Sedna's Orbit: How Gravity Keeps it Tied to Our Solar System

[Echo 6 Sierra]

If Sedna has such a crazy elliptical and lengthy orbit, what exactly ties it to our solar system? Gravity, OK, but is it just the Sun or all the planets combined that have the ability to pull the wee one back from its farthest point?

It just doesn't seem feasible that once it gets to the great length of its orbit that the gravity posessed by our SS would have the strength to pull it back.

Is it not considered a comet because of its size, composition, or frequency?

What keeps comets together when they have such velocity and are making "a turn" to head back towards the sun?

If Sedna is roughly the size of Pluto, could it be another "Pluto" in the making?

Will Pluto's orbit ever be more planar(?) like Earths and the rest of the planets?

 

[Phobos]

Welcome to Physics Forums, Echo 6 Sierra!

All the objects within the solar system contribute to the orbital mechanics of the system, but the sun takes center stage. A motivated individual here may calculate the relative contributions from the sun & each planet, but in the end, Sedna orbits the sun once every 10,500 years.

I agree it's amazing that objects so distant can still be held in around the sun, but gravity has in infinite reach and as long as the object isn't moving too fast or doesn't get too close to a more influential gravitational force (like a passing star) then it will stay in orbit around the sun. As you probably have heard, it is believed that there are solar system objects (comets) even further out than Sedna in the Oort Cloud. Out there, their relative velocity is miniscule and they stay in the neighborhood, so to speak. It only takes a slight nudge to change the orbit of an object in the Oort Cloud. We occassionally see these surprise visitors pass through the inner solar system only to return to the outer reaches of the solar system for a few thousand years more.

It's probably not called a comet due to its size. But there is a gray area in distinguishing between comets/asteroids/KBOs/planets.

Some comets do fall apart when they get near a planet, the sun, etc. Others have enough gravity or structural integrity to stay together. Another example is that some asteroids are like large rocks where as others are like gravel piles.

Sedna could be a Pluto-in-the-making (if it collides with other objects to gain more mass). There could also be other KBOs (Kuiper Belt Objects) that are Pluto sized or larger.

Hmm...not sure about the last one. I suppose it's possible over millions/billions of years.

 

[Nereid]

Will Pluto's orbit ever be more planar(?) like Earths and the rest of the planets?

Quick answer on this one: no.

Pluto's orbit is, more or less, stable within the solar system, due to a resonance with Neptune's. There are several other KBOs in similar stable orbits (called, IIRC, 'plutinos'), and other KBOs are in other resonance orbits. "Stable" in this context means that if, for whatever reason, the object strays from its orbit a bit, then the combined gravitational pull of the planets and Sun (mostly Sun and Neptune) will bring it back to its stable orbit (OK purists, it's a simplification!)

There are (at least) two significant caveats: "within the solar system" and "Oort cloud objects". For the former: rogue planets, passing stars, giant molecular clouds, ... a significant gravitational influence from outside the solar system could disrupt orbits, maybe in a big way (no, the combined gravitational pull of all Milky Way masses interior to the solar system doesn't count in this respect). For the latter: Oort objects are "within the solar system", but we know almost nothing about their number, size distribution, orbits, etc. Maybe there's one with an orbit which will, in a billion years or two, shake up Pluto's orbit?

 

[Echo 6 Sierra]

So, when we say "solar system", does that mean everything that is influenced by the Suns/Planets gravities? If so, why is the Oort cloud the Oort cloud? Is it circling the Sun in a belt-type configuration or is it a giant "shell" around us and trying to get to us due to gravity? If it's a shell thing, is it being held where it is by solar radiation or is there something else?

Thank you both.
E6S

 

[Nereid]

So, when we say "solar system", does that mean everything that is influenced by the Suns/Planets gravities?

Close; 'gravitationally bound to the Sun (and planets)' would be a somewhat better definition.

If so, why is the Oort cloud the Oort cloud? Is it circling the Sun in a belt-type configuration or is it a giant "shell" around us and trying to get to us due to gravity? If it's a shell thing, is it being held where it is by solar radiation or is there something else?

"Oort" after Jan Oort who first wrote a paper hypothesising its existence; "cloud" because it's blobby, with fuzzy edges, has considerable depth, ... and maybe 'cause it sounded better than something like 'quasi-spherical shell of uncertain extent'! :surprise:

Since there are no direct observations of the Oort cloud - either specific objects which comprise it or some integrated aspect - a purist could argue that it's 'just hypothetical'. However, when you do the math (likely orbits under the Sun+planets' gravity, plus nearby stars (etc)), a shell configuration seems best. Add calculated orbits of (some) very long period and 'new' comets, and you see that the shell configuration is consistent.

It's all just gravity; any dust will have long since been evaporated or departed (due to radiation and radiation pressure), leaving stuff that's 'just passing through'

Thank you both.

You're very welcome; I really enjoy this stuff, and it would seem quite a few others do too!

 

[Echo 6 Sierra]

So, would it be safe to say that the Oort cloud is what/where it is because gravity has it's influence on the things that make it up and at the same time it's being repelled by radiation from the Sun? Also, would the cloud continually gain mass by collecting things from space as they simply pass within the gravitational pull of the SS?

 

[russ_watters]

So, would it be safe to say that the Oort cloud is what/where it is because gravity has it's influence on the things that make it up and at the same time it's being repelled by radiation from the Sun? Also, would the cloud continually gain mass by collecting things from space as they simply pass within the gravitational pull of the SS?

Those forces (solar wind, light pressure) are insignificant even from earth.

You're not thinking of the Ort Cloud as being a stationary cloud of matter, are you? Its just a name for a region at the edge of the solar system where all the comets "reside" - And "reside" doesn't mean they stay there, they kinda just pass through in their normal orbits.

I really think you're making this more complicated than it is.

 

[Echo 6 Sierra]

Yes, I thought it was stationary. I also thought that with great exception everything stayed there, save the occasional comet.

I love this forum.

 

[Phobos]

Since there are no direct observations of the Oort cloud - either specific objects which comprise it or some integrated aspect - a purist could argue that it's 'just hypothetical'. However, when you do the math (likely orbits under the Sun+planets' gravity, plus nearby stars (etc)), a shell configuration seems best. Add calculated orbits of (some) very long period and 'new' comets, and you see that the shell configuration is consistent.

FWIW, I like to think that observation of long-period comets is a pretty good direct evidence for the Oort Cloud. Of course, that is not definitive proof and we need to locate several objects out at that distance (and calculate their orbits) to be sure.

 

[russ_watters]

Yes, I thought it was stationary. I also thought that with great exception everything stayed there, save the occasional comet.

I love this forum.

Glad we could help.

Here's the thing about highly eliptical orbits - the comets will spend most of their time out there in the ort cloud. That far away from the sun, they travel very, very slowly. They could spend hundreds of years out beyond the orbit of Pluto, then whip back around the inner solar system in a matter of a year.

Anyone have a website with a flash animation on this?

It may even be (speculating here) that there are many objects with more circular orbits and its just that every now and then a couple get purturbed and sent in towards the sun on an orbit of higher eccentricity. The asteroid belt is kinda like this (a lot more interesting a couple of billion years ago though).