The X Factor (recycled)

I have resisted reposting pre-ScienceBlog posts as the lazy way out, but seeing as how many of my fellow bloggers have done it, what the heck? This one comes from a year ago, on the heels of the discovery of "Xena," what might be a tenth planet. It seems appropriate given that newspaper columnists are doing the same kind of recycling as we anticipate the outcome of a big meeting in Prague later this month, when astronomers will announce just what it is that gets to be called a planet.

Astronomers have been finding new planets on an almost weekly basis for years. Until last week, though, they were all orbiting other stars. So the news that our solar system has 10 planets seems like a pretty big deal. After all, the last such addition, Pluto, came 75 years ago.

Well, yes and no.

Far be it for me, someone with no formal training in astronomy -- not even a first-year class that arts majors take to fulfill the mandatory science component of their degree -- to downplay the "Xena" story. Like I said, it's not every day that you get to name a new planet. (Although naming it after a fictional television warrior princess doesn't exactly strike me as couth.)

But for me, the real value of the event comes in its power to remind us of the need to qualify whatever we may believe about the universe as only our latest best guess.

This idea is something that is usually left to upper-level undergraduate science courses, if at all. In high school, we were all taught that the solar system has nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. I don't think anyone ever told me that there might be more, or that it wasn't so long ago that we thought there were fewer.

I do remember discovering -- on my own -- that Pluto isn't always the most distant planet. In fact, for most of my high school years, it was actually closer than Neptune, something that my teachers neglected to mention.

Eventually, anyone interested in the subject learns that the number of planets in orbit around Sol is anything but a stable, hard fact. Until 1781, everyone assumed the visible six were it, with Saturn occupying the most distant orbit. Advances in optics permitted William Herschel to spot Uranus. Sixty-five years later, Johann Gottfried Galle's patience and superior technology produced Neptune. And 84 years after that, in 1930, Clyde Tombaugh found Pluto.

Between then and now, all sorts of strange planet-like objects have turned up in the outer reaches of the solar system. There's Chiron, Sedna, Quaoar, and, just a couple of days before the Xena announcement, 2003 EL61.

What makes Xena special -- or, more precisely, the reason why Xena has made it off the specialty space websites and onto the mainstream media -- is that it's apparently bigger than Pluto. Which would mean it would qualify as a planet. Which is why stories in papers like London's Independent include quotes the likes of:

"Get out your pens and start rewriting the textbooks today," says an exultant Dr Michael Brown, of the California Institute of Technology, who made the discovery.

Brown goes on to note that "We are 100 per cent confident that this is the first object bigger than Pluto ever found in the outer solar system." He's making the point because all the other recent discoveries are significantly smaller than Pluto. But this assumes Pluto is some sort of objective benchmark for what qualifies as a planet. It isn't.

Astronomers have been arguing over the definition of a planet for decades. Many consider Pluto something less, in part because its orbit is so eccentric (an elongated oval, which is why it's sometimes closer than Neptune, sometimes farther) and tilted at an angle to that of the other planets (what's called the plane of ecliptic). That and it's so much smaller than the other planets.

Back in 2001, New York's Hayden Planetarium stirred up a lot of fuss by excluding Pluto from its list of planets, demoting the little guy to a mere "KBO," or Kuiper Belt Object. On the other hand, a recent cover of New Scientist suggested there may be as many as 23 "planets" in our system, most of them way out there, just waiting for technology to give us a chance to spot them.

My point is the forthcoming formal announcement by the International Astronomical Union on whether or not Xena is a planet will be an arbitrary decision, not a scientific one. There are simply too many technology-dependent criteria for scientists to agree on a common definition.

The inevitability of revision is what distinguishes the scientific method from other ways of understanding. Remember that the next time someone tells you how many natural elements there are in the universe, or what the lifespan of a dolphin is, or whether anything can travel faster than light.