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Gravity stretches out without limits although it decreases with the square of the distance involved.
Hi, I'm always suspicious that you already know this stuff and you're just asking to give us something to do - but that's ok, it's nice to have something to do, so thank you very much. It's quite a well crafted question but the trick you've put in here is that you've asked it in a way that is leading us to think the Oort cloud may be something that it isn't. Let's step back a bit and identify what the Oort cloud is.1. As I understand it, the Oort Cloud is essentially theorised to exist and theorised to be what we think it is..... The Oort cloud..... is theorized to be a cloud of billions of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU [Taken from Wikipedia, https://en.wikipedia.org/wiki/Oort_cloud ] Dutch astronomer Jan Oort proposed the existence of the cloud to explain (among other things) where long-period comets come from, and why they seem to come from all directions rather than along the orbital plane shared by the planets, asteroids and the Kuiper Belt.[Taken from NASA website, https://science.nasa.gov/solar-system/oort-cloud/facts/ ] - - - - - - - - - - - There just aren't any good images of the Oort cloud that I know of - but I don't know everything. Our most distant spacecraft, Voyager 1, may be on the inner edge of the Oort Cloud (Voyager 1 is reported to be about 140 AU away and the Oort cloud may start at around 200 AU) but it's only taken one path and it apparently stopped taking pictures way back in 1990. So, the most important thing to begin with is an understanding that the Oort cloud is just theorised rather than being something which we know exists and it extends out to some distance we haven't measured but instead we merely estimate from theory.2. Part of that theory is that the Oort cloud is going to be that bit of space (or all the icy planetessimals and stuff within this bit of space) that can be identified and treated as having the gravitational influence of our sun as the major gravitational influence that exists on them. To paraphrase that, we could define the outer edge of our solar system to be precisely where the Oort Cloud ends and understand this as the point at which our sun's gravity will no longer have dominion over an object. Anything further out experiences a more important gravitational pull to something else, e.g. to another star like Proxima Centauri that you mentioned or some other thing in the galaxy. Meanwhile, anything closer to our sun is going to be inside the region where our suns gravity should dominate over the gravitational pull toward any other body or place in the universe. If you're interested then you might wish to look up terms like a "Hill Sphere".[ Image taken from Wikipedia article about Hill Spheres ] Basically we can imagine massive bodies as creating some gravitational well around them. If there's a second massive body out there somewhere, then there is some place where another object (a third but generally less massive object) reaches the highest point of the gravitational well around the first massive object it can get to and it would then be in a region of lower gravitational potential and in the gravitational well of the other second massive object if it was any further out. The whole gravitational landscape can be thought of as having these sorts of valleys (gravitational wells) and dividing hill peaks (actually more like flat plateaus at the top of the hill rather than a sharp peak) between them. One way of defining the extent of the Oort cloud is to say it will be that region of space that is in our suns Hill Sphere, which is exactly how we would also typically define the extent or region of space that we say is in our solar system, which is exactly the same as saying that the gravitational well around our sun does appear to be the gravitational well any object in that region would be in. In principle then, any slow object will have to stay in that gravitational well because it doesn't have the kinetic energy to get over the hill. That doesn't mean that nothing ever leaves this region, indeed sometimes an object will get a bit of kick (a gravitational kick rather than something from a man with legs) from some other passing object, there will be enough of a boost in its kinetic energy and a dip in the gravitational landscape while that passing object is nearby so that the object will leave our solar system or start out on some nearly hyperbolic path towards our sun and be seen as a comet from planet earth etc. However, on the whole, most objects that are still going to be found in this region after 4.6 bn years (the approximate age of our solar system) will be fairly slow moving ones, so they will be stuck in our suns gravitational well. The really fast objects that were in that region can and probably will have got the freaking blazes out of there. Indeed there's something called "the virial theorem" that pretty much tells us this. So, when you ask your question:"What keeps it in check?" The answer has to be "It is our suns gravity". It's not magic or some amazing coincidence. Until we get better pictures and more spacecraft out there, "The Oort Cloud" and exactly how far out it might extend is just theory and it's a theory built very much on the notions of where (or how far out) our suns gravity will have dominion. Good question though, there's nothing wrong with a bit of mis-direction. I managed to persuade my children that I can slice a cucumber so thin it's like there's no cucumber in their sandwich.Best Wishes.
Apparently the outer edge of the oort cloud is estimated to stretch out as far as 100,000 to 200,000 astronomical units. That's halfway the distance to Proxima centauri.