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gshirk · Dec 4, 2007, 04:25 PM

How (and who) saw that there might be a tenth planet in our solar system?

albear · Dec 4, 2007, 04:32 PM

You mean, sedna? (thats the planets name )
I think through a telescope, and because it was following the pattern of an orbiting planet around our sun, but the orbit length is so huge that it takes so long that we didn't have the technology last time, like 500 years ago or something to tell that it was a planet, I've heard that it might not be just our sun that it orbits because the path is so huge it is likely that there is another sun at the other end to swing it back round either in one big oval or in a figure of eight we don't know.

Capuchin · Dec 4, 2007, 05:00 PM

Tenth planet?? We haven't discovered nine yet. ;)

Apart from the 8 planets, there are three currently recognised "dwarf planets". These are Eris, Pluto and Ceres.

Sedna will qualify as a dwarf planet if it proves to be in hydrostatic equilibrium, as is currently suspected.

There are as many as 50 other objects in the Kuiper belt and asteroid belt that are being examined for inclusion as dwarf planets, this is an area which is currently undergoing an overhaul of classification, with a lot of work to be done.

We don't know of any object that would qualify as a tenth... or even a ninth planet.

albear · Dec 4, 2007, 05:02 PM

Originally Posted by Capuchin

Tenth planet??? We haven't discovered nine yet. ;)

We had nine, but... well look at the pic

Capuchin · Dec 4, 2007, 05:08 PM

We had never discovered nine, not by today's criteria of what a planet is.

That's like saying
You: "oh i have ten cookies"
Me: "actually that tenth one is some dried up mud"
You: "oh, well i HAD ten cookies"

Or
You: "oh i have ten cookies"
Me: "actually a cookie is defined by the localised clusters of high densities of chocolate, your tenth cookie is in fact just a biscuit" (excuse the british and slightly arbitrary differentiation between cookie and biscuit, I'm just trying to illustrate the point)
You: "oh, well i HAD ten cookies"

In either case, you only ever had 9 cookies.

albear · Dec 4, 2007, 05:12 PM

Originally Posted by Capuchin

We had never discovered nine, not by today's criteria of what a planet is.

That's like saying
"oh i have ten cookies"
"actually that tenth one is some dried up mud"
"oh, well i HAD ten cookies"

No, not by today's criteria, but pluto is a planet dammit, and yea the planet sedna was (relativly recently) found to be orbiting our sun,

Capuchin · Dec 4, 2007, 05:15 PM

Neither pluto, nor sedna, is a planet. Please stop confusing the OP :)

albear · Dec 4, 2007, 05:19 PM

Originally Posted by Capuchin

neither pluto, nor sedna, is a planet. please stop confusing the OP :)

I thought sedna was a planet?
I know that pluto 'technically' isn't a planet anymore, because they changed the deffinition of what a planet is.
And speaking of the OP I see that you have not posted an answer to his question, 'technically'

Curlyben · Dec 4, 2007, 05:20 PM

Planet - Wikipedia, the free encyclopedia

A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its inner core, and has cleared its neighbouring region of planetesimals.

Capuchin · Dec 4, 2007, 05:21 PM

Pluto is not a planet, it's a dwarf planet. Nothing to do with "technically". It's not a planet.

Sedna is a potential dwarf planet, dependent on us gathering more information about whether it's in hydrostatic equilibrium (one of the criteria for being a dwarf planet).

In either case, Sedna will never be a planet under the current definition. The grandest title you could give to Sedna at the moment is "asteroid". It's referred to as a "trans-Neptunian object".

We only have 8 planets :)

albear · Dec 4, 2007, 05:24 PM

Pluto was until the change, considered a planet, I know it isn't anymore, so don't get annoyed at me.
As for sedna why not, can you give me clear reasoning

Capuchin · Dec 4, 2007, 05:31 PM

Yes, pluto was considered a planet, but it wasn't a planet. Just like you considered your tenth cookie to be a cookie, but it wasn't a cookie. How we define things today is how we define things.

Criteria for a planet (in the solar system):

* in orbit around the Sun
* has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
* has cleared the neighbourhood around its orbit.

