Trip to ninth planet might go to asteroid Pluto instead

It's not My Very Energetic Mother's solar system anymore.

What we used to think we knew about the planets has been getting a good shakeup lately. The old mnemonic for remembering the nine planets of the solar system (My Very Energetic Mother Just Served Us Nine Pizzas) won't work anymore.

Our understanding of the outer solar system has been changing so fast that in January, just when a mission to the last unexplored planet of the solar system gets launched, it may no longer be that. There might be 10 or more planets -- meaning a few more planets left to explore -- or there might be eight -- meaning US taxpayers are about to spend $550 million to tour a few large rocks.

First of all, there is the still-unresolved issue of what apparently is the newly-discovered 10th planet. Announced at the end of July, the faraway object called 2003 UB313 -- the most distant thing ever spotted in the solar system -- is a world that's larger than Pluto, and otherwise somewhat similar.

So, on the face of it, if Pluto is a planet -- and it's been that way, pretty much undisputed, for 75 years now -- then logically this new thing must be one also. So, according to 2003 UB313's discoverer Michael Brown of Caltech, now there are 10 planets.

But it gets more complicated. Just days before the discovery of the bigger-than-Pluto object, there was another report of a different object that's almost as big as Pluto, called 2003 EL61. Suddenly the boundaries start to seem a bit murkier: If UB313 is a planet because it's bigger than Pluto, does something that's just a bit smaller also qualify? And if not, just what is the cutoff?

These questions have been under sometimes heated discussion among astronomers and others for a few years now, but with the scheduled departure of the New Horizons mission, they have taken on new urgency. There is the question, for instance, of what to call 2003 UB313 -- a temporary designation that's got to go. (It's been nicknamed Xena for now, and its moon, discovered shortly afterward, is called Gabrielle, but neither of these qualifies under rules for names of celestial objects, and they were not intended to be anything but temporary.)

The rules for names, set by committees of the International Astronomical Union, are different depending on whether the object is considered a planet, or simply a member of the Kuiper Belt, a collection of millions of asteroids orbiting out past Neptune. Some large objects out there already have names, such as Sedna and Quaoar. But if perhaps dozens of these things are in the ballpark of Pluto's size or bigger, as many astronomers believe, then how can we possibly call these all planets?

This has led some astronomers, including Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, to suggest that Pluto should never really have been called a planet in the first place, and that it and the other objects discovered out in that region are all just Kuiper Belt Objects, or KBOs. There are only eight planets, in this scheme, and ever more shall be so.

Marsden is a member of a commission of the International Astronomical Union that's supposed to be deciding the fate of Pluto, 2003 UB313, and their kin. (They have not set a deadline for their decision and the whole process, according to some committee members, is a mess.) For Marsden, it comes down to this: There needs to be some rational basis for a definition, ''something that has some physical meaning, not just based on the feeling some people have for Pluto."

The most logical rationale, he thinks, is to stick with eight planets. But after seeing the storm of controversy that idea created -- including a flood of angry and distressed letters from schoolchildren and condemning editorials that followed the decision of New York's new Rose Center to leave out Pluto from their display of planets -- he would settle for a compromise, he says, such as accepting Pluto and anything larger as a planet.

MIT planetary scientist Richard Binzel supports that idea. ''Pluto has been the ninth planet for eight decades," he says, ''so it sets a historical precedent for what it is we call a planet. Pluto sets the planetary boundary." And, he says, this would be the least disruptive solution, fitting with what's described in everything from textbooks to postage stamps.

But others are insisting on a purely scientific definition. Alan Stern of the Southwest Research Institute in Boulder, Colo., thinks there's a clear-cut answer: It's all about the shape. Anything that's big enough to be round is a planet, he says. Smaller objects like asteroids tend to be lumpy and potato-shaped.

There is only one exception among the asteroids: Ceres, the largest- and first-discovered asteroid, is spherical. All the others are lumps. Stern thinks that's just fine: Call Ceres a planet, just as its discoverer John Hershel did in 1801, before changing his mind two years later after discovering other similar objects and deciding to identify them collectively as the asteroid belt. The roundness rule also would include a few other worlds in the brotherhood of planets. Not only Pluto, 2003 UB313 and 2003 EL61, but also Sedna, Quaoar, and perhaps one or two other KBOs whose sizes are not yet well determined.

All this confusion -- however it all works out -- just goes to show how little we know, and thus how important the new mission of exploration to Pluto will be.

When New Horizons, whose launch window opens Jan. 11, reaches Pluto in 2015 and then goes on to explore KBOs in the farthest region of our solar system, it will complete a task astronomers have been hungering for since the dawn of the space age.

Not only have big new objects been popping up in the Kuiper Belt, but Pluto itself, earlier this month, was discovered to have two extra moons in addition to Charon, which was discovered in 1978.

All of these objects are far enough from the sun's heat so they may have remained virtually unaltered since the solar system's birth 4 billion years ago, preserving a kind of fossil history of what our own planet was like back before the dawn of life -- regardless of what we call them.