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3\10 An orb by any other name-
 Debate over what constitutes a planet is far from settled
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Media Relations
University of California-Berkeley

Media Contacts:
Robert Sanders
(510) 643-6998, (510) 642-3734
rls{at}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.

(continued)

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