Astronomers Debate Status of Pluto and Xena

[LadyTevar]

Pluto on the chopping block
Astronomers meet to define 'planet'

PRAGUE, Czech Republic (AP) -- Nearly 2,500 astronomers from 75 countries gathered in Prague Monday to come up with a universal definition of what qualifies as a planet and possibly decide whether Pluto should keep its planet status.

For decades, the solar system has consisted of nine planets, even as scientists debated whether Pluto really belonged. Then the recent discovery of an object larger and farther away than Pluto threatened to throw this slice of the cosmos into chaos.

Among the possibilities at the 12-day meeting of the International Astronomical Union in the Czech Republic capital: Subtract Pluto or christen one more planet, and possibly dozens more.

But the decision won't be an easy one. Scientists attending the conference are split over whether Pluto should be excluded from the list of planets, Pavel Suchan of the meeting's local organization committee said.

"So far it looks like a stalemate," Suchan said. "One half wants Pluto to remain a planet, the other half says Pluto is not worth being called a planet."

Participants hope to set scientific criteria for what qualifies as a planet. Should planets be grouped by location, size or another marker? If planets are defined by their size, should they be bigger than Pluto or another arbitrary size? The latter could expand the solar system to 23, 39 or even 53 planets.

The debate intensified last summer when astronomer Michael Brown of the California Institute of Technology announced the discovery of a celestial object larger than Pluto. Like Pluto, it is a member of the Kuiper Belt, a mysterious disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. Brown nicknamed his find "Xena."

The Hubble Space Telescope measured the bright, rocky object officially known as 2003 UB313, at about 1,490 miles (2,300 kilometers) in diameter, roughly 70 miles (112 kilometers) longer than Pluto. At 9 billion miles (15 kilometers) from the sun, it is the farthest known object in the solar system.

The discovery stoked the planet debate that had been simmering since Pluto was spotted in 1930.

For years, Pluto's inclusion in the solar system has been controversial. Astronomers thought it was the same size as Earth, but later found it was smaller than Earth's moon. Pluto is also odd in other ways: With its elongated orbit and funky orbital plane, it acts more like other Kuiper Belt objects than traditional planets.

Some argue that if Pluto kept its crown, Xena should be the 10th planet by default -- it is, after all, bigger. Purists maintain that there are only eight traditional planets, and insist Pluto and Xena are poseurs.

Still others suggest a compromise that would divide planets into categories based on composition, similar to the way stars and galaxies are classified. Jupiter could be labeled a "gas giant planet," while Pluto and Xena could be "ice dwarf planets."

A decision on whether Pluto should be excluded or if "Xena" should be included on the list of planets will not be known before the end of the conference, Suchan said.

"We of course need the definition of a planet first."

[Mayabird]

Personally, I say it oughta be eight planets and then a bunch of KBOs. Philosophically, it doesn't really matter, since it's all a bunch of silly name-games and no matter what gets decided, people will just shrug and not think anything of it in a few decades. Honestly, I'm hoping for fist fights.

[DesertFly]

Personally, I say it oughta be eight planets and then a bunch of KBOs. Philosophically, it doesn't really matter, since it's all a bunch of silly name-games and no matter what gets decided, people will just shrug and not think anything of it in a few decades. Honestly, I'm hoping for fist fights.

No no no, fist fights aren't enough. There need to be gun duels at 50 paces at dawn as well. And personally, I think that Pluto shouldn't be a planet, and neither should Xena. In my mind, a planet is something that's big enough to have an atmosphere, mitigating circumstances aside, and only orbits a star, not another object. That makes the inner 8 planets (Mercury would be big enough, it's just so close to the sun that its atmosphere can't form), and excludes moons and little asteroids that pretend to be planets.

[Destructionator XV]

Beware, I will nitpick here.

and only orbits a star, not another object.

Problem with that from a strict sense, all objects orbit other things too, since gravity works both ways.

For example, the moon orbits earth, which is obvious, but the moon's gravity also moves the earth around as it goes around, causing the earth to move in a little sub orbit. If the mass of the earth and moon were closer, this would be even more pronounced, causing IIRC (it has been a while since I checked this, and I don't have the brain power to write a simulation or do the math at this time of night to confirm it) both objects to orbit an empty point between them. Binary stars should provide observational evidence of this; I'll look one up tomorrow.

