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Solar System has 8 planets (Pluto demoted)


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Old 24-08-2006, 18:46   #41
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there shouldnt be something like that according to me.im not talking about its scientific statement but i think little kids have been taught that there are 9 planets and its hard to get this info out of their minds.anyway its good to know that!!
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Old 25-08-2006, 00:59   #42
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Quote:
Originally Posted by freddie
Bah. Too bad. I thought nostalgia would win them over.

Actually one might say now that Mercury is a dwarf planet as well - I'm sure there are many planet like objects in the kuiper's belt which are bigger than Mercury & Pluto combined.

Aside from the definitional point Haku made about being the dominant body in its region of space, Mercury is a lot bigger than Pluto or Xena. It's over twice the diameter, eight times the volume and 25 times the mass. Mars is only about 40% wider than Mercury and twice the mass. Two moons in the solar system - Ganymede and Titan - are slightly bigger than Mercury, but they have less than half the mass. Mercury is a more substantial object than any of the moons in the solar system. Pluto is smaller and less massive than all seven big satellites in the solar system - Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton. Pluto has one-sixth the mass of our Moon.

It's unlikely that there's anything else much bigger than Xena in the Kuiper Belt - Mike Brown's surveyed most of the sky now and anything bigger would have to be in an even more inclined orbit than Xena.

There may be Earth or Mars sized planets much further out than the Kuiper Belt. There are two theories of planet formation. One, the disc instability theory, says that the giant planets formed very quickly out of instabilities in the disc of gas that surrounded the Sun as the solar system formed. It says there should be no big objects beyond Neptune. The other, the oligarchic theory, says that there were about 60 objects that formed and then collided together to create the planets we know. But in computer simulations several Mars to Earth-sized objects have near-misses with Jupiter and get thrown by its gravity into very distant and wild orbits. Neptune and Pluto are about five billion kilometres from the Sun. Sedna is about fifteen billion kilometres from the Sun (100 times as far as the Earth, or 100 Astronomical Units) and its orbit takes it out to about 150 billion kilometres (1000 AU). It's predicted that these planets, if they exist, will be somewhere between 1000 AU and 10 000 AU. Current telescopes can't find them, but there are plans to build an 8.4 metre telescope to scan the sky for them. If they exist, they'll be real planets, not minnows like Pluto. Maybe there are only 8 planets in the solar system, but maybe there will turn out to be twice as many - real planets. We'll be glad we didn't proclaim planet 9 to planet 90 to be tiny ice balls.
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Old 26-08-2006, 02:19   #43
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Quote:
Originally Posted by freddie
Actually one might say now that Mercury is a dwarf planet as well - I'm sure there are many planet like objects in the kuiper's belt which are bigger than Mercury & Pluto combined.
Michael Brown, discoverer of 2003 UB313 and supporter of the 8-planet proposal, has published a clear explanation on his web site (click on The eight planets: an explanation).
It is to be noted that if the original IAU proposal had passed, Michael Brown would have become the discoverer of 15 planets (and probably more as his work is not done), so the man is to be applauded for putting science before his personal glory.
However, he is now the discoverer of the biggest dwarf planet, and he hopes that 2003 UB313 will *at long last* gets its final name very soon now that the planet definition has been revised.
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Old 26-08-2006, 12:32   #44
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Quote:
Originally Posted by simon
It's unlikely that there's anything else much bigger than Xena in the Kuiper Belt - Mike Brown's surveyed most of the sky now and anything bigger would have to be in an even more inclined orbit than Xena.
Which still IS possible. What if there are a whole bunch of planetoid objects the size of Mercury or above under all kinds of weird orbital angles. Will that all for a yet another redefinition of the solar system as we know it? I just want something more permanent, if we're already redefining things.

There's also a question of other large onjects that might be found BEYOND the Kuiper Belt. Surely the gravitational pull of the sun extends far beyond what we know now as our solar system.
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Old 26-08-2006, 15:13   #45
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It's very unlikely that anything as big as Mercury exists in the Kuiper Belt because it would be eight times bigger than Triton, Xena and Pluto (Triton is Neptune's largest moon, but it's a captured KBO, slightly larger than Xena and Pluto). There certainly aren't going to be 'a whole bunch' of them that all happen to be at the celestial poles at this point in their super-inclined orbits. The fact that the three largest KBOs found even after most of the sky has been surveyed are all about the same size actually suggests that the biggest that objects formed in that region could grow to was about that size, just like there are physical reasons why Ganymede and Titan are as big as icy moons can grow.

Read Michael Brown's explanation that Haku pointed a link to. It would be a nonsense to classify Pluto and Xena, let alone dozens of smaller objects, as planets when they're just the largest members of a population of small bodies. Mercury is much bigger and it's not much smaller than Mars. It isn't part of a population of small objects like they are, it's the smallest of the big objects. Mercury is 25 times more massive than Xena and Pluto. It's several times more massive than the entire Kuiper Belt. It's the obvious place to draw the line. If something as massive as Mercury is found in Kuiper Belt, it should be a planet, but I'd bet money that one won't be. What's your problem?

I'd already discussed the possibility of finding big, planet-sized objects beyond the Kuiper Belt. They may exist and if they do they should be recognised as planets.

Last edited by simon; 26-08-2006 at 15:34.
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Old 26-08-2006, 20:58   #46
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Quote:
Originally Posted by simon
Read Michael Brown's explanation that Haku pointed a link to. It would be a nonsense to classify Pluto and Xena, let alone dozens of smaller objects, as planets when they're just the largest members of a population of small bodies. Mercury is much bigger and it's not much smaller than Mars. It isn't part of a population of small objects like they are, it's the smallest of the big objects. Mercury is 25 times more massive than Xena and Pluto. It's several times more massive than the entire Kuiper Belt. It's the obvious place to draw the line. If something as massive as Mercury is found in Kuiper Belt, it should be a planet, but I'd bet money that one won't be. What's your problem?
Hihi. I love your enormous conviction about a topic that's not yet completely explored. At least not up to an extent when one could claim something for certain like you seem to be. Tell you what... if they find something Mercury-size in Kuipers Belt you're buying me beer.
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Old 26-08-2006, 23:59   #47
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I'll take that bet for the Kuiper Belt because I'm pretty confident Mike Brown would have found anything that big by now, but not for the Scattered Disc beyond it. There could easily be something big lurking out there that's too dim to have been seen yet.
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Old 14-09-2006, 19:48   #48
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Xena renamed Eris

Xena has been given an official name by the IAU, to replace the nickname honouring the character played by Lucy Lawless. Mike Brown proposed and the IAU accepted the name Eris, after the Greek goddess of discord. He's called the name 'too perfect to resist', since his discovery sparked so much discord. The moon Gabrielle has been renamed Dysnomia after Eris's daughter, the spirit of lawlessness. So the actress no longer has a dwarf planet named after her character, but she does have a moon commemorating herself.

http://www.nytimes.com/2006/09/14/sc...rtner=homepage

http://www.newscientistspace.com/art...f-discord.html
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