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."

[Wicked Pilot]

I wonder if it was nicknamed Xena in a nod to the old 'Planet X'.

[Pick]

According to Wikipedia (I know, I know...)

The team refers to the object informally by the nickname Xena, after the television series Xena: Warrior Princess. They had saved this name for the first body they found that was larger than Pluto. The X of Xena is a reference to Percival Lowell's Planet X; they had wanted more female names for astronomical bodies, and this was the closest they could come to a mythological female name beginning with X.[citation needed] On the other hand, the team has also claimed that they chose the name because "We have always wanted to name something Xena",[10] seemingly implying that the name was chosen prior to the association with Planet X. The name Xena has been used by news media such as CNN, but it has not been proposed to the IAU.

[Steel]

Nope, i think it was actually really because the scientists who discovered it liked the show.

I really hope it becomes a planet, just imagine the fun in science classes!

[GrandMasterTerwynn]

Nope, i think it was actually really because the scientists who discovered it liked the show.

I really hope it becomes a planet, just imagine the fun in science classes!

If it becomes a planet, then every other large KBO will be eligible for full planet status as well. Instead of eight planets and a misidentified straggler, you'd have potentially dozens of planets. That won't be so fun . . . unless your idea of fun is making children's heads explode.

[Lord Zentei]

I've said it before, I'll say it again: we have eight true planets and one "honorary planet" representing the Kuiper belt. Problem solved.

[Alyeska]

The least arbitrary description of what makes a planet is something I read in a National Geopgraphic a while back.

A planet is a body that has sufficent mass to collect itself into a spherical shape.

That would clearly seperate planets from simple rocks in space. Of course it would increase the number of planets in the solar system by a hell of a lot.

[Spin Echo]

I have a hard time supporting that Pluto continue being referred to as a planet simply for historical reasons. I think some sort of Major and Minor Planet categories would better describe the solar system. You have Major planets be the Mercury through Neptune and Minor planets be all the odd balls that don't seem to be just space junk but not quite a full planet either.

Depending on how they have decide the criteria for planets, we could be acquiring a few other interesting planets as well. We have Sedna, that's nearly the size of Pluto but whose orbit swings between 76AU and 900AU, well outside the heliopause. There's also Buffy, half the size of Pluto, but has an almost circular orbit.

[Lord Zentei]

Of course in time we may have to face the prickly problem of distinguishing where the "minor" planets end and the "major" planets begin, if the mass-frequency graph is mostly continious. In that case, the distinction would become arbitrary anyway, and the "historic" approach might well be used if not by definition, then in practice.

[Mad]

Well, at least we know why aliens haven't contacted us yet. We've been giving them bad directions all along! "Yeah, we're the 3rd planet from the star with 9 planets." Moron humans can't even give correct directions to their own planet.

[Spin Echo]

Of course in time we may have to face the prickly problem of distinguishing where the "minor" planets end and the "major" planets begin, if the mass-frequency graph is mostly continious. In that case, the distinction would become arbitrary anyway, and the "historic" approach might well be used if not by definition, then in practice.

Yeah, I doubt we'll ever find the perfect place to draw the line between what is and isn't a planet. I'm not sure whether finding out more about other star systems would make the job easier or harder.

[wilfulton]

Well, at least we know why aliens haven't contacted us yet. We've been giving them bad directions all along! "Yeah, we're the 3rd planet from the star with 9 planets." Moron humans can't even give correct directions to their own planet.

Wait...I thought we were the center of the universe!

ROFL

[Admiral Valdemar]

You people need to buck up your reading comprehension skills. Since when did 9 billion miles equal 15 kilometres?

[Lord Zentei]

You people need to buck up your reading comprehension skills. Since when did 9 billion miles equal 15 kilometres?

That typo was toasted in a Testing thread.

[Admiral Valdemar]

I never go to Spamworld. This is just another in a line of bad CNN reporting concerning space. If it's not Columbia doing 18 times c or Japan planting a flag on the moon for China, it's this.

