Friday, August 25, 2006

Constellations, their Lore and Meaning

I hope you are making plans to be with us Friday evening, September 1, for the next in our series of astronomy presentations co-sponsored by Museum Astronomical Resource Society and the Science Library at MOSI.

Our presentation will be "Constellations, their Lore and Meaning." Though harder to see above our modern cities, the stars and their groupings still grace our nighttime skies, ever ready to tell us a story, tell us the time and give us direction on a clear night. Come learn how the constellations got their names and how they can help us today.

The presentation will be held Friday, September 1, from 7:00 PM to 8:00 PM in the Saunders Planetarium at MOSI in Tampa. As always, the presentation is free and open to the public. This is the fifth of our six presentations scheduled for 2006. We hope to see you there!

Clear Skies,
Jimmy Thomas

Thursday, August 24, 2006

The Votes Are In, Pluto Is Out

Hello, friends.

It is a momentous day, August 24, 2006. Following a week of heated debate at the IAU General Assembly in Prague, the results of the voting during today's closing ceremonies are known.

The designation of Pluto has changed from "planet" to "dwarf planet," as defined in item 2 of Resolution 5A, below. Pluto is joined in this designation by Ceres and 2003 UB313. Charon will not receive this "dwarf planet" designation, remaining simply a satellite of Pluto. The current solar system planetary count is a follows: 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and 3 dwarf planets (Ceres, Pluto, and 2003 UB313).

Please note that Pluto did not loose its "planet" designation because of its size, but because it did not fulfill its "planetary housekeeping" obligation of clearing out the neighborhood of its orbit. Since Pluto crosses the orbit of Neptune, it is evident that Pluto was not able to steak a claim for itself by gobbling up all of the adjacent matter and become the regional king of the hill, as the now eight solar system planets were able to do during their planetary formation.



For a look at the final draft of these resolutions, follow this link:
(http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html)

To view the full new release on results of the voting, follow this link:
(http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html)


Below is the text of the related IAU Resolutions 5A and 6A:

I should note that a proposed Resolution 5B recommend that the eight planets be described as “classical” planet, and proposed Resolution 6B recommended that the new class of Pluto-type bodies be called “plutonian objects.” However, neither of these resolutions was accepted during this meeting. The matter regarding Resolution 6B will be considered further by the IAU.


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RESOLUTIONS

Resolution 5A is the principal definition for the IAU usage of "planet" and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A
The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A "planet"(see footnote 1) is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape(see footnote 2) , (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects(see footnote 3) except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
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Footnote 1: The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
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IAU Resolution: Pluto

RESOLUTION 6A
The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

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Clear Skies,
Jimmy Thomas

Wednesday, August 16, 2006

Proposed New Definition for a Planet

Hello, friends.

For years, astronomers have been debating the definition of a planet. Today, August 16, the International Astronomical Union's Planet Definition Committee announced their proposal for a new, official definition of "planet." If the proposal is approved by a vote of IAU astronomers on August 24th, the number of planets in the Solar System would swell from nine to twelve.

The International Astronomical Union (IAU) is an organization 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. (URL: http://www.iau.org)

All of the IAU material on this announcement, including text, images and animation, is available at the following URL:

http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html


Below is the text from today's press release:

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The IAU draft definition of "planet" and "plutons"
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.

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Clear Skies,
Jimmy Thomas