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3\10 An orb by any other name-
 Debate over what constitutes a planet is far from settled
Part 2 of 3

Marcy and Fischer believe that consideration should be given to how an 
object formed, with the name planet reserved for objects forming in 
accretion disks around a star. In the early dust and gas cloud from 
which stars form, fluffy dust bunnies coalesce into bigger dust 
bunnies, until they're big enough for their own gravity to actively 
sweep in even more stuff. Anything that forms this way around a star 
should be called a planet, they argue. Stars and brown dwarfs form 
differently, in the middle of a swirling nebula, thus providing a way 
to differentiate planets from the rest.

But, Basri counters, "I don't think we should define what an object is 
based on how it formed, because I don't think we know enough about 
formation mechanisms, and you can't easily observe how things form."

No one now knows how brown dwarfs form, and to throw a wrench into 
things, there's some doubt that Jupiter formed the way the other 
planets did. Asks Basri, not entirely rhetorically: "Is Geoff going to 
stop calling Jupiter a planet if he discovers it was formed the way a 
brown dwarf is?"

A taxonomy of planets
---------------------
Marcy and Fischer believe that assigning a firm definition to planet 
may also lock astronomers into a taxonomy that will quickly become 
obsolete as we learn more about the varieties of planets in the 
galaxy.

"I think any time you try to draw sharp lines you get into trouble," 
said Fischer. "We should be a lot humbler and say we are calling these 
things planets because we have this historical precedent, this 
historical inertia.  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."

"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."

He anticipates that 70-80 percent of all stars will be found to have 
planets, most of these in multiple planet systems. And even though no 
Earth-sized planets have yet been discovered, the Milky Way galaxy 
could well harbor hundreds of millions of Earths.

"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. Are we really in 
the ultimate position right now where we should redefine things, 
because it freezes it in again? In a decade or two it may look 
incomplete again," Fischer said.

Basri scoffs at these objections. "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. 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."

When Neptune dominates
----------------------
Imke de Pater, who uses both radio telescopes and optical telescopes 
to study planets such as Jupiter and Neptune and volcanic activity on 
Jupiter's moon Io, also thinks that how a body forms should not make a 
difference in deciding whether a body is a planet.

"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," she proposes. "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."

Eugene Chiang, a new member of Marcy's Center for Integrative 
Planetary Studies, knows these swarms well.  He's part of a national 
team called the Deep Ecliptic Survey that is scanning the plane of the 
solar system in search of as many Kuiper Belt Objects as it can find. 
They've discovered some 250 since 1998, bringing the total known KBOs 
to about 600, all swarming beyond Neptune's orbit, 30 times farther 
from the sun than Earth.

Pluto, Chiang notes, is the largest of the Kuiper Belt Objects, and 
its orbit, like that of all the KBOs, is dominated by Neptune. In 
fact, it orbits in lock-step with Neptune: Pluto goes around the sun 
twice for every three Neptune orbits. A large class of such objects in
the Kuiper Belt has been dubbed Plutinos because they also inhabit 
this so-called 3:2 resonance. Of the 100 KBOs that Chiang has tracked 
well, 25 percent are in resonant orbits with Neptune.

"The asteroid belt is dominated by Jupiter, and the Kuiper Belt is 
dominated by Neptune," he says, and objects in neither of these belts 
should be called planets. In fact, because the Kuiper Belt is the 
source of many short-period comets that plunge through the interior 
solar system, Pluto could even be called a comet.

Chiang's interest in the KBOs with resonant orbits comes from his 
theory that planets migrate inward or outward after their initial 
formation. The many objects in resonant orbits with Neptune argue that 
it has migrated outward, he says, shepherding the KBOs with it and
locking many into resonances. The theory could explain some of the 
bizarre planetary systems that Marcy, Fischer, Paul Butler and others 
have found, in which large gas planets seem to be sitting awfully 
close to their star, in contrast to our own solar system, where the 
gas giants are far out. Early in a system's history, gravitational 
interactions between large gas planets and the gaseous disk or small 
objects called planetesimals can drive planets in or out, he said.

The case for Pluto 
------------------
None the less, Basri feels that Pluto needs to remain a planet, partly 
for historical reasons, but primarily because it fits a consistent and 
reasonable definition of a planetary mass object orbiting a fusor. And 
if we include Pluto, how can we exclude other Kuiper Belt Objects and 
asteroids that look almost identical?  There's really no difference 
between Mercury and Ceres, he says, so any consistent definition of a 
planet would have to include both. He suggests calling the eight 
undisputed planets "major planets" and the others, including Pluto, 
"minor planets" -- a usage once applied to the asteroids before their 
numbers skyrocketed. But they'd all still be planets.

"I've thought about this for two years now, and I think I've seen all 
the arguments, I've chewed on them for a long time, I've played with 
them. So I'm ready," he said. "That doesn't mean anyone else is."

Basri's proposed definition means that the number of planets in the 
solar system will continue to grow as more large objects are 
discovered in the Kuiper Belt.  The Caltech team that discovered the 
largest known KBO last year -- a body half the diameter of Pluto that
they named Quaoar (kwah-o-wahr), after a creation force in California 
Indian mythology -- estimates that they "should be able to find 5 to 
10 more of these really big Kuiper Belt Objects over the next couple
of years, including perhaps a couple [of] 'super-Plutos,'" according 
to their Web site. That means an eventual 25 planets.

Someday kids may be stumping their parents with planet names such as 
Vesta, Quaoar and Varuna, if not Ixion or Radamanthus. They'll be 
around for a while -- at least a few billion years -- so you might as 
well get used to them.

(continued)

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