Is Pluto a planet after all?

New Scientist ^ | July 27, 2009 | Stephen Battersby

[SunkenCiv]

Three years ago, the IAU decided to draw up the first scientific definition of the term planet. After days of stormy arguments at its general assembly in Prague, the delegates voted for a definition that excluded Pluto, downgrading it to the new category of dwarf planet.

The decision caused outrage among many members of the public who had grown up with nine planets, and among some astronomers who pointed out that only 4 per cent of the IAU's 10,000 members took part in the vote. The governors of Illinois saw the decision as a snub to Pluto's discoverer, Clyde Tombaugh, who was born in the state.

Next week the IAU's general assembly will convene for the first time since Pluto was axed from the list of planets. Surprisingly, IAU chief Karel van der Hucht does not expect anyone to challenge the ruling made in Prague, but Pluto fans can take heart: resistance remains strong.

If Pluto is reinstated, it will probably be thanks to discovery rather than debate. Mark Sykes of the Planetary Science Institute in Tucson, Arizona, believes that revelations within and beyond our solar system over the coming years will make the IAU's controversial definition of a planet untenable (see diagram). "We are in the midst of a conceptual revolution," he says. "We are shaking off the last vestiges of the mythological view of planets as special objects in the sky -- and the idea that there has to be a small number of them because they're special."

[SunkenCiv]

"To Pluto And Far Beyond" By David H. Levy, Parade, January 15, 2006 -- We don't have a dictionary definition yet that includes all the contingencies. In the wake of the new discovery, however, the International Astronomical Union has set up a group to develop a workable definition of planet. For our part, in consultation with several experienced planetary astronomers, Parade offers this definition: A planet is a body large enough that, when it formed, it condensed under its own gravity to be shaped like a sphere. It orbits a star directly and is not a moon of another planet.

[SunkenCiv]

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[ETL]

IAU definition of planet:

The definition of “planet” set in 2006 by the International Astronomical Union (IAU) states that in the Solar System a planet is a celestial body that:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has “cleared the neighbourhood” around its orbit.

A non-satellite body fulfilling only the first two of these criteria is classified as a “dwarf planet”, which is not a type of planet, while a non-satellite body fulfilling only the first criterion is termed a “small solar system body” (SSSB). Initial drafts planned to include dwarf planets as a subcategory of planets, but because this could potentially have led to the addition of several dozens of planets in the Solar System, this draft was eventually dropped. In 2006, it would only have led to the addition of three (Ceres, Eris and Makemake) and the reclassification of one (Pluto). The definition was a controversial one and has drawn both support and criticism from different astronomers, but has remained in use.

According to the definition, there are currently eight planets and five dwarf planets known in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, where smaller bodies cannot be found yet. Extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger.

Reasons for the debate:
Before the discoveries of the early 21st century, astronomers had no real need for a formal definition for planets. With the discovery of Pluto in 1930, astronomers considered the solar system to have nine planets, along with thousands of smaller bodies such as asteroids and comets. Pluto was thought to be larger than Mercury.

In 1978, the discovery of Pluto’s moon Charon radically changed this picture. By measuring Charon’s orbital period, astronomers could accurately calculate Pluto’s mass for the first time, which they found to be much smaller than expected.[1] Pluto’s mass was roughly one twentieth of Mercury’s, making it by far the smallest planet, smaller even than the Earth’s Moon, although it was still over ten times as massive as the largest asteroid, Ceres.

In the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt Objects, or KBOs.[2] Many of these shared some of Pluto’s key orbital characteristics and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, and some astronomers stopped referring to Pluto as a planet.[3] Pluto’s eccentric and inclined orbit, while very unusual for a planet, fits in well with the other KBOs. New York City’s newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth and Space in 2000.[4]

Starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna, and Eris) all comparable to Pluto in terms of size and orbit, it became clear that either they all had to be called planets or Pluto would have to be reclassified. Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other solar systems. In 2006, the matter came to a head with the measurement of the size of 2003 UB313. Eris (as it is now known) turned out to be slightly larger than Pluto, and so was thought to be equally deserving of the status of ‘planet’.[3]

http://en.wikipedia.org/wiki/2006_definition_of_planet

[b4its2late]

I vote it is.

[Lucius Cornelius Sulla]

That definition would include Pluto plus one or more other members of the Oort cloud plus Ceres in the Asteroid Belt, making for 11 or more Solar planets.

[Lancey Howard]

No, Pluto is a dog.

[SunkenCiv]

Thans ETL.

Mike Brown on the planet debate:
http://web.gps.caltech.edu/~mbrown/whatsaplanet/
http://www.nytimes.com/2005/08/23/science/space/23profile.html

dwarf planets:
http://www.gps.caltech.edu/~mbrown/dwarfplanets.html

Eris:
http://web.gps.caltech.edu/~mbrown/planetlila/

Quaoar:
http://www.chadtrujillo.com/quaoar/

Sedna:
http://web.gps.caltech.edu/~mbrown/sedna/

[GregoTX]

Someone is claiming they have obtained Plutos Birth Certificate, and it says Pluto is a planet.

[Zeppo]

It certainly is, that is if the Plutonian delegation to the IAU has any say in the matter...

[gitmo]

No, Pluto is a dog.

That's Goofy.

[dalereed]

“Someone is claiming they have obtained Plutos Birth Certificate, and it says Pluto is a planet.”

Checked with Disney and confirmed he’s a dog!

[SunkenCiv]

I recall how Mike Brown was pushing to have one of the bodies he discovered declared a tenth planet; and I recall also how, after the (indeed, poorly attended, political move to put the imprimatur on a preconceived outcome) vote three years ago, he became like a fanatical anti-smoker who used to be a three packs a day man, or perhaps like a reformed prostitute. Anyway, in a very short time (perhaps even by the end of this year, if anyone is indeed looking) telescope technology will have handed astronomers a number of new discoveries in the outer Solar System, and force the IAU to reexamine their flawed and phony definition.

http://www.mikebrownsplanets.com/2009/08/dont-try-to-blame-it-on-rio.html

http://twitter.com/plutokiller

[SunkenCiv]

At least. :’)

[SunkenCiv]

I wholeheartedly agree.

[SunkenCiv]

[SunkenCiv]

I canine imagine they’d do that.

[shibumi]

[SunkenCiv]

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[Onelifetogive]

...it became clear that either they all had to be called planets or Pluto would have to be reclassified.

Not true. Pluto could be "grandfathered" and then the standards could be tightened. It would always remain an interesting lesson on early astronomy.

[Not now, Not ever!]

No, Pluto is a dog.

Well that may be true, but if he had spoken up like Goofy did, I bet he would still be a planet!