[semi-OT] what is a planet?

Good day,

Found this little ditty press release in my mail this morning. For all you
vaccheads and gropos out there who are concerned
with either empire-building or what kind of body you'll be
fighting on, the whole definition of what a planet is is still up in the air.
I notice they didn't mention anything about a Dr Stone's (don't remember his
first name; worked at NASA JPL
way back) remarks from back in the late 70s/early 80s that "our
solar system is made up of a star, four planets, and debris."

Mk
---------------------------------------------------------
Media Relations
University of California-Berkeley

Media Contacts: Robert Sanders
(510) 643-6998, (510) 642-3734
rls@pa.urel.berkeley.edu

26 February 2003

An orb by any other name: Debate over what constitutes a planet is far from
settled

By Robert Sanders, Media Relations

BERKELEY -- 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.

What's in a name? Help us remember

For ages, teachers have been creating mnemonics to help
students remember the order of the planets. One well-
known version is "My Very Educated Mother Just Sent Us Nine Pizzas." Another
variant, mentioned in Robert Heinlein's book "Have Space Suit, Will Travel,"
goes: "Mother very thoughtfully made a jam sandwich under no protest."
(Thoughtfully stands for Terra, Earth's other name.) But with another five (or
more) potential planets, it's back to the drawing board. Please send us your
mnemonics (rls@pa.urel.berkeley.edu) for the latest solar system lineup:
Mercury, Venus, Earth (or Terra), Mars, Vesta, Ceres, Pallas, Jupiter, Saturn,
Uranus, Neptune, Pluto, Quaoar and Varuna. The best submissions will be
featured in a future issue of the NewsCenter,
     http://newscenter.berkeley.edu

Related links:

* Gibor Basri's thoughts on defining planets
  http://astron.berkeley.edu/%7Ebasri/defineplanet/index.html
* Geoff Marcy's Web site for extrasolar planets
  http://exoplanets.org/

[NOTE: Images supporting this release are available at
http://www.berkeley.edu/news/media/releases/2003/02/26_planet.shtml ]