[meteorite-list] What Is A Planet? Group Searches For Definition

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:56 2004
Message-ID: <200403301700.JAA12450_at_zagami.jpl.nasa.gov>


What is a planet? Group searches for definition

Sedna discovery prompts a debate

By Beth Daley
Boston Globe
March 30, 2004

After years of debate, astronomers worldwide are now planning to settle a
question that seems basic to every schoolchild: What is a planet?

Prompted by controversy over the status of Sedna -- a large and distant
object announced this month circling the sun -- the International
Astronomical Union is convening a group of top astronomers to draw up
official rules for which cosmic bodies are defined as planets.

Already, some speculate that there could be a popular casualty. Pluto, the
ninth planet, would fail many of the proposed criteria.

"There is a lot more evidence now that it really was a mistake to call Pluto
a planet," said Brian Marsden at the Harvard-Smithsonian Center for

Though the definition of a planet -- and the status of Pluto -- have been
tense topics in astronomy for years, nothing has energized the debate more
than the discovery of Sedna, officially known as 2003 VB12, the largest
planetlike object in the solar system since Pluto was first spotted in 1930.

Some astronomers say Sedna is clearly a planet, based on a simple
definition: Its own gravity has pulled it into a spherical shape. Others say
it can't be considered a planet because it is too small and has an orbit
very different from those of the existing planets.

As new technology allows detection of larger objects in the solar system's
outer reaches, some astronomers say an official rule about planet status is
long overdue. Without accurate ways to classify and sort the objects
circling the sun, it can be difficult to make predictions or draw general
conclusions about the solar system.

"There is some urgency now" to creating a definition, said astronomer Iwan
P. Williams. "It may well be that somebody will find a body 1.5 times the
size of Pluto."

Williams, a professor at Queen Mary and Westfield College, University of
London, will chair the IAU's group, made up of about 12 scientists that
began e-mail discussions last week. The group could make a recommendation
within several months, but no formal approval will be made until the IAU's
next general assembly in 2006.

"At long last we do need some definition of a planet," Williams said. "But
will it be culturally acceptable?"

In one sense, a new definition won't really change astronomy significantly.
Some astronomers equate it to the age-old discussion as to whether Australia
is an island or a continent: The distinction is mainly semantic.

But for astronomers -- and everyone else -- who has grown up on the model of
a nine-planet solar system, the new rules could shake what feels like a
basic truth.

According to one proposed definition, a planet is any large object whose own
shape is rounded by gravity. By that rule, the solar system is now home to
at least 50 planets, and could have far more. But other definitions would
draw the line far differently. According to one proposal, a "planet" must be
more massive than the total mass of all the other bodies in a similar orbit.
That would disqualify Pluto, leaving an eight-planet solar system.

Once it hardly seemed necessary to define a planet. They were first
described in ancient times as slowly moving celestial bodies against the
backdrop of stationary ones. In 1930, the last planet -- Pluto -- was
discovered, and while it was much smaller than the other planets and had a
strange orbit, the nine-planet solar system became a staple of school
curriculums the world over.

But as early as 1801, questions were percolating about new types of cosmic
debris that fell into a blurry area. That year, Ceres was discovered between
Mars and Jupiter. Though initially called a planet, it was later stripped of
that title and classified as an asteroid -- one of many in that part of the
solar system.

It wasn't until 1992, however, that scientists began taking a much closer
look at Pluto and the definition of planets. That year, an entirely new
group of bodies was discovered orbiting the sun beyond Neptune, in a region
called the Kuiper Belt. Maybe, some scientists speculated, Pluto was just
the largest of these objects -- more asteroid than planet.

The discussion was largely confined to academic circles until 1999, when
Marsden, who runs a Cambridge group called the Minor Planet Center,
suggested Pluto be given the honor of being named the 10,000th minor planet,
or large asteroid. Marsden saw it an issue of dual citizenship: Pluto would
be considered an asteroid by scientists, but could also maintain its public
role as a planet. Controversy over this suggestion was so fierce that the
International Astronomical Union had to issue a statement saying it had no
intention of calling Pluto anything but a planet.

Now the debate has started again. Since 1999, several large bodies have been
found in the outer solar system, which are larger than most asteroids but
smaller than planets. Sedna is the largest of these so far. Scientists say
it is probable that an object even bigger than Pluto may be discovered. So
compelling was this evidence that the Hayden Planetarium in New York
excludes Pluto as a planet in a display of the solar system.

But some astronomers say that's the wrong direction to go. S. Alan Stern,
director of the Southwest Research Institute's Department of Space Studies,
has suggested a more-inclusive definition of planets, one in which "gravity
rules." If an object is large enough for gravity to round its shape, then it
should be a planet, he said. It doesn't matter if that means there will be
50 planets, or hundreds.

"People say, `It's not like what I'm used to or what I was taught' -- so
what?" Stern said. "We have to get used to the fact that we still don't know
a lot."

For students everywhere who painstakingly learned the names of planets by a
mnemonic phrase, such as "My Very Educated Mother Just Served Us Nine
Pizzas," adding another 40 planets or more just might seem overwhelming.
Some astronomers, in turn, have suggested less-scientific definitions, but
ones that will keep Pluto in its place: Anything named a planet before 1950
remains one, for example; or any body within a certain boundary or size

"Pluto has drawn the line for being a planet. So for new discoveries, if
they are Pluto-sized or bigger: Welcome to the solar system," said Richard
P. Binzel, a planetary scientist at the Massachusetts of Institute of

Some scientists say even if the IAU's final rules exclude Pluto, it should
be allowed in through a grandfather clause.

David Rabinowitz, a Yale astronomer who helped discover Sedna, has been
speaking to student groups lately trying to describe this gray area of
planetary nomenclature. He jokingly said he thought to call Sedna-like
objects "planetary cast-offs" or "rejects." Now, however, he tells children
that maybe they should be called "pet planets."

"They are not part of the family," Rabinowitz said, "but you have a lot of
respect and love for them."
Received on Tue 30 Mar 2004 12:00:47 PM PST

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