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Planets
Could Increase from Nine to 12 Under New Definition |
Posted:
08.21.06
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Pluto's status as the ninth and smallest planet in our solar
system hangs in the balance as scientists debate what it really
takes to be a planet and whether the frigid celestial object makes
the cut.
Editor's Note: On Aug. 24, the International Astronomical
Union shifted course and demoted
Pluto to "dwarf planet" status. The eight planets are: Mercury,
Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune, the approved
IAU resolution said.
Printer-friendly version: PDF
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 It
is a debate that has been going on since Pluto's discovery in
1930. After considering its irregular orbit, peculiar orbital
plane, small size and proximity to the Kuiper asteroid belt, scientists
have been debating whether Pluto really should be called a "planet."
Under the simplest non-scientific definition, a planet is a non-stellar
object larger than an asteroid that moves against the celestial
background of stars. This definition describes the nine traditional
planets -- including Pluto.
Astronomers are now considering a proposal that would not only
solidify Pluto's place in the solar system but also set the number
of planets not at nine, but 12.
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New Definition |
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At a recent meeting in Prague, Czech Republic, the International
Astronomical Union made a first attempt at a scientific definition
for planets based on gravity.
 A
planet would be defined as a celestial object that orbits a star
but is not a star itself, that is not a satellite of another planet,
and that has sufficient mass to give the body a spherical shape
-- a phenomenon called hydrostatic equilibrium in which gravity
and thermal pressure are balanced.
Under this new definition the solar system's planets would include
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
and Pluto and three other celestial bodies: Ceres, a body in the
asteroid belt between Mars and Jupiter; the provisionally named
2003 UB313, a larger-than-Pluto Kuiper Belt object nicknamed Xena;
and Charon, now considered one of Pluto's satellites.
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Classes of
planets |
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The new classification of Charon points out a quirk in the new
definition.
Because of Charon's relative mass and distance to Pluto, it actually
orbits a point outside of Pluto because the center of mass of
Charon and Pluto lies outside of either body.
These two planets would be considered to comprise a "binary
planet" system.
The Earth's moon won't qualify as a planet, though it is large
and massive enough under the IAU proposal, because it clearly
orbits the Earth -- the center of mass of the Earth and moon is
found inside the Earth.
A new class of planets, called "plutons," would include
Pluto, Charon and 2003 UB313, as well as any other newly discovered
planets found in the Kuiper Belt.
Another class, the "dwarf planets," would include the
asteroid Ceres.
The IAU has more than a dozen possible objects on their planetary
watchlist, including asteroids, Kuiper Belt objects and an object
in a theoretical, distant region of comets called the Oort Cloud.
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Debate over
new definition |
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While supporters of the IAU resolution say the new definition
fills a scientific need in an increasingly complex solar system,
other experts have found faults.
Caltech astronomer Michael Brown, whose discovery of 2003 UB313
last summer brought the need for a planetary definition to the
forefront, told The New York Times that the new definition was
"a mess."
Brown opposes the plan because of its complexity.
Michael Shara, curator of astrophysics at the American Museum
of Natural History in New York, also disagreed with the new classifications.
 "The
plutons, and Ceres, and all these other bits of rubble, which
are fascinating objects, are nothing but aborted planets,"
he said on the Aug. 16 NewsHour with Jim Lehrer.
"The real planets of the solar system are the ones that
managed to sweep up all of the objects in their own neighborhood,"
a process called accretion, "to become complete planets."
"And under that definition, that much simpler definition,
which really explains and takes into account all the evolution
of the solar system, we have eight planets in the solar system,"
Shara said.
Experts believe the 2,500 members of the IAU will vote to adopt
the resolution -- implying the need to update textbooks and museum
displays -- but the new definition may not last.
"Did our committee think of everything, including extra-solar
system planets? Definitely not," IAU committee head Owen
Gingerich told Reuters.
"Science is an active enterprise, constantly bringing new
surprises. ... Undoubtedly some future IAU committee will have
to revisit this question."
--Compiled
by Adnaan Wasey for NewsHour Extra
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