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Re: Tenth Planet Found! Burn all Your Old Astronomy Books!
| quote: | Originally posted by Renegade
Which begs the question, what should the definition of a planet be? Do we have eight planets, ten planets, or is there some definition by which Pluto could remain included as a planet while Xena doesn't? |
Good question. Wikipedia doesn't even seem to have a definitive answer.
http://en.wikipedia.org/wiki/Planet
| quote: | | A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. Based on historical consensus, the International Astronomical Union (IAU) lists nine planets in our solar system. Since the term "planet" has no precise scientific definition, however, many astronomers contest that figure. Some say it should be lowered to eight by removing Pluto from the list, whilst others claim it should be raised to ten, fifteen, twenty, or even higher. |
| quote: | Interstellar planets
Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a likely hypothesis based on computer simulations of the origin and evolution of planetary systems, which often include the ejection of bodies of significant mass.
Such objects are not formally called planets, however, since the IAU has not defined the term "planet". |
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Definition and classification of planets
Main article: Definition of planet
Much like "continent", "planet" is a word without a precise definition, with history and culture playing as much of a role as geology and astrophysics. Recent definitions have been vague and imprecise; The American Heritage Dictionary, for instance, formerly defined a planet as:
A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the Sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.'
However, for some time that definition has been viewed by many as inadequate. The eight largest planets (which are also the eight nearest to the Sun) are universally recognised as such, and for this reason are often universally referred to as "major planets", but there is controversy over Pluto and other smaller objects. It is also worthwhile to remember that, before the invention of the telescope, the classical planets were seven in number: Mercury through Saturn, minus the Earth, plus the Sun and Moon (in certain cultures this is the reason for a seven day week).
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Suggested wide definitions
Since the discoveries of many of the objects in the Kuiper belt and around other stars, there has been a concerted push amongst scientists to come up with a precise definition of what constitutes a planet. In 1999, the IAU set up a working group to develop a scientifically plausible recommendation, but as of August, 2005 they had not reached a conclusion. After the discovery of 2003 UB313 (informally called "Xena"), a member of the committee, Alan Stern, has said that the group wanted "to get something done, pronto". He also informed journalists that a "consensus" in the group was moving towards the following definition:
A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior.
Note that this definition also covers disputes at the upper end of a planet's size, which provides the extra benefit of forming a barrier between planets and brown dwarfs. Many consider this definition the best option as it sets up divisions based on physical characteristics rather than an arbitrary size limit. It is also somewhat universal in its application where other definitions have been crafted mainly to sort our own solar system into simple categories (such as placing the size limit as just under Mars, Mercury or Pluto). Depending how it is interpreted, objects counted as planets under such a new system would include some or all of the objects listed above, with potentially many more yet to be found.
Gibor Basri, head of astronomy at the University of Berkeley, has suggested a similar definition and has also proposed the terms "fusor" (any object that achieves fusion in its core) and "planemo" (an object that is round from self-gravity but not a fusor) to help improve the astronomical nomenclature. Under Basri's definition:
A planet is a planemo orbiting a fusor
These definitions have the advantage of creating a group including larger moons (which share many characteristics with the smaller planets) and also covering large free-roaming objects, which some astronomers think should be included in the definition of a planet. Basri has also suggested 'liberal use of adjectives' such as "major", "beltway", "dwarf", "giant", "super" and "historical".[4] Others have suggested categories of planet/planemo based on composition such as "rock" (composed mainly of silicate), "gas" (composed mainly of hydrogen and helium), and "ice" (composed mainly of oxygen and carbon).
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Suggested narrow definitions
There are alternate suggestions which would instead reduce the number of planets in the system. Upon his discovery of Sedna, Mike Brown of Caltech suggested a definition which would exclude both Sedna and Pluto from being classified as planets, proposing the following:
A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit [5]
This definition generally plays down the importance of size, but instead focuses on the formation of the proposed planet. Under this definition, no Kuiper Belt objects (including Pluto) would be considered planets.
Brown's wish to "demote" Pluto prompted many to criticize him for setting out to create a purely scientific definition for a term which had an existing popular (albeit 'flawed') application. Upon his discovery of 2003 UB313, Brown indicated he had become a convert to this way of thinking, and proposed that whatever definition of planet be adopted, it should include both Pluto and any Kuiper Belt object found to be larger than Pluto. [6]
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Further classification
Astronomers distinguish between minor planets, such as asteroids, comets, and trans-Neptunian objects; and major (or true) planets.
Planets within Earth's solar system can be divided into categories according to composition.
* Terrestrial or rocky: Planets that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth, Mars
* Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ice giants, are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.
* Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Triton).
Many consider the Earth and its Moon to be a double planet, for several reasons:
* The Moon, as measured by its diameter, is 1.5 times larger than Pluto.
* The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon by a factor of approx. 2.2. (This is not a unique situation in the solar system. The Sun's gravity is also stronger than the primary's on Jupiter's moon S/2003 J 2; Uranus' moon S/2001 U 2; Neptune's moons S/2002 N 4 and Psamathe; and several asteroid moons. However, Luna is the sole case of this phenomenon affecting an object of planetary mass.)
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And a bit on stars...
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Star
From Wikipedia, the free encyclopedia
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For other uses of this term, see Star (disambiguation).
The Pleiades star cluster
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The Pleiades star cluster
A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Stellar astronomy is the study of stars. |
Whatever they ultimately are, their photographs always leave me awestruck.
Furthermore, Earth is really fucking small, which always makes me think it is all the more special that such a relatively tiny interstellar body can support such a diverse ocean of life.
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Tenth Planet Found! Burn all Your Old Astronomy Books!
Love it...there must be a desktop version somewhere... 
