why do Planets have elliptical orbits ,AND

[gttjohn]

why do Planets have elliptical orbits ,why do they just not have circular orbits ,I understand that the sun is like a bowling ball on a sheet of rubber and the planets in our solar system orbit the bowling ball as if i got a marble and rolled it onto the sheet of rubber but after a while the marble would have a circular orbit ,are we still at the stage where the marble was just thrown onto the rubber sheet ,and will we eventually all clump together towards the sun and burn up or will the sun eventually loose its gravity and we will be thrown into outer space


also why is the universe accelerating should it not be slowing down as the energy from the big bang looses strength

[gttjohn]

Sorry all i have just looked at the two stickys above my post but in laymans terms they are impossible for me to understand

[Rick88]

It just depends on the energy and momentum the planets possessed when they formed.
But they actually all have orbits which are very close to being circular, except for Mercury.

And yes, the expanse of the Universe *should* be slowing down if we only consider ordinary matter.
But there is also dark matter (and dark energy) to take into account, and that's what is causing the expansion of the Universe to accelerate.
Why, we do not know.

[tatiana]

Some planets have circular orbits while some have elliptical orbits based on the grvitational forces that exit between teh neighbring planets. Planets that are strong enough to exceed the gravitaional pull of a planet with follow a circular path unlike a plat that is to weak to exceed the gravitational force between another plaet will fall out and follow an elliptical orbit

[gttjohn]

thankyou

[gttjohn]

do we know what dark matter and dark energy is yet

[Rick88]

I dare say we don't have a clue.

[Vagn]

do we know what dark matter and dark energy is yet

We have some hypotheses for what dark matter is, such as axions, WIMPs and other exotic particles, such as scalar particles and axinos, but experimentally we aren't sure which is correct, WIMPs and axions are the most looked for thus far, and we have reasonably well known upper limits on density and mass for these as a result. It is also possible that the mechanics of large objects behave differently at large scales, which is called MOdified Newtonian Dynamics, or MOND however it doesn't apply itself particularly well to forming large scale structures, of the order of galactic clusters or larger.

Dark energy is completely unrelated to dark matter, but is named similarly due to the fact we don't know what it is, and thus it is 'dark', as far as I am aware, there is very little known about what dark energy actually is, which is a bit frustrating seeing as dark matter makes up about 71% of the Universe.

[Nik_2213]

IIRC, a true circular orbit is unstable to perturbation: Any third-object interaction, or even solar tides, will make the orbit slightly elliptical...
...
IMHO, that exasperating ~70% of dark matter may be revised downwards as more galaxies are found to have larger, dusty halos and thrice the expected number of red-dwarf stars. If we're looking for super-sym particles etc, we're into 'new physics' so, like finding neutrinos *do* have non-zero rest mass, it could up-end a lot of theory...

[tatiana]

Those solar tides and or even third object interactions i agree wiould cause the orbit to become slightly elliptical but isnt that the same as saying the gravitational pulls of oter forces, particularly other planets?

and what is non-zero-rest mass anyway?

[Joshua.B]

Dark energy is completely unrelated to dark matter

what about the dark fluid theory?
doesnt it propose that Dark Matter and Dark Energy are not seperate physical phenomena but that they are linked together.