You may remember that the Voyager 1 spacecraft officially “left the Solar System” back in 2012, bidding us all a fond farewell.
Specifically, Voyager 1 crossed a boundary known as the heliopause, beyond which the solar wind no longer blows. Out there is true interstellar space, in the sense that the ionized particles Voyager encounters and samples are more likely to come from other stars or other sources than our Sun.
However, Voyager 1 has not quite left the Solar System.
This depends a bit on how we define our terms (as so many things do in science), but by one fairly conventional definition, anything that orbits the Sun (or, like the Moon, anything that orbits something that orbits the Sun) is part of the Solar System. So taking that into account, just how big is the Solar System, and where exactly does the Solar System end?
- Earth: By definition, Earth orbits the Sun at an average distance of 1 astronomical unit (AU). Obviously the Solar System is larger than 1 AU.
- Neptune: As the most distant known planet, Neptune orbits the Sun at distance of roughly 30 AU. Neptune’s orbit is an important boundary line in astronomy, but it’s definitely not the end of the Solar System.
- The Kuiper belt: The Kuiper belt, the region Pluto calls home, extends from 30 AU to about 50 AU.
- The heliopause: Voyager 1 crossed the heliopause at about 121 AU.
- Voyager 1: As of the writing of this post, Voyager 1 is just over 134 AU from the Sun.
- Sednoids: By definition, sednoids come no closer to the Sun than 75 AU, but they’re free to travel much, much farther away than that. Sedna, the first known sednoid and the object for which this category of objects is named, reaches a maximum distance of about 936 AU from the Sun.
- Planet Nine: If such a planet exists (and that’s still a pretty big if), its orbit is estimated to range somewhere between 200 and 1,200 AU from the Sun.
- The Oort cloud: The Oort cloud has not yet been observed directly, but its existence is inferred from the orbits of long-period comets. It is believed the Oort cloud exists somewhere between 2,000 and 200,000 AU from the Sun.
By the way, Alpha Centauri is 4.35 light years away, which equals about 275,000 AU. If the Oort cloud does extend all the way out to 200,000 AU, that means our Oort cloud and Alpha Centauri’s Oort cloud might overlap (assuming Alpha Centaur has its own Oort cloud).
It would be interesting to know how two overlapping Oort clouds interact with one another and how many comets, asteroids, dwarf planets, etc have been exchanged from one cloud to the other. If only we had a space probe that could go check on that.
Planet Pailly 
Wow. When compared to the distance to alpha centauri, I never realized how far the the Oort cloud extends. Does that mean you could consider Sol as a distant part of the Centauri system? Or is the gravitational influence to weak?
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I think that’s sort of an unresolved question. Most astronomers seem to think Sol and Alpha Centauri are unrelated and just sort of passing by each other. A few have suggested that they might be orbiting each other, with Proxima Centauri thrown into the mix as well. In which case we could be part of a system of at least four stars.
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I’ve noted before that all definitions are ultimately relative. That said, I think when you say “solar system” to most people, they think about the planets, the Kuiper belt, and perhaps the inner Oort cloud (up to maybe 20,000 AU).
When we get to the point where we’re more than a light year out, orbits reach periods of millions of years and there becomes a good chance of passing stars stealing or introducing new objects. At two light years out, given that we’re at the intersection between the sun’s gravity and that of other stars, it’s questionable whether any object out that far will ever be able to complete a full orbit. I think any reference to these distant regions as the “solar system” should be qualified for clarity, but that’s just me 🙂
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Yes, a lot does depend on how we define our terms. Also, we don’t truly know the extent of the Oort cloud. If the Oort cloud does extend most of the way to Alpha Centauri, and if Alpha Centauri’s extends most of the way to us, then we really do need to rethink how we define the limits of a star system.
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