting satellites plus Ceres and Vesta, set them to scale but using a fourth-power of radius, so that places like Miranda and Vesta have visible detail while preserving the relation of which worlds are larger and which are smaller. And he learned all the names. I refer to the whole set as "planetas", but I add in that some of them are "lunas" of the others. I was pretty arbitrary in choosing the set, mainly going with the ones that had decent imagery available -- no Eris. And when the time comes to fill in more details on what the various places are like, we can fill those in. The total set numbers about 21, and I just don't describe the set as being of some magical size, the way English-speakers learn there are 26 letters and most of us learned there are 9 planets. To me, it's a lot less important for there to be a set of comfortingly fixed size or that the boundary be objective (I included Titan, Iapetus, and Enceladus, but not yet Dione or Rhea or any other Saturnian satellite). All I cared for was that he learned a bunch of them, that they were pretty, that he learned a tidbit or two about some of them -- hottest, biggest, smallest. Having known them all when he turned 2, I think he's gotten a pretty good introduction (he could see it from the chair where he eats). And I can't imagine anything that would have gunked it up more than having to explain categorization schemes.

It'll stick, JR. Check this:

"Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Pluto, Mercury, the Moon."

Memorized from the poster my Dad gave me when I was 2 or 3, in order of size as thought at the time. It sticks; and how I wish I still had that marvelous, magical poster! (Word of advice: Be sure to preserve these things for him, if you can; they are literally talismanic in later years.)

Gosh during that time, the "Moon" poster hanging on my bedroom wall was the one that came with the Pink Floyd Dark Side of the Moon LP. I still dream about green pyramids to this day.

Of course, Pluto was off by a bit

Nowadays it should be:

"Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Ganymede, Titan, Mercury, Callisto, Io, the Moon, Europa, Triton, Eris, Pluto."

When Pluto was discovered, we expected it (along with Triton) had a very low albedo (it turns out it has a very high albedo). Additionally, we thought the light from Pluto and Charon came from one object. I once saw an estimate suggesting its diameter was 6000 km!

I had a book with an illustration showing Pluto as a mirrorlike body of which we could only see the "reflected highlight" -- a circle of opposition surge -- while the majority of its apparent area was dark and unseeable.

The same book had an illustration of a volcano on Mercury in the "twilight zone" (permanent dusk, since the planet was thought to have tidally locked on the Sun) belching gas into a thin atmosphere, and snow clinging to rocky spires on Titan, with a ringed Saturn hanging in a blue sky. Beautiful fictions all around.

The admirable thing is how memorable images like that are. The real solar system has stuff that cool -- just not those particular things.

That book even managed to break the planet-only bias by including an illustration of Titan.

Per Jason's and Stu's complaint, it may be possible that book publishers know (the same way that advertisers who sell Cola know their line of work) that a book that *doesn't* feature the planets will lose out in sales to a book that gave Io, Europa, and Titan more attention than Mercury and Uranus. It's possible that a very important part of the lenses through which the population ends up viewing the solar system is a sales-time reaction to hunt for the "planets" (choir of heavenly angels sings) in all of their primacy and countability.

That wasn't actually my complaint; my complaint was over inappropriate, misleading and inaccurate images Full Stop, usually "false colour" images being used over beautiful real colour images. Case in point: whilst writing my latest book I had to fight for 3 days to get them to use a real colour photo of Victoria Crater instead of a false colour one. It didn't matter that the spread was called "Mars: the Red Planet", they wanted to use a false colour image showing VC in vivid blues and greens because it "looked more dramatic". I won, but only when I offered to let them use one of my colourisations of the same scene - with appropriate credit to NASA, etc, of course.

Remember to stick those guys in here (http://www.exoplanet.eu/catalog-all.php).

-Mike

I tend to think of them by mass, though:

Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars, Mercury, Ganymede, Titan, Callisto, Io, The Moon, Europa, Triton, Eris, Pluto, ...

http://en.wikipedia.org/wiki/List_of_Solar...objects_by_mass

--Greg

Cool article! I'm going to be thinking of planets as objects in the yottagram range from now on!

An upgrade possible for Charon..?

And the Earth hasn't got rid of the Moon. Does that mean we are now a dwarf planet?

I still don't know what "clearing the neighbourhood" means, but I should have thought it included ending up in an orbital relationship.



There is something rather arbitrary about this criterion; for one thing, it depends upon the diameter of the body, which in turn is going to depend upon materials and density -- and the relationship might not even be constant. What would be argued of a system where the common center of gravity floats above a solid or liquid surface, but well inside an atmosphere? What about a center of gravity which is sometimes above, sometimes below the surface? That might already be the case for some of those binary asteroids or irregularly-shaped KBOs with moons.

Name a criterion to separate 'planet' and 'moon' that isn't.

The "binary" definition is definitely a separate matter, but similarly interesting.

I really find it odd that it could actually depend upon the time in the cycle. Suppose the barycenter passed through the tallest mountain on a world, but nowhere else.

Moreover, the barycenter of Sun-Jupiter is outside the Sun. I think that's a nail in the coffin right there.

Masswise, too, things tend to be more profoundly disparate than the barycenter measurement indications. Sun and Jupiter. Earth and Moon. Those primaries are obviously much more massive than the secondaries. Charon is only 14% the mass of Pluto. Maybe mass fraction is a better statistic to use, with some arbitrary threshold. At least it wouldn't depend upon the time of day.