The IAU draft definition of "planet" and "plutons"
Dr.TeKtOn
2006. 8. 29. 13:43
16. August 2006, Prague
The
world's astronomers, under the auspices of the International
Astronomical Union (IAU), have concluded two years of work defining the
difference between "planets" and the smaller "solar system bodies" such
as comets and asteroids. If the definition is approved by the
astronomers gathered 14-25 August 2006 at the IAU General Assembly in
Prague, our Solar System will include 12 planets, with more to come:
eight classical planets that dominate the system, three planets in a
new and growing category of "plutons" - Pluto-like objects - and Ceres.
Pluto remains a planet and is the prototype for the new category of
"plutons."
With the advent of powerful new telescopes on the ground and in
space, planetary astronomy has gone though an exciting development over
the past decade. For thousands of years very little was known about the
planets other than they were objects that moved in the sky with respect
to the background of fixed stars. In fact the word "planet" comes from
the Greek word for "wanderer". But today hosts of newly discovered
large objects in the outer regions of our Solar System present a
challenge to our historically based definition of a "planet".
At
first glance one should think that it is easy to define what a planet
is - a large and round body. On second thought difficulties arise, as
one could ask "where is the lower limit?" - how large, and how round should an asteroid be before it becomes a planet - as well as "where is the upper limit?" - how large can a planet be before it becomes a brown dwarf or a star?
IAU President Ron Ekers explains the rational behind a planet definition: "Modern
science provides much more knowledge than the simple fact that objects
orbiting the Sun appear to move with respect to the background of fixed
stars. For example, recent new discoveries have been made of objects in
the outer regions of our Solar System that have sizes comparable to and
larger than Pluto. These discoveries have rightfully called into
question whether or not they should be considered as new 'planets.' "
The
International Astronomical Union has been the arbiter of planetary and
satellite nomenclature since its inception in 1919. The world's
astronomers, under the auspices of the IAU, have had official
deliberations on a new definition for the word "planet" for nearly two
years. IAU's top, the so-called Executive Committee, led by Ekers,
formed a Planet Definition Committee (PDC) comprised by seven persons
who were astronomers, writers, and historians with broad international
representation. This group of seven convened in Paris in late June and
early July 2006. They culminated the two year process by reaching a
unanimous consensus for a proposed new definition of the word "planet."
Owen Gingerich, the Chair of the Planet Definition Committee says: "In
July we had vigorous discussions of both the scientific and the
cultural/historical issues, and on the second morning several members
admitted that they had not slept well, worrying that we would not be
able to reach a consensus. But by the end of a long day, the miracle
had happened: we had reached a unanimous agreement."
The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states "A
planet is a celestial body that (a) has sufficient mass for its
self-gravity to overcome rigid body forces so that it assumes a
hydrostatic equilibrium (nearly round) shape, and (b) is in orbit
around a star, and is neither a star nor a satellite of a planet." Member of the Planet Definition Committee, Richard Binzel says: "Our
goal was to find a scientific basis for a new definition of planet and
we chose gravity as the determining factor. Nature decides whether or
not an object is a planet."
According to the new draft
definition, two conditions must be satisfied for an object to be called
a "planet." First, the object must be in orbit around a star, while not
being itself a star. Second, the object must be large enough (or more
technically correct, massive enough) for its own gravity to pull it
into a nearly spherical shape. The shape of objects with mass above 5 x
1020 kg and diameter greater than 800 km would normally be
determined by self-gravity, but all borderline cases would have to be
established by observation.
If the proposed Resolution is passed,
the 12 planets in our Solar System will be Mercury, Venus, Earth, Mars,
Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313.
The name 2003 UB313 is provisional, as a "real" name has not yet been
assigned to this object. A decision and announcement of a new name are
likely not to be made during the IAU General Assembly in Prague, but at
a later time. The naming procedures depend on the outcome of the
Resolution vote. There will most likely be more planets announced by
the IAU in the future. Currently a dozen "candidate planets" are listed
on IAU's "watchlist" which keeps changing as new objects are found and
the physics of the existing candidates becomes better known.
The
IAU draft Resolution also defines a new category of planet for official
use: "pluton". Plutons are distinguished from classical planets in that
they reside in orbits around the Sun that take longer than 200 years to
complete (i.e. they orbit beyond Neptune). Plutons typically have
orbits that are highly tilted with respect to the classical planets
(technically referred to as a large orbital inclination). Plutons also
typically have orbits that are far from being perfectly circular
(technically referred to as having a large orbital eccentricity). All
of these distinguishing characteristics for plutons are scientifically
interesting in that they suggest a different origin from the classical
planets.
The draft "Planet Definition" Resolution will be
discussed and refined during the General Assembly and then it (plus
four other Resolutions) will be presented for voting at the 2nd session
of the GA 24 August between 14:00 and 17:30 CEST.
Notes for editors
The
IAU is the international astronomical organisation that brings together
distinguished astronomers from all nations of the world. IAU's mission
is to promote and safeguard the science of astronomy in all its aspects
through international cooperation. Founded in 1919, the IAU is the
world's largest professional body for astronomers. The IAU General
Assembly is held every three years and is one of the largest and most
diverse meetings in the astronomical community's calendar.
Science contacts (the persons below can all be reached via the IAU GA Press Office, see bottom)
Owen Gingerich IAU Planet Definition Committee Chair Tel: via the Press Room +420-261-177-075
Iwan Williams President, IAU Division III Planetary Systems Sciences Tel: via the Press Room +420-261-177-075
Ron Ekers IAU President Tel: via the Press Room +420-261-177-075
Catherine Cesarsky IAU President-Elect and member of the Planet Definition Committee Tel: via the Press Room +420-261-177-075
Richard Binzel Member of the Planet Definition Committee Tel: via the Press Room +420-261-177-075
Dava Sobel Author and historian, member of the Planet Definition Committee via the IAU Press Office Tel: +420-261-177-075
Mike Brown Discoverer of 2003 UB313 and a host of other planet candidates Via Caltech Media Relations Tel: +1-626-395-3226 E-mail: mbrown@caltech.edu
PIO contact Lars Lindberg Christensen IAU Press Officer IAU GA 2006 Press office, Meeting Room 3.2 Prague Congress Center Tel: +420-261-177-075 Cellular: +49-173-3872-621 E-mail: lars@eso.org