[Alyeska]

Of course in time we may have to face the prickly problem of distinguishing where the "minor" planets end and the "major" planets begin, if the mass-frequency graph is mostly continious. In that case, the distinction would become arbitrary anyway, and the "historic" approach might well be used if not by definition, then in practice.

Yeah, I doubt we'll ever find the perfect place to draw the line between what is and isn't a planet. I'm not sure whether finding out more about other star systems would make the job easier or harder.

Except I just posted a perfectly logical distinction between planets and other bodies. Enough mass and a spherical shape.

[General Trelane (Retired)]

Except I just posted a perfectly logical distinction between planets and other bodies. Enough mass and a spherical shape.

I've usually seen that criterion coupled with a requirement to orbit the sun directly. That would avoid reclassifying Terra and Luna as a dual-planet system.

[Grandmaster Jogurt]

Except I just posted a perfectly logical distinction between planets and other bodies. Enough mass and a spherical shape.

I've usually seen that criterion coupled with a requirement to orbit the sun directly. That would avoid reclassifying Terra and Luna as a dual-planet system.

From the definition I remember, a double planet is a two-body system wherin the barycenter is within neither object. Since the barycenter of the Earth-Luna system is within Earth, it wouldn't be considered a double planet even under Alyeska's system.

[Grandmaster Jogurt]

Ghetto Edit: Actually, since Luna has a greater attraction to Sol than Earth, General Trelane's system could actually put Earth-Luna into the double-planet category, rather than the other way around!

[Alyeska]

Hell, the Moon can be considered a planet for all I care. What really makes it different then any other body out there? The only difference is that it orbits Earth rather then orbiting the sun.

Making a distinction purely on orbits is about as bad as the whole arbitrary size issue.

[Lord Zentei]

Hell, the Moon can be considered a planet for all I care. What really makes it different then any other body out there? The only difference is that it orbits Earth rather then orbiting the sun.

Making a distinction purely on orbits is about as bad as the whole arbitrary size issue.

Well, the Moon used to be classified a planet prior to the Heliocentric system winning out. Hell, so was the Sun.

As for the spherical shape argument: that might very well work (and is rather less arbitrary than a mass based definition), though you would have borderline cases there as well, of course.

[Spin Echo]

Yeah, I doubt we'll ever find the perfect place to draw the line between what is and isn't a planet. I'm not sure whether finding out more about other star systems would make the job easier or harder.

Except I just posted a perfectly logical distinction between planets and other bodies. Enough mass and a spherical shape.

So what qualifies as enough mass? Vesta is located in the main asteroid belt with a diameter of 540 km. It's nearly spherical and its surface shows evidence of past volcanic activity. The asteroid Ida has a tiny spherical moon Dactyl, only 1.5km in diameter. If an object Dactyl's size were orbiting the sun instead of an asteroid, is it large enough to count as a planet?

Also, deciding how spherical is spherical enough turns out to be a tricky question. Spherical objects tend not to be actually spherical but oblate spheroids. Icy bodies tend to form spheres more easily, so will become spherical at much smaller sizes than rocky bodies. Ceres, a 1000km diameter asteroid in the main belt is spherical while KBO 2003 EL61 is more massive but an ellipsoid due the fact it completes a rotation every four hours. Which one wins out there, mass or how spherical it is?

If there was one quick, easy answer, the astronomers would have settled upon it by now.

[kheegster]

Beeb

Planets plan boosts tally to 12

By Paul Rincon
Science reporter, BBC News


The number of planets around the Sun could rise from nine to 12 - with more on the way - if experts approve a radical new vision of our Solar System.

An endorsement by astronomers meeting in Prague would require school and university textbooks to be rewritten.

The proposal recognises eight classical planets, three planets belonging to a new category called "plutons" and the largest asteroid Ceres.

Pluto remains a planet, but becomes the basis for the new pluton category.

The plan has been drawn up by the International Astronomical Union (IAU) with the aim of settling the question of what does and does not count as a planet.

Some 2,500 astronomers gathered at the IAU General Assembly in Prague will vote on the plan next Thursday.

