Celestial shake-up

Misfit
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Topic 191658

Pluto's 76-year run as a planet may be over

San Diego Union-Tribune editorial

August 5, 2006

When the General Assembly of the International Astronomers Union meets in Prague on Aug. 14, the world will be watching, or at least the celestially intrigued will be watching. That's because the astronomers will debate the status of Pluto, the large icy rock with the wildly out-of-kilter orbit that was dubiously declared to be the ninth planet in 1930.

The matter is of no small interest locally, because the debate over Pluto's status is linked to last year's discovery at the Palomar Observatory of another large icy rock, nicknamed Xena, that certainly would qualify as the 10th planet based on the standards used to evaluate Pluto. The problem, it turns out, is that so would 13 more “transneptunian� icy rocks.

Those with parochial pride in Palomar suggest the 13 others could be ignored based on an arbitrary size cutoff, giving Xena planetary status and making the observatory a crucial historical footnote. But scientific classifications should be logical and coherent, not an exercise in boosterism.

So the question for astronomers boils down to this: Use a narrow definition and go back to the pre-1930 view that the solar system has eight planets, or relax standards and decree there are 23. While the latter may appeal to Michael Jordan and his legions of fans (non-sports fans: Jordan famously wore No. 23), our view is it would be a tremendous hardship for fourth-graders assigned to learn the names of 23 planets.

This rationale may not be substantive, but it has (some) logic and coherence to it. So here's for tough standards and Pluto's retreat from the solar system stage. Eight is enough.

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DanNeely
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Celestial shake-up

They've tried to do this several times before, so far without success.

{sung}Tradition{/sung}

Odysseus
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RE: They've tried to do

Message 43556 in response to message 43555

Quote:
They've tried to do this several times before, so far without success.
{sung}Tradition{/sung}


The number of conventionally recognized planets was reduced at least once, sometime in the XIX century I believe: the first few objects to be discovered between Mars and Jupiter (Ceres, Pallas, et al.) were classified as planets at the time, but later they were relegated to their present status of asteroids or “minor planets�.

Misfit
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A plan to expand the solar

Misfit
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Pluto and other brave new

Pluto and other brave new worlds

By Dava Sobel; author of “Longitude,� “Galileo's Daughter� and “The Planets,� served as the sole non-scientist on the Planet Definition Committee.

August 17, 2006

Pluto has become the butt of jokes lately, replacing Uranus as the solar system's laughingstock – and all because scientists find themselves forced, at last, to come to terms with the meaning of the word “planet.�

Tacit definitions have existed since ancient times, when planetai, meaning wanderers, applied to seven moving lights in the sky: the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. But telescopes have revealed more objects in the solar system than were dreamed of in ancient philosophy, and new discoveries demand strict, useful terminology that will help astronomers categorize a host of newfound worlds.

Pluto, discovered in 1930, was hailed as a planet before its true nature came to light. In time, Pluto proved to be far smaller than any of the other planets, and very unlike them in the way it orbits the sun at an exaggerated tilt.

Even so, there seemed no need to coin a new designation for Pluto, and it held on to its planet classification. But in 1992 astronomers made the first of what now amount to several hundred sightings of other solar system bodies at the distance of Pluto and beyond. Suddenly there was reason to reclassify Pluto as a member of this new society, which quickly became known as “trans-Neptunian objects� or “Kuiper Belt objects,� in honor of Gerard Kuiper (1905-1973), who had predicted a vast zone of small bodies in the environs of Pluto.

As astronomers began to debate the issue, it spilled over into popular awareness and ignited considerable heat, for the planets are held in common, in awe, by all humankind. What might have constituted a purely scientific discussion, akin to deciding whether a particular tree was coniferous or deciduous, instead became public discourse.

