Our solar system, as it existed until Pluto's ouster on Aug. 24, 2006. (NASA)

So long Pluto, it's been a gas. You've served us well over the 70-odd years since you were discovered in 1930, slinging about in that funny, elongated orbit you have, way out there on the outer fringe of our solar system.

When we were kids, you were our favourite planet. What child wouldn't like the smallest planet in the universe, especially when we were able to hang it on the classroom mobile so far from all those other big gassy things elbowing about so close to the sun?

The fact that you shared a name with one of the kindest cartoon characters of the 20th century only made you that much more endearing.

Certainly you didn't deserve all that scorn heaped upon you in recent weeks as the International Astronomical Union (IAU) went about the business of trying to define what exactly is a planet.

Time magazine was the meanest: "Get Pluto out of here!" it bellowed. Though it must be said that all over the world writers, and even learned scientists, had fun at your expense.

They said you were too small — smaller than our own moon, in fact, and we would never call that old rock a planet. They called you puny and cold, and they questioned your roundness. (Did you even have enough gravitational heft to leave you squished into a spherical shape?)

But mostly they doubted your "orbital dominance." It's that quirky tilted orbit again, Pluto. To be a planet in today's universe you have to be able to clear other objects out of your orbital zone.

She was almost a planet. Xena, formally known as 2003 UB313, is shown here with satellite Gabrielle, orbiting through the Kuiper Belt at the outer edge of the solar system. (Michael Brown/W.M. Keck Observatory/AP)

An absence of orbital dominance

Planets today are the SUVs of the solar system and you, Pluto, were just too nice. You tilted and dodged when you should have bullied. You deked, politely, across the orbit of Neptune as you made your long trek, 248 years to slingshot around the sun.

It was this same lack of orbital dominance, it appears, that doomed the hopes of three other planetary wannabes: Your moon, Charon, which followed you about like a devoted sidekick; Ceres, which used to be a planet until it was demoted to a large asteroid in the 1800s; and the recently spotted 2003 UB313 (in the outer solar system), which its California discoverer would like to name Xena, after a different sort of cartoon character.

For a time it looked like all three were going to be admitted to the new world order and be called planets. The neighbourhood, so to speak, would grow from the nine most of us living on this earth have memorized at one point or another, to 12, which would have been fitting — the universe is, after all, expanding.

That was the position of the executive committee of the IAU. It took almost two years to come up with the not inelegant definition that any mostly round space object that is at least 800 kilometres in diameter, orbits the sun and has a mass roughly one-12,000th that of Earth is a planet. Period.

Alas, that was before some 2,500 astronomers from 75 countries gathered for the full IAU meeting in Prague in August 2006. And before the editorial writers, the axe-grinders and the Pluto-crats waded in with their many objections.

Their main one was that if you kept Pluto and let Ceres and Xena into the club (given their size, how could you not?), then you were basically opening the door to at least 50 other potential planets of roughly the same girth and mass. Most of them are like you Pluto, just wandering ice balls and comets zooming about in the Kuiper Belt, an icy disk-shaped region that some call our solar system's "final frontier."

This just wouldn't do. (There goes the neighbourhood.) Besides, how could anyone older than 12 or younger than 45 ever hope to memorize such a bulging list of heavenly bodies.

The new heavenly order

The solution, now, is to divide the solar system into three sets of planetary inhabitants:

• The eight full-scale, sun-orbiting planets, including Earth, of course.
• A secondary and probably larger group of so-called dwarf planets, including Pluto, Ceres, Xena and probably dozens of others of a certain size and pretense. (Some astronomers are suggesting they be called Plutonians, in deference to Pluto's earlier standing.)
• The many — perhaps several hundred thousand — "smaller solar system bodies" like identifiable comets and asteroids whose banal characterization aptly depicts the void that is the human imagination.

Of the eight remaining planets, they are, in order of their proximity to the sun:

• Mercury, the eighth-largest and second-most dense planet, also has a somewhat quirky orbit. It is also the quickest to skirt across the heavens and is named after the Roman god of travel, commerce and (oddly) thievery.
• Venus, is the sixth-largest planet. It is also the brightest and has a particularly circular orbit. Some call it Earth's sister planet because the two are remarkably similar in size, though its atmosphere is much more dense. Others say it is where women come from, though clearly they are saying that in jest.
• Earth, the third rock from the sun and fifth in size, was formed some 4.6 billion years ago and has evolved a certain life form that likes to think it can classify all there is to classify in the universe.
• Mars, or the Red Planet because of its distinctive colouration holds a special place in the imagination of humankind because of comic books, hints of water on its well-grooved surface and an unquenchable scientific ambition that it may be just the planet to colonize at some point down the road. Named after the Greek god of war (hence a place where it is said men come from), Mars enjoys an average temperature of -55 C, though it can rise to a balmy 27 C on a really good day.
• Jupiter is the big guy on the block, more than twice the size of all other planets combined and roughly 318 times the size of Earth. A huge gas ball (roughly 90 per cent hydrogen and 10 per cent helium) Jupiter is believed to have a solid rock core but no one is entirely sure.
• Saturn, the one with the distinctive ring, is the second-largest planet (120,000 km in diameter) in the solar system and also a massive hunk of gas. The least dense of the planets, its specific gravity is, in fact, less than that of water.
• Uranus is the third-largest planet in the system and the one that every school kid jokes about. It is a big chunk of rock and ice and was the first to be discovered in modern times, in 1781.
• Neptune, the fourth-largest by diameter is often hidden behind Uranus (now children) and so was not formally discovered until 1846. Another gas planet, it is (with Xena and Pluto out of the picture) the most distant planet from the sun.

So, little Pluto, this is the celestial company you will be leaving — four big rocks and an equal number of giant gas balls — as you make your eccentric zig-zaggy way through the interstellar darkness. Maybe that's why we like you so much: You are just like us.