-- would become instant planets. More would surely be added to the list each year as new discoveries are made. More than 600 KBOs have been detected so far, but researchers extrapolate the limited sky surveys done so far to estimate there are about 100,000 of them bigger than 62-miles (100 kilometers). Enough larger KBOs exist to grow the solar system's planet count, based on Basri's definition, to two dozen within two years, according to estimates.The asteroid Ceres would also have to be reclassified as a planet under Basri's plan. At 930 kilometers (580 miles) wide the Texas-sized rock was the first asteroid ever discovered back in 1801. At the time, some astronomers thought it was a planet, until other asteroids were discovered. Like most other asteroids, Ceres orbits the Sun in a belt between Mars and Jupiter.
Pluto: Just a rock
Basri said he would be surprised if the IAU has not adopted a definition along the lines of his idea within a decade.
Other astronomers will be surprised if the IAU does formally increase the planet count.
Berkeley researchers Imke de Pater and Eugene Chiang would prefer simply to toss Pluto clear out of the planet picture and go back to eight. Its just a big rock, they say, the largest of the KBOs. In fact, Pluto's orbit, like those of other KBOs, is controlled by Neptune's gravitational presence. Pluto is yanked around the sun twice for every three trips made by Neptune. Astronomers call this an orbital resonance of 3:2.
The Kuiper Belt, interestingly, is seen as a comet reservoir, a swarm of objects that formed together in non-planetary manner, each object standing to be kicked toward the inner solar system at any time to become a comet.
"I would say a planet is a body in orbit about a star, but not forming part of a larger swarm, like the asteroids in the asteroid belt or the Kuiper Belt Objects," de Pater said. "A planet also would have to be in a stable orbit for a few billion years -- it shouldn't be a KBO in transit to becoming a comet."
The middle road in the argument, which is winning so far, goes like this: Pluto is a planet, and the public would be confused and even upset to change that. Leave it as the ninth and final planet, but scientifically keep in mind that it's a KBO and don't otherwise increase the count of planets in our solar system.
The larger argument
Around other stars, the definition of a planet gets even murkier. An IAU working group has come to some agreements. A planet is something that orbits a star but does not, like a moon, orbit another planet.
That definition has proved inadequate in the face of new discoveries.
In recent years, astronomers have found dozens of gas giant planets that are much more massive than Jupiter. Though they are not quite heavy enough to jumpstart the thermonuclear fusion that powers a real star, these gargantuan objects strain the definition of planets as we once knew them.
Confusion results in part because astronomers had, back in the mid-1990s, agreed on a loose definition for middleweight objects called brown dwarfs. These failed stars are not massive enough to ignite hydrogen, but they do radiate more than Jupiter. No mass cutoff was set to distinguish between large planets and small brown dwarfs.
Basri suggests it be set at 13 times the mass of Jupiter, or roughly 4,000 Earth masses. Anything bigger can cause deuterium to fuse in the object's core, generating the sort of heat and low-level light typically associated with brown dwarfs.
Geoffrey Marcy, the Letterman guest and co-discover of that 17-Jupiter-mass object, disagrees with Basri. Marcy, who also works at the University of California, Berkeley, says an additional factor must be considered: How did the object form?
If a large object condenses into being at the same time a companion star forms, then the object might be called a brown dwarf, Marcy suggests. But if the object forms later, out of the detritus of star formation (as did all the planets in our solar system) then it deserves consideration as a planet, regardless of mass.
Marcy thinks it's too early to commit to any firm taxonomy.
"It's way too early to define a planet," Marcy said. "No one would have predicted 10 years ago that we'd have any extrasolar planets. Even though we have now found more than 100 of them, these are still the early days in planet hunting."
One of Marcy's colleagues, Debra Fischer, also at Berkeley, agrees that there is more to learn before firm definitions are set. She worries that a quick resolution to the debate could set astronomers' up for a repeat of the definition problems they face today.
"It's a little arrogant, I think, for us to imagine that we understand what the full spectrum is going to shake out to be," Fischer said. "Are we really in the ultimate position right now where we should redefine things, because it freezes it in again?"
Fischer said drawing sharp lines could cause trouble in a decade or two.
"Let's admit that at either end, the high-mass end and low-mass end, this has been completely arbitrary, and that some things don't fit with our classification scheme."
Basri disagrees
Basri stands his ground.
"It's like saying we shouldn't define what a star is until we understand all about star formation and weird binary stars, and so on," Basri said. "If we define a planet based on the basic observable properties of these objects, people can later apply all sorts of adjectives to them as they are understood better, without changing what they are basically talking about."
Alan Boss, who chairs the IAU Working Group on Extrasolar Planets to which Basri plans to submit his proposal, thinks Nature should decide.
Boss, an extrasolar planet theorist at the Carnegie Institution of Washington, shared his personal views with SPACE.com, making clear that his thoughts do not represent the IAU. Boss' views are in line with what Marcy and Fischer expressed.
"It would be nice to name things based on how they formed," Boss said, "even though that is the hard way to go about it." In the end, he said, "it would be best to group things together based on how Nature makes them, because that will probably tell us more about how they formed."
Boss said it might turn out that sun-like stars typically have companions with masses up to 20 times that of Jupiter, but seldom anything more massive. If that were true, "then I would be tempted to let Nature make the decision for us and call all those objects planets."
Regarding Pluto, Boss brings us right back to where we started.
"It seems clear to me, based on what we know about the present solar system, that there are only eight planets, and several populations of generally smaller objects."
Pluto is unlikely to be officially delisted as a planet, most experts agree. A suggestion along those lines generated a public outcry back in 1999, prompting the IAU to issue a press release stating its regret that "incomplete or misleading" press reports on the status of Pluto "appear to have caused widespread public concern." The statement went on to assure that "no proposal to change the status of Pluto as the ninth planet" had been made within the IAU.
When NASA's New Horizons probe visits Pluto and the Kuiper Belt in 2015, it will almost surely become crystal clear -- if it hasn't already before then -- that Pluto the planet is also just a common rock, very unplanetlike in most astronomers' eyes, roaming in a sea of planetary wannabes.
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