On one side were major-planet purists who felt that bodies smaller than 1,500 miles in diameter (a size calculated to eliminate Pluto) should be dropped from the planet list. On the other side were Plutophiles who objected to arbitrary size discrimination. One Pluto specialist asked pointedly, “Is a dachshund not a dog?�

The lack of consensus on the “planet� definition struck people both within and outside the planetary science community as ludicrous, though several everyday terms we all think we understand are similarly vague. “Life,� for instance, poses semantic problems for biologists – as well as for exobiologists, who hope to identify it if and when they find it on Mars or Europa.

In 2005, after a Kuiper Belt object tentatively named “Xena� turned out to be larger than Pluto, the question changed from “Should Pluto continue to be called the ninth planet?� to “Are there 10 planets in all?� Also in 2005, Ceres, a small body discovered more than 200 years ago between Mars and Jupiter and long dismissed as a “minor planet� or “asteroid,� was observed by the Hubble telescope to be more or less round. Scientifically, a roundish object carries more weight than a potato-shaped one, because roundness signifies the greater mass required to pull itself into a ball, or “hydrostatic equilibrium.� Round Ceres raised the question, “Are there perhaps 11 planets?�

Not for the first time, but with new urgency in 2006, the International Astronomical Union, or IAU, impaneled a committee to define both the word “planet� and the status of Pluto.

Our committee – seven in number, like the planets of old – met at the Paris Observatory in late June and reached a unanimous agreement. In short: A planet is a body in orbit around a star (as opposed to orbiting another planet) and big enough for gravity to make it round.

The full text of our proposed definition is being released this week, to be discussed by astronomers from around the world, now in Prague at the IAU General Assembly, and voted upon next week. If approved, our resolution will not only leave Pluto in place but will also add “Xena� (2003 UB313) and Ceres to the current census of planets – with room for additions as future discoveries warrant.

What's more, Pluto will lend its name to a newly defined category of planets – the “plutons� – which differ from the other planets by virtue of their highly inclined, elongated orbits, which take more than two centuries to complete and that suggest a different origin. As the prototype of this class, Pluto may still attract funny remarks, but it will have gained new significance.

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Bird-Dog
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edit

edit

debugas
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one more interesting aspect

Message 43560 in response to message 43559

one more interesting aspect of all this is how one makes distinction between a planet and a satellite ? It is well known that moon and earth rotate around the common masses' center (which is for this pair is inside our planet) now imagine if the moon's mass was bigger (close to earth's mass) which of the two would be called planet and which would be called satellite then?

Isn't it the case with Pluto and its satellite Charon (which is half Pluto's diameter and an eighth of Pluto's mass)?

Quote:

Pluto's satellite Charon has a diameter of approximately 1200 km (700 mi.) and is the largest satellite in the solar system in proportion to the size of its planet. Charon orbits Pluto at a distance of 20,000 km (12,000 mi.) with exactly the same period, keeping the same face directed toward Pluto, in what is called locked synchronous rotation. As a result, Pluto and Charon are often referred to as a double planet system.
Odysseus
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RE: one more interesting

Message 43561 in response to message 43560

Quote:

one more interesting aspect of all this is how one makes distinction between a planet and a satellite ? It is well known that moon and earth rotate around the common masses' center (which is for this pair is inside our planet) now imagine if the moon's mass was bigger (close to earth's mass) which of the two would be called planet and which would be called satellite then?

Isn't it the case with Pluto and its satellite Charon (which is half Pluto's diameter and an eighth of Pluto's mass)?


Yes; you seem to have the answer already. The criterion they’re using to distinguish binary planets from planet-with-satellite systems is the location of the barycentre about which both objects appear to revolve. If it’s outside the larger body, as in the Pluto–Charon system, both are considered planets, but if it’s inside, as in the Earth–Moon system, the smaller body is considered a satellite.

Not that my opinion counts, but I don’t see this definition to be any less arbitrary than the others that have been suggested. For instance, it means that the denser the primary, other things being equal, the more likely that the secondary will be deemed a planet! OTOH I suppose it makes some practical sense, in terms of detecting invisible objects by the size of the ‘wobble’ they induce in the motion of a visible partner: a “satellite� won’t ever appear to displace its primary by more than the latter’s radius, while a “planet� will.

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