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

albear

pluto was until the change, considered a planet, i know it isnt anymore, so dont get annoyed at me.
as for sedna why not, can you give me clear reasoning

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.

albear

lets discus sedna, we dont 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 ive been informed

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. There are probably hundreds of other bigger objects with similar orbits that we havent 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

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

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.

albear

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

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

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 dont think so

Capuchin

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.