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On August 24 -- if everything goes as expected -- the people of planet Earth will wake up to a new world of 12 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and 2003 UB313. And, according to the International Astronomical Union (IAU), there are more to come.
Of course the "new" planets have always been there, at least in our timeframe, but it's only been with the advent of powerful new telescopes on the ground and in space and a new definition of the word "planet" -- that the latest 3 spherical bodies are being recognized as such.
Under the auspices of the IAU, a specially selected committee recently concluded 2 years of work in redefining what, exactly, should be deemed a planet, as well as what the difference is between planets and smaller solar system bodies, such as comets and asteroids.
For decades, a debate about whether or not Pluto is a true, bona fide planet has raged, and now it appears the tiny, distant planet discovered by Clyde Tombaugh in 1930, is a winner. In fact, Pluto not only remains a planet, but now serves as the prototype for the new category of "plutons" -- although its not official just yet.
More On Pluto
* Pluto and Charon
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* The New Horizons mission
* Planetary Radio Interview with Alan Stern, co-discoverer of Pluto's new moons
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* Enter your photo in the New Horizons Digital Time Capsule
Astronomers at the IAU General Assembly, now underway in Prague, Czech Republic, must approve the new definition by vote August 24.
The word "planet" comes from the Greek word for "wanderer" and, in fact, 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. During recent decades, however, new technologies have allowed astronomers to uncover numerous large objects in the outer regions of our solar system, and the historical definition of a "planet" just wasn't cutting it anymore.
"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," noted IAU President Ron Ekers, Federation Fellow, CSIRO Australia Telescope National Facility. "These discoveries have rightfully called into question whether or not they should be considered as new 'planets.'"
Defining a planet may seem easy: a planet is a large, round body. But setting the particulars is extremely tricky. Just how large and how round does an asteroid have to be before it becomes a planet? What is the lower limit? Conversely, where is the upper limit -- how large can a planet be before it becomes a brown dwarf or a star?
The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. During the last 2 years, the world's astronomers have had official deliberations about a new definition. The results of those deliberations were channeled to a Planet Definition Committee (PDC) that was appointed by the organization's executive committee, led by Ekers. The PDC is comprised of 7 esteemed individuals from the milieu of planetary science:
* Owen Gingerich, the committee chair, professor emeritus of astronomy and of the history of science at Harvard University and a senior astronomer emeritus at the Smithsonian Astrophysical Observatory;
* Iwan Williams, president IAU division III planetary systems sciences, professor of Mathematics and Astronomy at Queen Mary University of London;
* Catherine Cesarsky, IAU president-elect, director general of ESO [European Organization for Astronomical Research in the Southern Hemisphere];
* Richard Binzel, professor of planetary science at the Massachusetts Institute of Technology (MIT)
* Dava Sobel, author (Galileo's Daughter, The Planets) and historian;
* Mike Brown, professor of planetary astronomy at the California Institute of Technology (Caltech), discoverer of 2003 UB313 as well as a host of other planet candidates; and
* Ekers.
This group convened in Paris in late June and early July 2006 to wrap-up the two year process by reaching a unanimous consensus for a proposed new definition of the word "planet."
"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," sais Gingerich. "But by the end of a long day, the miracle had happened: we had reached a unanimous agreement."
Some of the most heated discussions arose over whether "to draw an arbitrary line or use physics," Binzel told The Planetary Society via email from Prague. "We chose physics. 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."
The new definition is, then, based on humanity's modern ability to measure the physical properties of an object to determine its true nature as opposed to the simple definition from past millennia that an object is a planet if it moves against the background of fixed stars. An object is, therefore, now defined as a planet based on its intrinsic physical nature.
The part of "IAU Resolution 5 for GA-XXVI" that describes the new planet definition, reads: 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.
Simply stated, 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 accurately massive enough for its own gravity to pull it into a nearly spherical shape.
Ceres was called a planet when it was first discovered in 1801 orbiting in what is known as the asteroid belt between Mars and Jupiter. Because 19th century astronomers could not resolve the size and shape of Ceres, and because numerous other bodies were discovered in the same region, Ceres soon lost its planetary status. For more than a century, Ceres has been referred to as an asteroid or minor planet.
Recent Hubble Space Telescope (HST) images, however, resolve the size and shape of Ceres and reveal that it is nearly spherical. More technically, Ceres is found to have a shape that is in a state of hydrostatic equilibrium under self-gravity. Hence, Ceres is a planet because it satisfies the proposed IAU definition of planet.
Beyond honing the definition of a planet, the IAU draft resolution also describes a new category of planet -- pluton -- for official use. Pluto (from which the new category gets its name), its companion Charon, and 2003 UB313, a provisional name given to a large object shown by HST to be as large as, or larger than Pluto, are plutons and are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete. In other words, they orbit beyond Neptune. In addition, plutons typically have orbits that are highly tilted with respect to the classical planets, which is technically referred to as a large orbital inclination. These more distant bodies also typically have orbits that are far from being perfectly circular, or in other words, have a large orbital eccentricity.
Historically, Pluto was indeed the ninth planet to be discovered, but now it is also known as the first pluton, with its satellite Charon being the second pluton. The classical planets,meanwhile, can be numbered by their distance from the Sun, and there is no change in their order. Plutons, on the other hand, may due to their high eccentricity change their relative distances from the Sun with time and, as a result, their order.
Although 2003 UB313 has been popularly called Xena, that is not an official IAU name. A decision and announcement of the new name will come at a later time. It will mark the first time the organization has actually named a planet.
What makes all of the distinguishing characteristics for plutons scientifically interesting is that they suggest a different origin from the classical planets.
If the proposed resolution is passed on August 24th, our solar system will grow by 3 planets. The name 2003 UB313 is provisional, and a "real" name will be assigned to this object 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," according to an official press release. Currently a dozen or so "candidate planets" are listed on IAU's "watchlist." But that list is fluid as new objects are found and the physics of the existing candidates becomes better known. By the end of this century, there could well be "up to 200 new planets," Binzel predicted.
The draft "Planet Definition" resolution will be discussed and refined during the General Assembly. Then, it and four other resolutions will be presented for voting August 24th. So far, said Binzel, "early returns are good."
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. The meeting ends August 25.
Source
So, now we have 12. Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, Xena.
That is, if everything goes according to plan on the 24th of August.
they will have to rewrite all their zodiac stuff.
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