Several Johns Hopkins University astronomers are
describing the recent decision to strip Pluto of its
planetary status as a "muddled" ruling that is unlikely to
settle ongoing debates over how to define a planet and
whether the term should apply to Pluto. In an informal
poll, only one astronomer was pleased to hear about Pluto's
new status.
Their reactions came after a vote by the International
Astronomical Union, meeting in Prague, that defined a
planet as "a celestial body that is in orbit around the
sun, has sufficient mass for its self-gravity to overcome
rigid body forces so that it assumes a ... nearly round
shape and has cleared the neighborhood around its orbit."
Because Pluto does not meet the last criterion, the IAU
demoted it to "dwarf planet" status.
The decision leaves the solar system with only eight
planets. Since its discovery in 1930, Pluto has been
considered the system's ninth.
Following are some comments about the IAU decision
from astronomers in the Krieger School's
Henry A. Rowland
Department of Physics and Astronomy and at the
Applied Physics
Laboratory. APL is managing the mission of NASA's New
Horizons spacecraft, which was launched earlier this year
on a 9-1/2-year journey to explore the distant region that
includes Pluto.
Andrew Cheng, APL: "I think the IAU vote is a
muddled compromise that will not settle the question of
'What is a planet?' Pluto is not a 'planet' according to
resolution 1, but it is a 'dwarf planet' by resolution 2.
So is it a 'planet'? I thought so before and still think so
now, but those who did not think so before can now point to
the IAU definition and say that Pluto is really not a
planet but a sort of second-class citizen.
"Actually, that is the same situation that has
prevailed with Ceres, other asteroids and comets for many
years — centuries in the case of some of these
objects. Those objects were known as minor planets before,
but now a few of the minor planets have been promoted to
'dwarf planets.' So I suppose I should be happy that Pluto
wasn't demoted all the way into the minor planet
category."
Harold "Hal" Weaver, APL and New Horizons project
scientist: "I don't expect the hoopla over the demotion
of Pluto from the realm of 'classical planets' to have any
effect on the conduct of the New Horizons mission. The
scientific investigation of Pluto remains an important
component of our effort to understand the processes that
shaped the outer solar system, even if some of the objects
in that region defy our efforts to categorize them. The New
Horizons mission remains as viable as ever because it will
provide the initial reconnaissance of one of the solar
system's newly discovered frontiers.
"Regarding the resolution itself, I'm with Andy Cheng
in concluding that the situation is still somewhat muddled.
What exactly is meant by a planet 'clearing its
neighborhood'? Since many 'plutinos,' including Pluto,
cross Neptune's orbit, I'd say Neptune's neighborhood still
needs some clearing! It just seems a bit risky to me to
base a definition on a theoretical construct —
'dynamically cleared regions' — that's only
approximate at best and may change significantly as our
understanding of planet formation evolves over time.
"I further note that there have been particularly
large swings in the theories of outer solar system
dynamical evolution during the past decade. What was
'conventional wisdom' five years ago has been replaced with
the latest fad, and I don't expect that situation to change
any time soon."
Karl Glazebrook, professor in Physics and
Astronomy: "What is meant by 'clearing its orbit'? How
does this relate to having an orbit overlapping Neptune?
Clearly Neptune has not cleared its orbit. They should have
gone with something clean like a size criterion. Seems to
me like a muddled compromise which will just cause more
problems — what about some of the weird orbits
extra-solar planets have? — and the issue will have
to be revisited."
William P. Blair, research professor in Physics and
Astronomy and chief of observatory operations for NASA's
Far Ultraviolet Spectroscopic Explorer Satellite, operated
by Johns Hopkins: "I think the demotion of Pluto into
the realm of other minor objects outside the orbit of
Neptune is the most consistent thing to do to straighten
out the nomenclature of our solar system. However, I don't
find the wording of the official planet definition to be
very clear, and hence it will continue to be open to
interpretation.
"I find it comforting to know, though, that Pluto
hasn't changed just because of our nomenclature. It is the
same today as it was yesterday, and as it has been for
thousands of years. It is still the most accessible of the
objects beyond Neptune that we can study, and studying it
should reveal much new information about the outer solar
system."
Richard Conn Henry, professor in Physics and
Astronomy: "I am delighted that rationality has
prevailed! Keep in mind that our own sun is a dwarf star
... and Pluto is now a dwarf planet! Pluto is an extremely
interesting and important object, and I am overjoyed that
NASA's New Horizons mission is on its way to Pluto! Hurrah
for Pluto, first dwarf planet to be visited by a NASA
mission!"