Recent debates over whether or not the Voyager 1 spacecraft has 'left the solar system' typically leave out some critical details. The limits of the Sun's particle radiation is not the physical edge of the contents of the solar system, but it is the point of changeover to the exceedingly tenuous atmosphere of matter and magnetic fields that fills the space between the stars in our galaxy.
It's been a hot topic recently because the Voyager 1 spacecraft, after 36 years of cruising away from us, now seems to be passing through a zone described as the 'magnetic highway', where the Sun's magnetic field lines connect to those pervading interstellar space in our galaxy. This marks a transition where Voyager is going to move beyond the heliopause - where the endless flood of particle radiation streaming from our sun - stuff like electrons and protons - has become sufficiently diluted that it no longer pushes through the tenuous particle radiation and matter between the stars. This is the entry point to the interstellar medium, in a sense the atmosphere of the Milky Way.
But this has led so some misleading comments about 'the end of our solar system'. NASA recently released a statement on March 20th, 2013 to try to clear up the confusion about Voyager's whereabouts:
"The Voyager team is aware of reports today that NASA's Voyager 1 has left the solar system," said Edward Stone, Voyager project scientist based at the California Institute of Technology, Pasadena, Calif. "It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space. In December 2012, the Voyager science team reported that Voyager 1 is within a new region called 'the magnetic highway' where energetic particles changed dramatically. A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed."
However, I feel compelled to write this post because even NASA's statement doesn't quite manage to explain what's different between 'leaving the solar system' or 'reaching interstellar space' - and there is a difference, for this solar system or any other.
Like any normal star, the Sun creates what is in effect a bubble of its own effluvia. It does this by exerting a pressure against the interstellar atmosphere, but the further away you get the weaker this pressure. Exactly where the solar pressure become equal to the surrounding pressure depends on a lot of factors. It depends on things like magnetic fields that interact with electrically charged matter, it also depends a great deal on exactly where we are in the galaxy and the local density of this interstellar atmosphere - which is always changing as we follow our lengthy galactic orbit, a journey of about 230 million years.
Regions of denser interstellar atmosphere, or medium, are expected to drastically shrink the size of the Sun's bubble, bringing the heliopause in close. Variations in the Sun's activity will also effect this. In other words, the heliopause is not some fixed 'edge' to our solar system, and it just happens to occur out beyond all the major planets at this time.
It is however the place at which the Sun's extended atmosphere gives way to the atmosphere of the galaxy at large. So while one can say that beyond this is interstellar space, that's a little confusing. It doesn't mean that one is leaving the solar system, just that one is now bathed in a different medium - the interstellar medium, not the solar medium.
But if the heliopause, and Voyager 1's present location just within it, is not the edge of the solar system, where does the solar system actually end?
It's a good question. In my opinion the most physically sensible marker is the distance from the Sun at which the Sun's gravity can no longer sustain objects in long-term stable orbits. In other words it's the distance at which the gentle tug of other stars, and even the subtle variations in the net gravitational field of all the matter in the Milky Way, can perturb and destabilize the trajectory of any lonely, cold, bodies out there.
The problem is, much as with the heliopause, that this may not always be at the same distance from the Sun. Again, our movement around the galaxy and the hither-and-dither of other stars and objects result in a slowly shifting gravitational landscape. Nonetheless, rough estimation suggests that it's about 1 light year away. Not coincidently this is also the speculated outer edge of the Oort cloud - a vast structure consisting of trillions of icy chunks of detritus flung outwards while our planets formed some 4.5 billion years ago.
The Oort cloud is the likely origin of very long-period comets - objects that come falling inwards with orbits that may take anywhere from hundreds of years to millions of years to complete. For example, Comet West was last seen in 1976. If you want to see it again you'll need to wait about 6 million years, since it's now swinging back out to its far point of some 1.1 light years away.
All of which means that while poor old Voyager may have begun to taste a little fresh galactic breeze in its face, it's still a very, very long way from passing beyond the wealth of islets that separate us from truly open interstellar space.
In fact, since one light year is about 63, 240 astronomical units (AU) from the Sun, and Voyager is currently about 124 AU from the Sun, moving away at approximately 3.6 AU a year, it will take another 17, 500 years (give or take) for it to move beyond the outer Oort cloud.
Reports of Voyager having left the solar system are therefore a little premature.