At about three different points in my life (with significant gaps in between) I remember news reports of Science discovering a tenth planet in our Solar System (I realize we have folks posting from lots of different location: I’m referring to the Solar System that has Earth in it).
After the initial reports: Bupkiss (occurs to me that I’m not sure how to spell “bupkiss”)
So what’s the deal? Beyond Pluto- what?
I’ve also heard it suggested that Pluto isn’t a “real” planet. What up?
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There is a ring of debris beyond Neptune called the Kuiper belt. It consists of cold icy objects left over from the earliest days of the solar system. Pluto’s “planethood” is sometimes questioned on the grounds that it might just be a particularly large object of this type. Essentially a giant comet. There is some possibility that there might even be another such object or objects beyond pluto. The light is so dim out there that it would very hard to see from here. The possible size and position of such object(s) can be somewhat constrained by gravitational calculations however.
Yeah, there’s no tenth planet. Plenty of conspiracy surrounds a supposed Planet X but it doesn’t exist.
This site has a lot of great info about Pluto and the rest of the solar system. Essentially, once we discovered the Kuiper Belt we realized Pluto may just be the largest member of it, but since it has been classified as a planet in the past, it is considered one now.
A couple of largish objects have been discovered out there in the Kuiper belt, but not as large as Pluto.
There is no evidence for a very large planet out there, sometimes referred to as Planet X.
It was decided a few years back (by the International Astronomical Union, the organisation in charge of these things officially in the scientific community) that we should continue to call Pluto a planet instead of just a large comet, and that our solar system still officially has nine planets. There had been some talk about taking away its official planet designation because of its makeup and its proximity to the comet objects in the Kuiper belt. I think the reason they let it remain a planet is because it’s big enough that its gravity pulls it into a spheroid shape, and it doesn’t stay lumpy like an asteroid or peanut-shaped like Halley’s Comet. 
So far, that seems to be the working definition of planets: they orbit stars; they aren’t moons orbiting other planets; they’re big enough to be round; and they’re small enough that they don’t start nuclear fusion and become stars themselves.
Does anyone have anything new to add? Or do all of the above arguments still apply to the “new” information in the linked article?
Personal opinion: unless its diameter is greater than Pluto’s, a body will not be characterized as a planet.
So “Sedna” just misses out.
Apparently, the new object (which IMO they should have named ‘Segway’, after the Norse god of Well-meaning, but ultimately useless inventions) may be larger than Pluto.
Well, if Pluto is considered a planet, this one should be as well. That said, I’m not sure if pluto should be considered a planet. But they are both in the same league
Either neither are planets or both are. I can’t see a defense of pluto is a planet but this new thing isnt.
I’ve also read that some scientists think that Pluto may have once been a moon orbiting Neptune and that a passing comet or something may have knocked it out into it’s own orbit around the sun.
Don’t forget that Pluto has a moon, Charon, which is a big point in favor of its planet-ness.
The only people who seem to care about the classification of bodies as planets or not are those who aren’t astronomers. Astronomers care more about what the things are and don’t worry themselves about a bit of arcane and probably obsolete terminology.
A “planet” isn’t an official type of thing. It’s just a convenient way to refer to a limited number of bodies. And it’s just a convention, the way continent is. Is Australia a continent or a large island? Is Greenland? Does it make any real difference outside of trivia games?
The number of planets will stay at nine, because they make a nice compact visual group useful for instructional purposes. Trying to fit the spectrum of astronomical bodies into tightly defined exclusive categories doesn’t even work as well as trying to make the electromagnetic spectrum fit our preconceptions. (Quick: where’s the boundary between visible and ultraviolet light?)
The Kuiper belt is interesting enough in its own right to clump all the stuff in it together, without trying to backfit some of them into the category of planets.
And then we can all get back to worrying about whether those big-ass objects circling other stars are planets, brown dwarfs, failed stars, or continents. 
Apparently this object is slightly smaller then pluto(2000 km in diameter to pluto’s 2300). Mercury is only twice that size. I think the question is: What is the defintion for a planet? At what size do we cut them off and why?
Well, call it a “planet” and it plays heck with your horoscope. Who wants to define what “Sedna is rising in Cancer” means?
IOW, and as Exapno says, it entirely arbitrary. Go ahead, call Pluto a planet and say that Sedna isn’t if it floats your boat. I personally think of Pluto as the largest Kuiper Belt Object, which is quite important on its own.
This what’s-a-planet debate has already got so fuzzy that it seems highly unlikely that anything newly discovered will be properly accepted. Sedna will remain the poor cousin of the planets. Perhaps someone will write it a little musical number all of its own, but it’ll probably have to be Rolf Harris.
Also there must surely be a “brown dwarf” joke but I can’t for the life of me work out what it is.
By that definition, Ceres and several of the other asteroids are planets. And most likely Sedna and and several of the Kuiper Belt objects (KBOs) are too.
The most siginficant thing about sedna is not its planethood, but that it isn’t in the Kuiper Belt but rather further out. The Kuiper Belt seems to have a fairly sharp edge of about 50 AU. That’s in terms of semi-major axes; some KBOs are further away at aphelion.
Sedna, by contrast never gets closer that about 75 AU and has a semi-major axis of something like 500 AU. It also has a very elliptical orbit. The discoverers think it’s an inner-Oort Cloud object rather than a KBO. See here Sedna (2003 VB12)
Here’s a question for you: If a team of NASA astronauts were today standing on this planetoid (whatever), would the ambient light be enough for them to see surface features unaided, or are we talking midnight-in-a-coal-mine dark?
Another question: On another thread, someone said that NASA’s old Apollo spacesuits (used on the Moon), wouldn’t hack it on, say, Jupiter’s moons, where the temperature is so cold that it would literally suck the heat out of the spacesuit. True? If so, what alternative might there be?
Yeah, but isn’t Charon so large compared to Pluto that the two bodies can be said to orbit each other?
To quote the International Society of Non-Astronomers Against Pluto:
“Remember… real planets may wobble, but they never waltz.”
Or the other way around. It’s often thought that Neptune’s moon Triton was once a pluto-like planet that was captured by Neptune’s gravity.
I’ve been in a cave with the lights off, and I can say without question it’s going to be brighter than that. Currently, Sedna is some 90 AU away from the sun. Since light decreases as the square of the distance, the sun will be only 1/8100 as bright as it is here on Earth. Or a bit brighter, allowing for the fact that the sun is seen through the filter of the atmosphere here on Earth. At any rate, that’s not very bright and I’d definitely bring a flashlight.
Obviously, the suits would need more insulation. The main conduit for heat loss would be through the soles of the feet (unless they fell down), so you want to put lots more insulation there. Personally, I’d like about 20 cm or so of shuttle tile material on the bottom of my feet if I were to make such a trip.
[QUOTE=Carnac the Magnificent!]
Here’s a question for you: If a team of NASA astronauts were today standing on this planetoid (whatever), would the ambient light be enough for them to see surface features unaided, or are we talking midnight-in-a-coal-mine dark?
according to Mike Brown, an astronomer quoted in the Washington Post, “Sedna is so far out that from its distance the sun could be completely blocked out with the head of a pin, he said.” my guess is that there is not enough ambeint light…