Oh, we humans do love a cleanly defined boundary, don’t we?
They make things easier, after all. If we’re trying to categorize something, knowing what labeled bin to put it in is handy. If we’re looking for trends, then sharp boundaries are even better because they let us compare things in a single category to see how they change.
This tendency, though, can lead to trouble. It can mislead us or cause confusion. Especially when we take something that is fundamentally fuzzy and indistinct and try to ram its square peg into a round hole.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Take, for example, the solar system.
If you picture it in your head, you likely see the sun in the center and a retinue of planets orbiting it. At some point, perhaps around four billion to five billion kilometers out, roughly equivalent to the orbital distance of Neptune, you might draw an imaginary line: everything within that line is inside the solar system, and everything beyond that line is outside it.
You may see where I’m going with this. That line you drew in your head is arbitrary and, I dare say, even wrong.
First, well past this distance, there are icy bodies called trans-Neptunian objects (TNOs) that are nonetheless still bound to the sun by gravity. Some TNOs orbit the sun in a flattish torus called the Kuiper belt, and others orbit much, much farther out from a very roughly spherical halo called the Oort cloud that potentially stretches for a trillion kilometers around our star. On that scale, even the outer planets orbiting the sun seem huddled close-in.
And second, well, setting such outer limits depends on how you define what the solar system is and what’s outside it.
I was reminded of this because of a space news story that came out just last week, and it’s good news (a rare gem): engineers have been able to get Voyager 1 talking to Earth again. The deep-space probe was launched in 1977 and is now a staggering 24 billion kilometers from Earth, which is more than 160 times farther away from our planet than the sun is. Last November the spacecraft suffered a hardware glitch that scrambled its communications, and engineers had to get clever by rerouting software around the bad component. After they uploaded the fix, Voyager 1 appears to be working better, and they expect it will be back to full operational duty in the next few months.
This reminded me of something that happened in September 2013, when Voyager 1 was “just” 19 billion kilometers from Earth: NASA announced that the spacecraft had entered interstellar space in August 2012. At the time, a lot of people talked about how Voyager had finally “left the solar system.”
And here’s where we run into that second issue of where the solar system “ends.” By any real definition, even the fuzzy ones, Voyager 1 was still well within the solar system—certainly, it was (and still is, and will be for some time) closer to the sun than most of the TNOs in the black depths of space—yet NASA was correct: Voyager 1 is also in interstellar space.
How can this be?
This confusion arises because of two different ways of thinking of what defines the solar system. In this case, we’re comparing the sun’s gravitational influence, exerted upon the objects orbiting it, and its magnetic influence, delivered to deep space by its solar wind.
The solar wind is a stream of subatomic particles the sun continuously blows into space. It flows away from the sun at high speed, nearly two million kilometers per hour, and consists of electrons, protons, neutrons and some heavier atomic nuclei as well. It’s not clear what accelerates the wind to such high speeds. Scientists know the sun’s magnetism is the driving force, but the exact mechanism still isn’t understood.
If space were truly empty, the solar wind would expand forever, flowing out into the galaxy and, because it moves at such high speed, eventually exiting the Milky Way entirely. But space—despite the name—is not empty. The vast volume between the stars does in fact have matter in it. It’s not much, to be sure: roughly one subatomic particle per cubic centimeter on average (although that can change hugely depending on where exactly in space you are). The air you’re breathing right now is some 10 quintillion times denser, so this interstellar matter is thin gruel indeed, but it’s enough.
As the solar wind plows into this ethereally thin cosmic vapor, it loses momentum and slow down, eventually coming to a halt. This region where it stalls out, poetically called the heliopause, marks the exterior boundary of the heliosphere, the volume of space dominated by the sun’s solar wind. Within the heliopause region, the sun’s magnetic influence wanes and that of the interstellar medium—the material between the stars—strengthens.
This shift is just what Voyager 1 detected in 2012. Several measurements showed that the interstellar medium dominated the region of space the spacecraft was passing through and that it had left the heliosphere behind.
So while Voyager 1 was still well inside the solar system, the space around it was influenced more by the galaxy itself than the sun.
As usual, when dealing with scientific matters, you need to be careful to define your terms.
And in the interest of open scientific honesty, I’ll admit I’ve made this mistake myself. I wrote in early 2013 that Voyager 1 had left the solar system when, in fact, NASA said at the time that it had not. (This happened so often in media over the years that the webcomic xkcd, in its usual cheeky style, had something to say about this topic as well. NASA later confirmed after reviewing its data that the spacecraft had actually entered interstellar space in 2012.) But I also pointed out at the time how hard-and-fast definitions of even where the sun’s heliosphere ends are complicated and difficult to pin down. These regions are squishy and in flux, lacking any easily measured delineation.
If any of this sounds familiar, that’s because it’s reminiscent of pondering where Earth’s atmosphere ends and outer space begins—a quandary encapsulated by the debate over something called the Kármán line. I covered this in a recent article, and there are some similarities; in both cases, we’re dealing with a sort-of atmosphere—the heliosphere carved out by the solar wind and Earth’s enveloping shroud of air—and where it impinges on the environment of deeper space. The difference here is that Earth’s atmosphere fades away gradually with altitude, blending seamlessly with the near vacuum of space, whereas the heliosphere does have a boundary. That neutral zone (to borrow a Star Trek–ism) is wide, certainly—tens of billions of kilometers through—but it’s small compared to the immense size of the heliosphere itself.
With Voyager 1 having long passed the interstellar version of the Kármán line, it’s well on its way into the galaxy. It will hopefully continue to take measurements of the interstellar medium and begin transmitting them back to Earth soon once again. Even after 46 years, it’s still breaking boundaries.