My nine-year-old daughter can’t get over the fact that, as of now, we can’t detect any boundary to the universe. Sounding a bit like a mesmerized stoner, she rarely misses an opportunity to mention how much it blows her mind to try to imagine a borderless universe. So she’ll be either relieved or disappointed to learn that unlike the vast expanse of the entire universe, our solar system does have a border. What’s more, unlike the imaginary political borders humans draw on maps, our solar system’s edge is likely defined by a physical ‘wall’ of faintly-glowing hydrogen atoms.
The space filled by solar wind
This model is based on the idea that our Sun is constantly emitting not just light and heat, but charged particles as well. These high-speed protons, electrons and other ions that are blasted out of the Sun’s corona are collectively called solar wind. Solar wind isn’t emitted at a perfectly regular rate, but there’s enough of it being pushed out across the solar system that it’s almost like the Sun is inflating a large bubble from the inside.
That bubble is called the heliosphere, and seems to have a definable edge. 100 times further than the Earth is from the Sun, solar wind starts to encounter stray hydrogen atoms from outside the solar system. These neutral, interstellar atoms provide just a bit of resistance to the solar wind, and the two types of particles collide often enough to create a detectable amount of ultra-violet light. The meeting area between stray particles probably isn’t the most clear-cut boundary you might imagine, but the area where our Sun’s output meets objects from outer space should at least be directly observable.
Spotted by spacecraft
This light from the edge of the heliosphere, or heliopause, was first detected by the Voyager 1 spacecraft 30 years ago when that craft exited the solar system. Voyager 2 is seeing similar evidence, although it won’t actually cross the heliopause until 2030. The New Horizons spacecraft has started picking up traces of evidence that further support those observations, even though it’s still in the Kuiper belt.
The clearest proof of this model would be attainable when a spacecraft passes through the heliopause. Once on the other side, the ultraviolet light would immediately drop off, sitting squarely behind something like Voyager 2 as it continues to travel away from the Sun. Scientist note that this may not turn out to be the case, and that there may be an unknown source of ultraviolet light in the space beyond our solar system. However, with New Horizons backing up what has previously been found by Voyager 1 and 2, there is a strong likelihood that our Sun’s sphere of influence does a pretty good job of marking itself at the edge of the solar system.
Source: New Horizons may have seen a glow at the solar system’s edge by Lisa Grossman, Science News