For Now, Pluto Holds Its Place in Solar System
By DENNIS OVERBYE
Published: August 16, 2006
Pluto dodged a bullet today.
In the hope of ending years of wrangling, a committee of astronomers and
historians has proposed a new definition of the word “planet” that would
expand at a stroke the family of planets from 9 to 12 and leave textbooks
and charts in thousands of classrooms out of date.
But astronomers immediately began to wrangle about it.
“It’s a mess,” said Michael E. Brown of the
California Institute of Technology.
Among the chosen few within the solar system would be not only Pluto,
whose status has been challenged in recent years, but also Ceres, the
largest asteroid; 2003 UB313, nicknamed Xena, an object discovered by Dr.
Brown in 2005 orbiting far beyond Pluto in the outer solar system; and even
Pluto’s largest moon, Charon.
In addition, at least a dozen more solar system objects are waiting in
the wings for more data to see if they fit the new definition of planethood,
which is that an object be massive enough that gravity has formed it into a
sphere and that it circles a star and not some other planet.
The definition, they said, would apply both inside and outside the solar
The new definition was to be announced today in Prague, where some 2,500
astronomers are meeting in the triannual assembly of the International
Astronomical Union. It is the work of the group’s Planet Definition
Committee, whose chairman is Owen Gingerich, a
Harvard astronomer. The astronomers will vote on the definition on Aug.
In a statement, Dr. Gingerich said this might not be the last word on
what a planet is. “Science is an active enterprise,” he said, “constantly
bringing new surprises.”
So it was no surprise that as word of the decision leaked out yesterday,
reaction from astronomers suggested that the argument was far from over.
“This will be the talk of the town in Prague,” said Alan P. Boss, a
planetary theorist at the Carnegie Institution of Washington, who said the
new definition, with four paragraphs and four footnotes, read as if it had
been written by lawyers, not scientists. “I don’t think this is the one were
Neil deGrasse Tyson, director of the
Hayden Planetarium, which was raked over the coals five years ago for
demoting Pluto in an exhibit in its new Rose Center at the
American Museum of Natural History in New York, was clearly disappointed
in the committee’s work. “I’m happy there’s finally a definition that’s
unambiguous,” Dr. Tyson said. “There hasn’t been one in 2,500 years.”
But roundness, he said, is not a very interesting attribute to use in
classifying astronomical bodies. “A Plutophile is well served by this
definition,” he said. “It is one of the few that allow you to utter Pluto
and Jupiter in the same breath.”
But Alan Stern of the Southwest Research Institute in Boulder, Colo.,
called the definition “a nice solution that works both inside and outside
the solar system.”
Everybody agrees that a little clarity is needed when it comes to
categorizing the members of the solar system. The proposed definition would
come as a relief to schoolchildren and others who have rallied to the cause
The planet (if that is what it is) has been an oddball ever since Clyde
Tombaugh spied it wandering in the outer reaches of the solar system beyond
Neptune in 1930. Not only is it much smaller than the other eight planets,
only a fiftieth the mass of Earth, but its orbit is unusually elliptical and
inclined to the plane that marks the orbits of the other planets. In recent
decades, however, other objects with orbits like Pluto’s have been
discovered in the Kuiper Belt, a junkyard of icy debris beyond Neptune.
Many astronomers began to argue that it made more sense to think of Pluto
as a Kuiper Belt object, a minor planet instead of a planet. When it was
reported that the Hayden Planetarium had done just that in its new Rose
Center, which opened in 2000, a firestorm erupted. Schoolchildren rushed to
the defense of lonely little Pluto.
Two years ago, the International Astronomical Union appointed a group to
come up with a definition that would resolve this tension. The group, led by
Ivan Williams of Queen Mary University in London, deadlocked. This year a
new group with broader roots took up the problem. After a sleepless night in
Paris this spring, what Dr. Gingerich calls a miracle took place: “We had
reached unanimous agreement.”
In a nod to the idea of classifying Pluto with the Kuiper Belt, the group
proposed calling planets with elongated orbits beyond Neptune “Plutons,”
while emphasizing that they would still be planets.
But Dr. Brown pointed out that at least 43 other publicly known objects
in the Kuiper Belt were big enough to fit the planet definition, and that
his group was sitting on a list of dozens more.
Dr. Boss said, “We’re going to have more planets inside the solar system
than we have outside.”
He added, “Being a planet used to be an old boys’ club, with eight or
Dr. Boss and Dr. Brown were especially critical of a feature of the new
definition that would bestow planetary status on Charon, a moon of Pluto.
With a diameter of about 700 miles, Charon is big enough for gravity to
crush all other forces and make it round, but so are some of Jupiter’s and
Saturn’s moons, as well as our own.
The difference, according to the definition, is that the center of
gravity for Pluto and Charon is between them, not inside either one. So
technically, Charon is not orbiting Pluto but is orbiting the center of
gravity of the two bodies. The center of gravity for the Earth and its moon,
on the other hand, is inside the Earth. Dr. Boss calls this “a legalistic
Dr. Brown said, “That one doesn’t pass the smell test.”
“I really hoped something good would come of this,” he said. “They proved
“It is sad,” he added. “Clarity would have been nice.”
Dr. Stern, however, who is the principal scientist on the New Horizons
space mission to Pluto, said the new definition was logical and not
It makes sense, he said, that there could be dozens of planets in the
solar system. The new discoveries in the Kuiper Belt have put Pluto in
context, he said. “Pluto is no longer the misfit,” Dr. Stern said. “It is
closer to average than the Earth.”
He added: “Nature is much richer than our imagination. Life is tough,
life is complicated. Get over it.”
Not everybody cares about the great planet debate.
Geoffrey W. Marcy of the
University of California, Berkeley, a widely known hunter of planets
around other stars, said in an e-mail message, “I am not attending the I.A.U.
meeting, nor do I care about the outcome of any vote about whether Pluto and
Xena are ‘planets.’ ”
“The universe,” Dr. Marcy added, “contains so much beauty and so many
mysteries that we astronomers already have our hands full figuring out how
it all came about.”