Having Pups Over Pluto And The Planetary Misfits Of
The Kuipers
As king of the Kuipers should Pluto remain an
honourary planet until something bigger is found
beyond Neptune
by Robert Sanders
for Berkeley NewsCenter
Berkeley - Mar 12, 2003
Ask any kid how many planets are in our solar system,
and you'll get a firm answer: nine. But knock on a few
doors in Berkeley's astronomy department, and you'll
hear, amid the hemming and hawing, a whole range of
numbers.
Professor Gibor Basri, who plans soon to propose a
formal definition of a planet to the international
body that names astronomical objects, argues that
there are at least 14 planets, and perhaps as many as
20. To the well-known list of nine he adds several
large asteroids and more distant objects from the
rocky swarm called the Kuiper Belt circling beyond the
orbit of Neptune.
Professor Imke de Pater and Assistant Professor Eugene
Chiang, on the other hand, toss out Pluto without a
backward glance. It's just a big rock, they say, a
former member of the Kuiper Belt, puppy-dogging
Neptune around the solar system.
Not so fast, says Professor Alex Filippenko. The
International Astronomical Union (IAU), which rules on
names for astronomical bodies, has officially said
that Pluto remains a planet, at least for the time
being.
Thus, officially, there are nine. He cavils a bit,
however, making it clear to his students that Pluto is
"more fundamentally a Kuiper Belt Object (KBO), though
an unusually large one."
Professor Geoffrey Marcy and research astronomer Debra
Fischer, both "planet hunters" within the department,
also prefer to keep the number at nine, noting that
the sun, though it probably had 12 or 14 planets in
the past, will in five billion years probably lose
Mercury and Pluto, bringing the count down to seven.
Moons, fusors, brown dwarfs
This difference of opinion within the astronomy
department is part of a larger debate in the
astronomical community over what constitutes a planet.
It provides endless hours of beer-hall debate and
Friday-afternoon tea-time chat, with little hope for
resolution in the near future.
"It's something of an embarrassment that we currently
have no definition of what a planet is," Basri said.
"People like to classify things. We live on a planet;
it would be nice to know what that was."
The IAU has sidestepped any formal definition,
largely, Basri says, because a good definition would
eject Pluto from the list and relegate it to a "minor
planet" or, even worse, a comet. Basri has come up
with a definition that keeps Pluto in the fold, but
necessarily brings in other objects that until now
have not been considered planets -- objects with names
such as Vesta, Pallas and Ceres, now considered
asteroids, or KBOs such as Varuna.
He's now preparing a formal definition to put before
the IAU Working Group on Extra-Solar Planets, and has
posted an article on his Web site that lays out his
definition and arguments as to why it should be
adopted.
"By 10 years from now, I'd be a little surprised if
the IAU had not adopted something along the lines I'm
proposing," Basri said. "It's reasonable."
Most astronomers and the IAU agree that planets should
be orbiting a star -- or more precisely, an object
that is big enough to ignite hydrogen fusion in its
core (what Basri calls a fusor). The IAU Working Group
also excludes anything, like a star, that is big
enough to manage core fusion itself. The consensus
thus excludes moons, even those such as Ganymede,
which is almost as large as Mars but which happens to
be orbiting the planet Jupiter rather than a star.
The definition also excludes failed stars called brown
dwarfs, which are too small to be stars but too big to
be planets. These are the subjects of Basri's
research.
In 1995, he was the first to obtain a spectrum
confirming that brown dwarfs exist, and he has
concentrated on tests that can distinguish brown
dwarfs from low-mass stars.
This work naturally led him to focus on mass as a way
to distinguish between planets and non-planets. He
proposes a natural upper limit for a "planetary mass
object" of about 13 times the mass of Jupiter, or
about 4,000 Earths. At this size, gravity will cause
an object to give off heat, as happens with Jupiter,
but the pressure at the core is a bit too cool to fuse
the element easiest to fuse, deuterium or heavy
hydrogen. Because anything bigger, including stars and
brown dwarfs, is able to fuse deuterium, Basri argues
that it makes sense to define a "planetary mass
object" -- or planemo, as he has dubbed them -- as an
object too small to achieve any fusion.
A natural lower limit to the mass of a planemo, Basri
says, would be a body large enough for self-gravity to
squash it into a round shape. On average, that would
be about 700 kilometers in diameter, though that
number is squishy -- an iron wrecking ball like
Mercury could be smaller and round, while icy planets
like Pluto would need to be larger to achieve
roundness. This limit excludes all but a few asteroids
and KBOs, most of which bear a resemblance to
potatoes.
"The upper limit of a planetary mass is the fusion
boundary, and the lower limit is roundness," he said.
"This definition does not depend on either
circumstance or origin."
Basri then throws in the other traditional property of
planets to reach a final definition: a planet is a
planemo orbiting a fusor.
"If you take this definition," he says, "you don't
have any trouble what to call these objects,"
including many of the new extrasolar planets that
Geoff Marcy and Debra Fischer are discovering.
Marcy disagrees. In his search for planets around
other stars -- he and his colleagues have found about
two-thirds of all known extrasolar planets -- he has
come across planet systems that aren't so neat. Two
years ago, his team discovered two bodies orbiting the
star HD168443 -- one with a mass about 7.6 times that
of Jupiter, and one 17 times Jupiter. Basri would call
this a planetary system with one large gas planet and
one brown dwarf companion -- sort of a failed binary
star system, where one "star" wasn't big enough to
make the grade.
Talk show host David Letterman, an astronomy buff,
quizzed Marcy about these two objects when he was a
guest in April 2001. Marcy admitted that the larger of
the objects is "so large it doesn't even seem like a
planet. We don't know what to call it. Is it a planet?
Is it a star? Is it something in between?
We're befuddled."
"Well, what the hell are we going to do?" asked
Letterman.
"We're screwed," Marcy admitted.
"Run for your life, everybody," Letterman quipped.
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.
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Your enemy is not surrounding your country — your enemy is ruling your
country. And the day he and his regime are removed from power will be
the day of your liberation." -George W. Bush 1/29/03
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