Sedna ticks only the first of these boxes, we are looking for further evidence to ensure it is in hydrostatic equilibrium before we classify it. Pluto has ticked 1 and 2, but not 3. There are many other objects around the Plutonian orbit.

Objects which tick all 3 boxes are classified as planets, objects which tick only boxes 1 and 2 are dwarf planets, objects which tick less than 2 of the boxes are nothing special.

albear · Dec 4, 2007, 05:36 PM

Originally Posted by Capuchin

Yes, pluto was considered a planet, but it wasnt a planet. Just like you considered your tenth cookie to be a cookie, but it wasnt a cookie. How we define things today is how we define things.

Criteria for a planet (in the solar system):

* in orbit around the Sun
* has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
* has cleared the neighbourhood around its orbit.

Sedna ticks only the first of these boxes, we are looking for further evidence to ensure it is in hydrostatic equilibrium before we classify it. Pluto has ticked 1 and 2, but not 3. There are many other objects around the Plutonian orbit.

Objects which tick all 3 boxes are classified as planets, objects which tick only boxes 1 and 2 are dwarf planets, objects which tick less than 2 of the boxes are nothing special.

Lets discus sedna, we don't know the path of its orbit, it could be oval(ly) shaped *if*it orbited only our sun, but if since its orbit is huge it could be orbiting another sun, which in theory could make it have a figure of 8 orbit, sort of, or so I've been informed

Capuchin · Dec 4, 2007, 05:41 PM

Its orbit is calculated to have a perihelion of 76AU and an aphelion of 975AU. That's way too small for there to be another star. If there was another star that close, we'd sure know about it.

It just has a normal highly elliptical orbit. There are probably hundreds of other bigger objects with similar orbits that we haven't been able to detect because they are near their aphelion, where they spend most of their lives. (if you remember Kepler's laws).

I need to sleep now but I'll pick this up in the morning if you still want to discuss :)

albear · Dec 4, 2007, 05:43 PM

Originally Posted by Capuchin

Its orbit is calculated to have a perihelion of 76AU and an aphelion of 975AU. That's way too small for there to be another star. If there was another star that close, we'd sure know about it.

It just has a normal highly elliptical orbit.

Not a sun that gives off light, but I forget what its called I think a neutron star, and no we might not.

Capuchin · Dec 4, 2007, 05:46 PM

Everything gives off light, except black holes. If there was a black hole there we would be able to see the gravitational lensing. The other star (or whatever) would be close enough to affect the orbit of our star and therefore us, we would be in a binary system. We would have detected it. Easily.

albear · Dec 4, 2007, 05:52 PM

Originally Posted by Capuchin

Everything gives off light, except black holes. If there was a black hole there we would be able to see the gravitational lensing. The other star (or whatever) would be close enough to affect the orbit of our star and therefore us, we would be in a binary system. We would have detected it. Easily.

Hhhhmmmmmm, fair enough. Ill admit that my knowledge on this has reached its limits and cannot think of a counter point at the moment and so I'l go away and do some research on it.

But pluto should be a planet!!

Capuchin · Dec 4, 2007, 05:55 PM

Should be is fine, you can argue that all you want :)

Neutron stars are detected by their very characteristic pulses of radiation, by the way.

Here's a nice map of the Sedna orbit, as you can see, there are no objects that we know of near to it (certain other objects like ceres etc have been left off this plot). If there was another gravitational source, as you say, then we would be able to infer it's presence from it's orbit, and from the orbit of our planets, too. You can also see on this how messy Pluto's orbit is, hence why it is not a Planet. It hasn't wiped it's orbit clean of other objects.:

albear · Dec 4, 2007, 05:57 PM

Exactly, 'that we know of, I have seen the orbit of sedna and are you telling me that the suns gravity is such that it can bring back a 'planet from that distance, I don't think so

Capuchin · Dec 4, 2007, 05:59 PM

Yes, that's exactly what I'm saying. What is the biggest force acting on it? Gravity from our Sun. There isn't another star for 266871 AU, that's 300 times the furthest distance that Sedna gets from our sun.