What Counts as the Edge of the Solar System?

After reading this XKCD comic I wondered: what counts as the edge of the solar system? Will Voyager leave the solar solar system any more times? - Question posed by @_TK_O.

The comic referred to by @_TK_O

There's no hard-and-fast definition of the edge of our solar system, but most of the proposed definitions hinge on some aspect of the Sun's influence losing out to external forces. Here are some milestones on the way:

The Orbit of Pluto

Our solar system's most famous not-planet, Pluto, orbits at an average distance of just under 40 AU from the Sun. The Voyager spacecraft both passed the orbit of Pluto in 1989. This isn't widely thought of as being a good delimiter for the edge of the solar system because, whilst all of the big stuff is inside this perimeter, there's still plenty more stuff out there that can be said to be under the influence of the Sun in some way or another - The Kuiper belt extends beyond this distance, for example.

The Termination Shock

This is where the battle between the solar wind (the outflow of charged particles from the Sun) and the local interstellar medium starts to become apparent. The solar wind slows down to subsonic speeds, and a shockwave is formed, much like the one you see in an empty sink if you run a tap into it**. Voyager 1 passed through this point in 2004 / 2005 at a distance of between 75 and 90 AU.

The Heliosheath

The termination shock is the boundary to a region known as the heliosheath. Interaction with the interstellar medium causes further slowing of the solar wind, along with increased turbulence.

The Heliopause

This is the point in the heliosheath at which the solar wind's speed is reduced to zero as it is no longer strong enough to hold back the stellar winds of surrounding stars. Rapidly changing levels of radiation implied that Voyager 1 cleared the heliopause in August 2012, having entered it in 2010.

The Magnetic Highway

Between July and August, Voyager 1 appeared to enter and re-enter a region (implying that the edge of the region itself was washing backwards and forwards, like a tide) that became known as the magnetic highway. This region is where the Sun's magnetic field lines connect with interstellar field lines, allowing charged particles from within our heliosphere to leave to the interstellar medium, and those from the interstellar medium to enter our heliosphere.

A change in the direction of the magnetic field lines is an indicator that NASA is looking for with regards to having officially left the Solar system and, as of 20 March 2013, with Voyager 1 at around 120 AU from the Sun, this has still not been detected.

The Oort Cloud

Even out where the solar wind is a distant fairytale, the Sun still exerts an influence gravitationally. The Oort cloud is a hypothesised spherical region of space containing many billions of cometary objects - it is from this region that long-period comets are thought to originate. The inside of the Oort cloud is thought to begin at a minimum of 2,000 AU away: that's nearly 200 times further from the Sun than Voyager 1 has currently travelled. The outermost edge of the Oort cloud is thought to be 50,000 AU from us, which is getting on for a light year away.

To put this in perspective, Voyager 1 could take anything up to 28,000 years to get that far.

* Lying between Mars and Jupiter.

** The ring surrounding the point at which the water hits the sink is the 'termination shock' for this system, and happens where the outflowing water slows down to a point at which it is overcome by the pressure of the water flowing back down the basin.