If you say the orbit should be an ellipse with the sun at one of the focal points, then it gets better, as the main planets follow that close enough (the perturbations in their orbits caused by their moons are negligible in comparison), and moons would not, since while they do orbit the sun, their path doesn't look like an ellipse anymore (it looks like a flower pattern, actually). I don't think this is a perfect explanation either, as it would include comets, which obviously aren't planets, but that is where your massive enough for atmosphere requirement could step in, but I think that is imperfect too, since moons can have atmospheres (Io, Europa, Callisto, Enceladus, Titan, Ganymede, and Triton all do).

I'll think about it more once I get my sleep; I am barely awake right now.

[DesertFly]

Destructionator, I see what you're saying, and you're right. But I wouldn't neccessarily call a planet "orbiting" a moon. Sure, its orbit is affected by the moon, but the point they both orbit around is still inside the planet.

However, taking your revised definition as a starting point, we can change it to:

A planet is a stellar object with enough mass to hold an atmosphere, mitigating circumstances aside, which primarily orbits a star.

Good enough for you?

[Batman]

Bah. I say keep Pluto and make Xena a planet if only for the wackiness of having a planet called 'Xena' (of course the wusses will probably rename it either way. Spoilsports).
Make it 'primarily orbits a star (or several) and isn't smaller than Pluto (or whatever arbitrary marker you want to set)'.
Or how about 'isn't smaller than x and/or has a material composition of y, and primarily orbits star(s).
Yes, I want to keep Pluto. How'd you know?

[Destructionator XV]

Good enough for you?

Yeah, I think that works. Of course, we'll have to wait for real scientists to make a definition to know the fate of Pluto.

[LadyTevar]

Proposal would increase planets from 9 to 12
'Big Bang' expansion would keep Pluto in the mix

PRAGUE, Czech Republic (AP) -- Our solar system would have 12 planets instead of nine under a proposed "Big Bang" expansion by leading astronomers, changing what billions of schoolchildren are taught about their corner of the cosmos.

Much-maligned Pluto would remain a planet -- and its largest moon plus two other heavenly bodies would join Earth's neighborhood -- under a draft resolution to be formally presented Wednesday to the International Astronomical Union, the arbiter of what is and is not a planet.

"Yes, Virginia, Pluto is a planet," quipped Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology.

The proposal could change, however: Binzel and the other nearly 2,500 astronomers from 75 nations meeting in Prague to hammer out a universal definition of a planet will hold two brainstorming sessions before they vote on the resolution next week. But the draft comes from the IAU's executive committee, which only submits recommendations likely to get two-thirds approval from the group.

Besides reaffirming the status of puny Pluto -- whose detractors insist should not be a planet at all -- the new lineup would include 2003 UB313, the farthest-known object in the solar system and nicknamed Xena; Pluto's largest moon, Charon; and the asteroid Ceres, which was a planet in the 1800s before it was demoted.

The panel also proposed a new category of planets called "plutons," referring to Pluto-like objects that reside in the Kuiper Belt, a mysterious, disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. Pluto itself and two of the potential newcomers -- Charon and 2003 UB313 -- would be plutons.

Astronomers also were being asked to get rid of the term "minor planets," which long has been used to collectively describe asteroids, comets and other non-planetary objects. Instead, those would become collectively known as "small solar system bodies."

If the resolution is approved, the 12 planets in our solar system listed in order of their proximity to the sun would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and the provisionally named 2003 UB313. Its discoverer, Michael Brown of the California Institute of Technology, nicknamed it Xena after the warrior princess of TV fame, but it likely would be rechristened something else later, the panel said.

The galactic shift would force publishers to update encyclopedias and school textbooks, and elementary school teachers to rejigger the planet mobiles hanging from classroom ceilings. Far outside the realm of science, astrologers accustomed to making predictions based on the classic nine might have to tweak their formulas.

Even if the list of planets is officially lengthened when astronomers vote on Aug. 24, it is not likely to stay that way for long: The IAU has a "watchlist" of at least a dozen other potential candidates that could become planets once more is known about their sizes and orbits.

