As is often the case in astronomy, a seemingly simple question does not necessarily have a simple answer. For example, where does our Solar System end? Furthermore, where does our stellar neighborhood stop, and where does the vast expanse of space begin?
This question actually caused quite a fuss in the Summer of 2013, when many news outlets proclaimed that the Voyager 1 spacecraft had finally and officially left the Solar System. While it’s clear that Voyager has traveled farther than any man-made spacecraft has before, it turns out there are several different ways of deciding where our Solar System actually ends.
Unsurprisingly, space is pretty complicated.
In our particular Solar System, there are eight planets (including Earth), many dwarf planets, countless asteroids, and comets, which all share one common denominator that makes us members of the coolest club in the cosmological neighborhood: our gravitational attraction to the Sun.
It’s easy to assume that the farthest celestial bodies orbiting our Sun are the things that determine the edge of our Solar System - which, yes, sounds simple and intuitive, but the Sun’s gravitational attraction starts to weaken near the “edge” of the Solar System, at a point we call the Oort Cloud, which is an enormous and odd collection of icy objects orbiting the Sun, 150 trillion kilometers away.
So that must be where the Solar System ends, right? Well, yes, but that’s not where Voyager is.
If we use the Oort Cloud as our definition of the boundary, then Voyager actually has a really, really, really long way to go. It won’t even enter the Cloud for another 300 years, and it won’t pass out of it for another 30,000 years.
Still, this isn’t even the threshold that got the media’s knickers in a twist. Another way of defining the Solar System is not by the Sun’s gravitational influence, but by the influence of its radiation.
Consider what’s out beyond the farthest planet, Neptune. The local interstellar medium, as it’s officially known, is made up of matter - gas and dust and particles hanging out in the areas of space in between star systems. Now, the Sun pushes this interstellar medium out of the way with a high-speed stream of charged particles called the solar wind.
This stream flows in all directions from the Sun, forming a bubble called the heliosphere.
The boundary of this bubble is where the solar wind can no longer overpower the outside pressure of the interstellar medium. And as the solar wind begins to interact more with the interstellar medium, it slows abruptly, forming a shock wave known as the termination shock.
Just beyond the shock wave, the Solar System appears to slam on its breaks, like a driver sending everyone flying forward. The area where this radiation bunches up is called the heliosheath. This continues until the pressures of the solar wind and the interstellar medium balance out, forming the final boundary known as the heliopause.
So it’s through this last border, the heliopause, that Voyager crossed in August of 2013. Ed Stone - a Voyager Project Scientist and Caltech alumni - calls it “a mixed transitional region of interstellar space."
Of course, that doesn’t have the same ring to it as “Voyager has left the building." But it's sufficient to say that Voyager has in fact entered interstellar space - it just has a rough ride ahead of it through the Oort cloud, and it’s going to be feeling the Sun’s gravitational pull for tens of thousands of years.
In the meantime, enjoy the journey, and cross your fingers in hopes that the information Voyager will bring back to us will be unprecedented. And be sure to subscribe to SkyFeed, so you’ll be notified when I give you an update in about 300 years.
Also, be sure to follow my social media (Instagram: @astrolia) for daily space memes and things.
Sources: This story was originally published on SciShow Space. I am republishing a lightly edited version on SkyFeed in light of interest in the subject. Hofmeister, Caitlin. “Where Does the Solar System End?” SciShow Space, YouTube. 22 April 2014. Web video.
Citation: Rovira, Lia N. "Where Does the Solar System End?" SkyFeed. 29 Jan, 2018. Web article.