Where does the Solar System end?
This is a question I've heard many times in the past, and a quick search of the site says it hasn't been asked here, so I figured I might as well ask (and answer) it. I know that it is rare for someone to ask and answer their own question, but I think it could work here, and I welcome input (including other answers) from anyone and everyone here.
The Sun is roughly 4 light-years away from the closest star system, the Alpha Centauri system. The planets in our Solar System, however, aren't even close to that far away from the Sun. Where does our Solar System end? Is the edge considered to be the orbit of Neptune, the Kuiper Belt, the Oort Cloud, or something else?
Note: this question on Physics SE is similar, but the answers posted here go in different directions.
Brilliant question - something that has intrigued me (and many others) for a long while
If we could estimate the frequency of close encounters of distance $D$ by stars of mass $M$ and the probability of such an encounter ejecting an object of orbital radius $R$ then we could average over billions of years come up with a statement like: "Objects at $R \gt R_L$ have an $80$% chance of being ejected while objects at $R \lt R_L$ have an $80$% chance of not being ejected. Has anything like that been done?
According to the Case Western Reserve University webpage The Edge of the Solar System (2006) an important consideration is that
The whole concept of an "edge" is somewhat inaccurate as far as the solar system is concerned, for there is no physical boundary to it - there is no wall past which there's a sign that says, "Solar System Ends Here." There are, however, specific regions of space that include outlying members of our solar system, and a region beyond-which the Sun can no longer hold any influence.
The last part of that definition appears to be a viable definition of the edge of the solar system. Specifically,
valid boundary region for the "edge" of the solar system is the heliopause. This is the region of space where the sun's solar wind meets that of other stars. It is a fluctuating boundary that is estimated to be approximately 17.6 billion miles (120 A.U.) away. Note that this is within the Oort Cloud.
Though the article above is a bit dated, the notion of the heliopause has been still of interest to scientists, particularly how far away it is - hence, the interest in the continuing Voyager missions, which states on the website, that it has 3 phases:
- Termination Shock
Passage through the termination shock ended the termination shock phase and began the heliosheath exploration phase. Voyager 1 crossed the termination shock at 94 AU in December 2004 and Voyager 2 crossed at 84 AU in August 2007.
(AU = Astronomical Unit = mean Earth-sun distance = 150,000,000km)
the spacecraft has been operating in the heliosheath environment which is still dominated by the Sun's magnetic field and particles contained in the solar wind.
As of September 2013, Voyager 1 was at a distance of 18.7 Billion Kilometers (125.3 AU) from the sun and Voyager 2 at a distance of 15.3 Billion kilometers (102.6 AU).
A very important thing to note from the Voyager page is that
The thickness of the heliosheath is uncertain and could be tens of AU thick taking several years to traverse.
- Interstellar space, which NASA's Voyager page has defined as
Passage through the heliopause begins the interstellar exploration phase with the spacecraft operating in an interstellar wind dominated environment.
The Voyager mission page provide the follow diagram of the parameters listed above
It is a bit complicated as we do not know the full extent of what the dynamics are like out there, a recent observation reported in the article A big surprise from the edge of the Solar System, reveal that the edge may be blurred by
a strange realm of frothy magnetic bubbles,
Which is suggested in the article could be a mixing of solar and interstellar winds and magnetic fields, stating:
On one hand, the bubbles would seem to be a very porous shield, allowing many cosmic rays through the gaps. On the other hand, cosmic rays could get trapped inside the bubbles, which would make the froth a very good shield indeed.
@HDE226868 - thank you! I got the picture from the Voyager missions page, the 2nd link in this answer.
Sorry it took me so long to accept, but I wanted to wait a while and see what other answers (none!) were forthcoming. Great answer.
@HDE226868 no apology needed - it is a good strategy to wait and see for a while.