[meteorite-list] Why Planets Will Never Be Defined

[Ron Baalke]

http://www.space.com/scienceastronomy/061121_exoplanet_definition.html

Why Planets Will Never Be Defined
By Robert Roy Britt 
space.com
21 November 2006

Before the dust even settled after the Great Pluto War at the
International Astronomical Union (IAU)'s General Assembly in Prague, one
thing became clear: There will never be an accepted scientific
definition for the term "planet."

Rather than crafting an acceptable definition, the IAU alienated
members, put the group's authority in jeopardy and fueled schisms among
astronomers on theoretical grounds and even nationality.

And the whole affair was scientifically pointless, many astronomers say.

The controversial planet-definition resolution, passed Aug. 24 in a vote 
of just 424 IAU members, will not stand as worded. Some 300 astronomers 
have pledged not to use it, and many others say it must be redone to 
eliminate contradictions. It will be reworked, at the least, and 
possibly overturned at the 2009 IAU General Assembly in Rio de
Janeiro, Brazil.

Meanwhile, the debate - which the IAU limited to defining round things in
our solar system  - was a neighborhood nomenclature brawl amid a universal 
war of words. Any terminology that might be relevant to our little solar 
system will be laughably inadequate if applied across the galaxy.

Shortly after the Prague vote, I posed a series of questions about the
new definition's merits and shortcomings to several astronomers, among
them Geoff Marcy at the University of California, Berkeley. Marcy and
his colleagues have found more planets beyond our solar system than any
other team.

"Your questions imply that a definition of the word 'planet' is useful
scientifically. That is a view not shared by many professional planetary
scientists," Marcy replied. "The astrophysics of planetary bodies is so
rich and complex that defining 'planet' has never been an issue under
discussion among professionals. So, some of your questions read to me
like the old phrase 'When did you stop beating your wife?' The taxonomy
of asteroids, comets, moons, planet and brown dwarfs is far too
limited to capture the diversity of their origins and internal
constitutions."

Diverse indeed. During 2006, the tally of known extrasolar planets
surpassed 200, and the range of sizes and setups illustrates why a
universal definition is impossible in light of the fact that scientists
are sharply divided on what to call Pluto.

Arguing since 1990

The debate over what constitutes a planet flared up after the 1990
discovery of the first round objects orbiting another star. The three
so-called pulsar planets are about the same size as Earth. They are 
often forgotten in discussions about exoplanets. Some astronomers 
don't see them as planets at all, because they orbit a fast-spinning, 
dead star that cannot support life.

Other worlds several times the mass of Jupiter float freely in space;
they have no host star. Are they planets? Other oddities abound.

"It is a little-known fact that nearly 25 percent of the known
extrasolar planets are in binary- or multiple-star systems," said
Stephen Kortenkamp , a research associate at the University of Maryland.
"That further complicates the notion of creating a universal definition
of planet."

One day during what Kortenkamp calls the Great Pluto War, he browsed his
dictionary. "I see lots of words that have multiple definitions,
depending on the context in which they are used," he told me back then.
"I don't see why the word 'planet' can't be treated the same way."

Kortenkamp figures "planet" means one thing in our solar system and
something else around other stars, and also has varying meanings for
geologists or planet-hunters or the public. "The IAU would have been
better off with this approach rather than trying to dictate a single
definition for what is really a cultural term that means different
things to different groups of people," he said.

The known setups are a tiny sample of what's out there. There are
perhaps 250 billion planets in our galaxy, says Gregory Laughlin, an
exoplanet hunter and planetary system theorist at the University of
California, Santa Cruz.

Eventually, astronomers could find two Earth-size objects orbiting each
other around a center of gravity in the space between them, Laughlin
said. Other worlds might be accompanied by planet-size "Trojans" that
move with them in a horseshoe-shaped pattern. The present IAU
definition, requiring a planet to clear out the path of its orbit, is
not set up to handle such offbeat configurations.

It's also possible two planet-mass objects could be found orbiting each
other with no star involved.

Complicating the idea of planet-hood are the very massive objects that
have been the easiest to find with current technology. There are dozens
of them, each several times the heft of Jupiter, and many bump up
against the mass range of brown dwarfs to create yet another fuzzy 
area of definition.

Brown dwarfs are big balls of gas that can be up to 70 times as massive
as Jupiter but not massive enough to carry out the thermonuclear fusion
of hydrogen that powers real stars. Generally, the lower cutoff is
thought to be at 13 times the mass of Jupiter, a level that triggers the
fusion of deuterium, which gives brown dwarfs a warm glow that Jupiter
can't muster. Thing is, astronomers don't know how gas-giant planets are
born nor what conditions create a planetary mass object versus a brown
dwarf.

In many astronomers' minds, formation scenarios must play a role in any
useful planet definition. The current IAU definition does not even
address formation.

'Major rifts'

The Great Pluto War alienated many of the roughly 10,000 professional
astronomers around the world who did not have a chance to cast a vote.
It also created "two major rifts" among astronomers, said David
Morrison, an astronomer at NASA's Ames Research Center who was among the
few who did vote.

"Most important was a rift between astronomers who study physical
properties of objects and those who study orbits (dynamics)," Morrison
told me. "The dynamicists dominated at the IAU, and many of them would
not accept any definition that was based solely on physical properties
such as size."

"The second division was along national lines," Morrison explained.
"Some astronomers seemed irritated by perceived American domination of
the process. Some felt, with considerable justification in my opinion,
that some Americans astronomers defended Pluto as a planet in large part
because an American had discovered it. As in so many other international
contexts, there can be reaction against perceived American arrogance."

In an interview with SPACE.com published in September, IAU
president Catherine Cesarsky said there is no reason to question the
governing body's authority. But when asked if that authority had been
weakened, she also said: "It is too early to tell."

In the broadest terms, a planet could be thought of as anything from an
800-kilometer-wide (500-mile-wide) round rock orbiting a dead star to a
colossal gas ball floating alone in space. No accepted definition will
be possible unless the IAU democratizes the decision-making process by
allowing all members to vote.

Even then, defining and categorizing all these different worlds is seen
as impossible by some astronomers. Many think it is simply irrelevant,
or, as Geoff Marcy puts it: "Categorizing them does not magically add
insight."

This article appeared in the Winter issue of Ad Astra
magazine.