"The solar system is a middle-aged star, and like all middle-aged things, its waistline is expanding," said Jack Horkheimer, director of the Miami Space Transit Planetarium in the United States and host of Public Broadcasting's Stargazer television show.

Opponents of Pluto, which was named a planet in 1930, still might spoil for a fight. Earth's moon is larger; so is 2003 UB313 (Xena), about 70 miles (113 kilometers) wider.

But the IAU said Pluto meets its proposed new definition of a planet: any round object larger than 800 kilometers (nearly 500 miles) in diameter that orbits the sun and has a mass roughly one-12,000th that of Earth. Moons and asteroids will make the grade if they meet those basic tests.

Roundness is key, experts said, because it indicates an object has enough self-gravity to pull itself into a spherical shape. Yet Earth's moon would not qualify because the two bodies' common center of gravity lies below the surface of the Earth.

"There are as many opinions about Pluto as there are astronomers," Binzel said. "But Pluto has gravity on its side. By the physics of our proposed definition, Pluto makes it by a long shot."

IAU President Ronald D. Ekers said the draft definition, two years in the making, was an attempt to reach a cosmic consensus and end decades of quarreling. "We don't want an American version, a European version and a Japanese version" of what constitutes a planet, he said.

Neil deGrasse Tyson, director of the Hayden Planetarium at New York's American Museum of Natural History -- miscast as a "Pluto-hater," he contends, merely because Pluto was excluded from a planetarium solar system exhibit -- said the new guidelines would clear up the fuzzier aspects of the Milky Way.

"For the first time since ancient Greece, we have an unambiguous definition," he said. "Now, when an object is debated as a possible planet, the answer can be swift and clear."

[Scottish Ninja]

Calling Charon a planet just seems utterly ridiculous to me. If we're going to do that, why don't we call our moon a planet too? Titan? Europa? They seem to qualify as planets too, then.

[LadyTevar]

It's explained in the article itself why the moons are going to be considered as planets, Ninja.

Yet Earth's moon would not qualify because the two bodies' common center of gravity lies below the surface of the Earth.

In other worlds, the central point the Moon (as well as Titan and Europa) rotates around is within the planetary crust, unlike Pluto/Charon which rotate around a point located in the space between them.

[frigidmagi]

Maybe I'm an asshole but I'm a firm believer in the idea that if you found it, you get to name it. So let's call the damn thing Xena and tell the panel to get a life.

[Scottish Ninja]

Ahh. I didn't read it carefully the first time around. So Charon and Pluto would be considered much more like binary planets than a planet and a moon.

That makes more sense, sorry for the confusion.

[Batman]

Maybe I'm an asshole but I'm a firm believer in the idea that if you found it, you get to name it. So let's call the damn thing Xena and tell the panel to get a life.

Seconded, thirded and fourthed. And if we find another one let's name it MacGyver.

[Destructionator XV]

Steven Colbert, on his show, just said something rather humorous: all you need to know about the planets is this little sentence:

My Very Educated Mother Just Said Uh-oh! No Pluto!

Somewhat unrelated, but I lol'd.

edit for those who need explaination:

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

It is a trick we use at least here in the States to remember the names of the planets as young children.

My Very Excellent Mother Just Sent Us Nine Pizzas

Is one variation on the original.

[Batman]

That joke apparently needs context.

[Josh]

Mnemonic device, Bats, using the first letter of each planet's name.

Sentimentally, I'd rather keep Pluto a planet. However, the point does stand that we need to figure the cutoff before we go deeming every big chunk of rock a planet.

Should I break out the obligatory Brunching Shuttlecocks link again?

[Scottish Ninja]

You may have to. I don't think I've heard of it.

[Josh]

http://www.brunching.com/conversationpluto.html
http://www.brunching.com/morepluto.html

Pluto does not take well to his status being questioned.

[Ali Sama]

size comparison.
http://en.wikipedia.org/wiki/Image:2006-16-d-print.jpg

[Hotfoot]

I personally like the idea of having the planets split into three groups of four. There's a nice symmetry to it, and each group really is distinct from the others.

The fact that you've got Rock, Gas, and Ice goes well with Space Empires IV planet types has nothing to do with it.