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SciShow Space explains how different experts define our the boundaries of our solar system and why it's way more complicated (and interesting) than it sounds.

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I’m Caitlin Hofmeister and welcome to SciShow Space. 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? As in where does our stellar neighborhood stop and where does the vast expanse of space start? This question actually caused quite a kerfuffle in the summer of 2013 when many news outlets proclaimed that the Voyager 1 spacecraft, launched in 1977, had finally and officially left the solar system. While it’s clear that Voyager has traveled farther than any human-made thing before, it turns out you can’t actually say once and for all that it has crossed out of our solar system because there are different ways of deciding where our solar system actually ends. Space: turns out it’s pretty complicated.


So there are seven other planets, many dwarf planets, countless asteroids and comets, and, of course, Earth all orbiting the Sun, and we all have one thing in common that makes us members of the coolest club in the cosmological neighborhood: our gravitational attraction to the Sun. By most thinking, just orbiting the Sun is an easy and intuitive way to define the Solar System and its limits. But, the Sun’s gravitational attraction starts to fizzle out in part of the Solar System called the Oort cloud which is an enormous and odd collection of icy objects orbiting the Sun 150 trillion kilometers away. That’s 100,000 times farther from the Sun than we are. 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. By then I’ll just be, like, a preserved head in a glass jar... if there are glass jars.

But 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 outside the Solar System. The local interstellar medium, as it’s officially known, is made up of matter; gas and dust and particles that hang 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 kind of piles up suddenly like a driver slamming on the brakes and 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, putting the spacecraft into what one astronomer called “a mixed transitional region of interstellar space." Of course, that doesn’t have the same ring to it as “Voyager has left the building." So while we know Voyager 1 has entered interstellar space, saying it’s left the Solar System is actually misleading. It still 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. So now you know what the experts know and you can sit back and enjoy the rest of Voyager’s journey, and we’ll give you an update in about 300 years.

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