In the light of today's news regarding the 800 mile object of icy debris dubbed "Quaoar", discovered in the Kuiper Belt, astronomers say the case for Pluto as a planet weakens.
Back in elementary school, a planet did not emit any light, had its own atmosphere, and had satellites (one or more moons) orbiting it. Apparently this is now incorrect?
How does Pluto not qualify as a planet? What is the definition of a planet?
A planet is a body that orbits the sun and produces only reflected light. They usually have moons. We have the 'traditional' planets, the five besides earth known to the ancients and visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn.
In this fact lies the answer to your question. The definition was made when only these were known - i.e. before the telescope.
Uranus was discovered in 1781. A ring of minor planets (the asteroid belt) was discovered in 1801 orbiting between Mars and Jupiter. In 1846 Neptune was discovered.
Astronomers noted the mathematical relationships between the distances of the planets from the sun (including the asteroid belt taking the place of a 'planet'). They also noted that the orbits of some planets were slightly irregular, indicating perhaps that unseen bodies were affecting the orbits gravitationally. This led to a search for more planets. In 1930 Pluto was discovered as a result of this search, but now we know that this was more or less accidental.
In 1950 astronomers predicted a cloud of bodies (called the Oort Cloud) which surrounds our solar system far beyond Pluto. These bodies do not circle the sun in a 'ring' orbit, as do the planets and the asteroid belt, but surround it on all sides. The reason for this conclusion is that most comets (of which the Oort cloud is thought to be the source) come in from all sides.
They also predicted another belt of bodies, called the Kuiper Belt, between Pluto and the Oort Cloud, but much closer than the Oort Cloud. This belt was predicted because some comets do follow the path of the ecliptic (the same path as the planets.) So the Kuiper Belt behaves as does the asteroid belt.
The existence of the Kuiper Belt has been proved; but the existence of the Oort Cloud is theoretical.
So the question arose, since Pluto is probably part of the Kuiper Belt (and not necessarily the closest part, but only the most visible) is it correct to call it a planet? We do not call the largest body in the asteroid belt, Ceres (discovered 1801), a planet.
So you see all this has caused astronomers to reconsider the definition. Pluto definitely orbits the sun, and has at least one moon; but is it a planet?
According to a popular theory of the formation of solar systems, particles are drawn from space due to the gravitational effect of the sun. Some may have been thrown off by the sun as it evolved. At any rate, the particles also are drawn to each other by gravity, (accrete) and thus form larger bodies. The asteroid belt is either a planet in the process of accretion, or an exploded planet.
This theory also explains the Kuiper Belt and the Oort Cloud.
Posts: 6257 | Location: British Columbia, Canada | Registered: 06-11-02
The link provided by Minn. states, in part, 'One complicating factor: Believe it or not, astronomers have never fully agreed on the definition of a planet. It's one of those "we know it when we see it" things, though apparently not always.'
The Rose Center for Earth and Space at the American Museum of Natural History in New York City has been telling visitors since Jamuary/February of 2001 that Pluto is no longer considered a planet in the Solar System. Yet two newspaper articles, Space.com (link at top of Astronomy board page), and Discovery,com report something to the effect of "Quaoar" weakening Pluto's case for being a planet.
So is it, or isn't it? Do not inquire of the local astronomers or science teachers/professors, because the issue seems to be undecided at this point. Science, like the information cyperexpressway is highly kinetic and always changing. Time will only reveal, when some of the dust of the theoretical Oort cloud diminishes, whether science will also concur.
babs, I appreciate your well-stated and thorough reply. Your response was more informative than any of the articles that I have perused in the past two days. Thank you!
