You might find yourself looking at a (to-scale) diagram of the planets of the solar system (and Pluto), such as the following:
and think to yourself “Wow, Pluto is so much farther out from the sun than the Earth is. The solar system is so massive!”. And while you would be correct in your statement, the orbits of the sun’s outer planets (and dwarf planets) pale in comparison to the true extent of our solar system. Enter: the OORT CLOUD.
Actually, the Oort Cloud is going to have to wait. First, we’re going to have to cover comets. For a comet to show its characteristic tail, it has to pass (relatively) close to the sun. This implies that its orbit must be highly elliptical so that it can be near the sun for a short period of time before moving far enough away that it reverts to its less aesthetic ball-of-ice-and-rock form.
Everyone knows about Halley’s comet, which becomes visible from Earth every 76 years. Combining this lengthy period with the above fact that comet orbits must be very elliptical, one can imagine that Halley’s comet is, at its furthest extent away from the sun, far away. An in fact it manages to reach a bit farther than the orbit of Neptune before crashing back toward the sun.
But, in the grand scheme of things, Halley’s comet actually has a fairly short period. The famous Hale-Bopp comet has a period of over 2500 years and, as you might guess, reaches much farther away from the sun than Halley’s comet: 183 AU, or roughly 6 times farther than Neptune’s Aphelion.
But wait, there’s more! This unassuming ball of rock:
is Sedna, a minor planet whose aphelion is 936 AU, or 30 times Neptune’s aphelion.
But even beyond Sedna, there is the Oort Cloud. The Oort Cloud is a theoretical cloud of gas and icy bodies which exists at the very edge of the Sun’s gravitational influence. This cloud is conjectured to exist at up to 200,000 AU away from the sun. In other words, the outer reaches of the Oort Cloud, should it exists according to these projected specifications, would be over six thousand times farther away from from the sun is than Neptune. This is the true extent of our solar system, beyond which point the sun’s gravity is no longer sufficient to pull bodies along with it as it wanders through the galaxy.
The Humongous Big Universe 
I’ve read a bit about the Oort Cloud, and I’ve been fascinated by the possibilities it poses for interaction with other star systems. For example, a 2015 paper in the Astrophysical Journal of Letters (linked below) contends that a close pass from a low-mass binary system has perturbed Oort Cloud objects in the past. What do you think of this? It’s easy to think of our Solar System as wholly separate from all other solar systems, but with a reach as far as the Oort Cloud it seems inevitable that some interactions take place.
Paper link: https://authors.library.caltech.edu/55650/1/2041-8205_800_1_L17.pdf
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I’ve known that it’s projected that at some point in the future another star will come very close to our solar system, but I didn’t know that scientists think this has happened before. I didn’t end up including it in the post because it was getting a little lengthy, but the concept of the Hill Sphere, which is the sphere around a planet / star where it is gravitationally dominant (see: https://en.wikipedia.org/wiki/Hill_sphere), would definitely come into play in these interactions of objects in the Oort Cloud. I imagine the physics of this would get pretty complicated if there’s both the sun and a binary system, but it’d probably be cool to do a simulation to see what body is going to have “control” over the Oort cloud at any point in time.
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TA here!
Great post! I like the conversation going on here about other objects coming into our Oort Cloud – the idea that something can get that “close” to our solar system is definitely something that intrigues astronomers. But, even though the Oort Cloud extends out to massive distances, it’s VERY empty, and like the paper linked above mentions, the “dynamically active” part only extends to around 20,000 AU. That’s still really far out from our part of the solar system! But does mean that most objects, even coming within the outer Oort Cloud, won’t have much of an effect at all.
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