Only 8 Planets in the Solar System...

[LadyTevar]

Pluto gets the boot
Pluto no longer a planet, say astronomers

PRAGUE, Czech Republic (AP) -- Leading astronomers declared Thursday that Pluto is no longer a planet under historic new guidelines that downsize the solar system from nine planets to eight.

After a tumultuous week of clashing over the essence of the cosmos, the International Astronomical Union stripped Pluto of the planetary status it has held since its discovery in 1930. The new definition of what is -- and isn't -- a planet fills a centuries-old black hole for scientists who have labored since Copernicus without one.

Although astronomers applauded after the vote, Jocelyn Bell Burnell -- a specialist in neutron stars from Northern Ireland who oversaw the proceedings -- urged those who might be "quite disappointed" to look on the bright side.

"It could be argued that we are creating an umbrella called 'planet' under which the dwarf planets exist," she said, drawing laughter by waving a stuffed Pluto of Walt Disney fame beneath a real umbrella.

The decision by the prestigious international group spells out the basic tests that celestial objects will have to meet before they can be considered for admission to the elite cosmic club.

For now, membership will be restricted to the eight "classical" planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Much-maligned Pluto doesn't make the grade under the new rules for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.

Instead, it will be reclassified in a new category of "dwarf planets," similar to what long have been termed "minor planets." The definition also lays out a third class of lesser objects that orbit the sun -- "small solar system bodies," a term that will apply to numerous asteroids, comets and other natural satellites.

It was unclear how Pluto's demotion might affect the mission of NASA's New Horizons spacecraft, which earlier this year began a 91/2-year journey to the oddball object to unearth more of its secrets.

The decision at a conference of 2,500 astronomers from 75 countries was a dramatic shift from just a week ago, when the group's leaders floated a proposal that would have reaffirmed Pluto's planetary status and made planets of its largest moon and two other objects. (Watch why some think planet size doesn't matter -- 3:39)

That plan proved highly unpopular, splitting astronomers into factions and triggering days of sometimes combative debate that led to Pluto's undoing.

Now, two of the objects that at one point were cruising toward possible full-fledged planethood will join Pluto as dwarfs: the asteroid Ceres, which was a planet in the 1800s before it got demoted, and 2003 UB313, an icy object slightly larger than Pluto whose discoverer, Michael Brown of the California Institute of Technology, has nicknamed "Xena."

Charon, the largest of Pluto's three moons, is no longer under consideration for any special designation.

Brown was pleased by the decision. He had argued that Pluto and similar bodies didn't deserve planet status, saying that would "take the magic out of the solar system."

"UB313 is the largest dwarf planet. That's kind of cool," he said.

[Dark Silver]

So...we no longer have 9 planets...

Still say they could have left Pluto, 9 was such a nice number...

[LadyTevar]

I'd have rather had 12 planets, than only 8

[Stofsk]

I prefer 8 planets. The four inner terrestrial planets and the four outer gasgiants. Pluto's only a planet if you consider a ball of rock half the size of Australia a planet.

[Batman]

I think Pluto should appeal this decision. I want more planets Valendamnit.
And while including Ceres feels iffy (that thing has been an asteroid for all my life) if that's what it takes, so what?
And what's with the 'clearing its orbit' thingy? Doesn't that mean that only Mercury and Venus are actually planets, what with all the others having moons polluting their orbits?
Do Pluto and Neptune ever really get close enough to seriously affect each other?
And I wouldn't exactly call Mercury or Venus terrestrial

[Comrade Tortoise]

Can we cut the processed meat please. Talk of how many planets we prefer is...well it fits the bill ever so slightly. I dont mind the humor, but keep it in context with substantive discussion

[Destructionator XV]

And while including Ceres feels iffy (that thing has been an asteroid for all my life) if that's what it takes, so what?

In science, things aren't determined by tradition, history, or what you want.

And what's with the 'clearing its orbit' thingy?

Probably means it takes local objects into its own gravity well so they are either absorbed by the planet or brought into planetary orbit.

But of course, the people who worked out this definition are way smarter than you and I combined, and they probably know exactly what it means, but it would take some time to explain to us lay people.

And I wouldn't exactly call Mercury or Venus terrestrial

Well, astronomers do, and they also include Mars in there, as all four inner planets are rocky as opposed to gaseous.

Any even if we ignored the people way smarter than us with many more years of specialized education in the field which both of us lack, why not Venus? It is almost the same size as Earth, has a significant atmosphere of mainly CO2, and a similar internal structure. Big difference between the two is probably in rotation; Venus rotates much more slowly and in the opposite direction. Earth and Venus may well be the two most similar bodies in the solar system.

[Mayabird]

Earth and Venus are the most similar bodies in the solar system. Similar size and composition, similar orbits, and atmospheres. Compared to the other planets it's much closer. "Terrestrial" basically means that the planet is rocky rather than gaseous.

I am happy with this decision. Pluto was just one of the larger pieces of Kuiper Belt stuff orbiting the sun in the outer regions of the solar system. And if it makes you feel better, there's a probe heading out to it for closer study.

[Josh]

It's going to be an adjustment. Personally, I'll probably always think of Pluto as a planet, regardless of the ruling.

It is really strange about how this hits you at such a fundamental level. I was always fascinated by Pluto as being the 'furthest out there', even with all the Planet X calculations floating around.

(I'll admit, I always wanted to have a nice hermit retreat out there, too. Seemed like a good place to get away from it all.)

[DesertFly]

Well, it's still going to be there, just the classification for it has changed. As for me, once I started learning about the solar system, Pluto just seemed out of place. I'm happy to see it go.

[Stofsk]

Big difference between the two is probably in rotation; Venus rotates much more slowly and in the opposite direction. Earth and Venus may well be the two most similar bodies in the solar system.

There are other, major differences. Venus has no magnetic field, and it's climate is insane - both of which mean we probably won't get to colonise it.

[Josh]

Bah. Insane climate just means it's a challenge!