New era

"For the first time in more than 75 years, we will be able to discover new planets in our Solar System. This is a fascinating prospect," said Richard Binzel, a member of the IAU planet definition committee which put together the proposal.

Dr Andrew Coates of the Mullard Space Science Laboratory in Dorking said he thought the plan was "a good compromise".


He explained: "It keeps the idea of eight classical planets, while Pluto is allowed to retain its status. But other objects are allowed in, which I suppose makes life more interesting."

Experts have been divided over whether Pluto - further away and considerably smaller than the eight other planets in our Solar System - deserves the title.

Since the early 1990s, astronomers have found several other objects of comparable size to Pluto in an outer region of the Solar System called the Kuiper Belt.

Some astronomers believe Pluto belongs with this population of "icy dwarfs", not with the objects we call planets.

Allowances could once be made for Pluto on account of its size. At just 2,360km (1,467 miles) across, Pluto is significantly smaller than the other planets. But until recently, it was still the biggest known object in the Kuiper Belt.

That changed with the discovery of 2003 UB313 by Professor Mike Brown and colleagues at the California Institute of Technology (Caltech). After being measured with the Hubble Space Telescope, it was shown to be some 3,000km (1,864 miles) in diameter, making it larger than the ninth planet.

Kicked upstairs?

The IAU draft resolution recognises eight "classical" planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - three "plutons" - Pluto, Charon and UB313 - and the asteroid Ceres.

Charon is currently described as a moon of Pluto, but because of its size some experts consider it a twin planet.

Professor Owen Gingerich, who chairs the IAU planet definition committee, said: "In a sense we're demoting Pluto by taking it off the list of classical planets. But we're promoting it by making it the prototype of this new category of plutons."

Dr Coates commented: "Something had to be done about the definition. It does change the textbooks somewhat, but it also demonstrates that this is a vibrant area of research.

"The surprise is Ceres, because most people thought of it as an asteroid."

Ceres is the largest object in the asteroid belt between Mars and Jupiter, and like a planet is spherical in shape.

Seeking endorsement

The basis for this re-evaluation is a new scientific definition of a planet which uses gravity as the determining factor.

According to this definition, two conditions must be satisfied for an object to qualify as a planet:

* The object must be in orbit around a star, but must not itself be a star
* It must have enough mass for the body's own gravity to pull it into a nearly spherical shape

On whether he was confident the resolution would be passed, Professor Gingerich told the BBC News website: "It will be a very awkward situation if they don't.

"On Sunday afternoon, we proposed it out of the blue for the division chairmen and they voted unanimously that they would be prepared to back it. That's a good cross-section of astronomers.

"I'm sure it will be controversial to those with a stake in some other solution, but I hope we will get an overwhelming endorsement."

More objects are likely to be announced as planets in the future. 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.

These include the distant objects Sedna, Orcus, Quaoar and 2003 EL61 and the asteroids Vesta, Pallas and Hygiea.

The IAU spent two years debating the matter among its membership. A seven-member committee was set up to consider the findings and produce a draft proposal.

The body has been responsible for the naming of planets and moons since 1919.

[Spin Echo]

There's a bit more information in this article here

Apparently anything with a diameter over 500 miles and a roughly spherical shape is now considered a planet. I think this is just going to cause trouble down the road. The diameter should have been bigger. I agree with the chap here:

Michael Brown, a planetary astronomer at the California Institute of Technology in Pasadena and the discoverer of Xena, doesn't like the revised definition of a planet.

There are already 53 round objects larger than 500 miles across that would qualify as plutons, he said. The number is growing rapidly as he and other scientists detect more objects in the Kuiper Belt.

"Instead of nine or 12, there will be a couple hundred 'planets' in the next few years," Brown said. "The simplest solution is the one that makes most people cringe: Admit that we made a mistake in 1930 by calling Pluto a planet."

[Spin Echo]

Apparently anything with a diameter over 500 miles and a roughly spherical shape is now considered a planet. I think this is just going to cause trouble down the road. The diameter should have been bigger. I agree with the chap here:

Ghetto edit: And orbiting a star is on the list of criteria, obviously.