Where Does the Solar System End?

Humans want to explore the farthest reaches of outer space in the eventual hopes of finding other habitable, Earth-like planets. These planets could harbor life and put an end to the debate on whether or not we’re alone in the universe.

However, navigating to a distant star system means first leaving our own solar system. But where exactly is the boundary between our solar system and interstellar space?

Over the centuries we’ve accrued a vast amount of knowledge about our own solar system. We’ve put an age to the sun, classified all of the planets, charted trajectories in orbit to a precise degree, discovered the moons orbiting those planets, and even seen pictures of other worlds.

Despite this, we’re still not exactly in agreement as to what the solar system is, or more specifically, where it ends. It doesn’t have a clear edge or border, and deciding on one depends on what criteria you’re using. Identifying a cut-off point is important for classifying sizes of distant solar systems and having consistent measurements of our own. This question is made more difficult by the fact that we don’t have our own solar system completely mapped yet.

The Oort Cloud is the farthest structure around the Sun, home to billions of icy rocks that occasionally fall towards the Sun and become comets. But that’s purely hypothetical, as we’ve never even seen it.

Voyager 1 is the most distant spacecraft that we have in space, but even it is some 300 years away from reaching the starting point of the Oort Cloud. In the meantime, both Voyager 1 and 2 have run into formations that scientists had no idea about.

Back in 2007, Voyager 1 came across massive, foamy magnetic bubbles about 100 million miles wide. These gigantic structures, around 9 billion miles from Earth, came as a complete shock to the scientist. They later realized that the bubbles were actually part of the Sun’s magnetic field. Because the Sun spins, the outer edges of this field become crunched and knotted.

The discovery shows that there could still be parts of our own solar system that we’re not even aware of yet. Still, we have a few sets of criteria for deciding the end of the solar system. If you take it by mass, Neptune, the last planet in orbit around our Sun, could be considered an ending point. Although this cuts out the Kuiper belt, there is some logic behind it.

The Sun accounts for 99% of all the mass in the solar system itself, and the remaining 1% of the mass is overwhelmingly occupied by Jupiter and the gas giants. Comparatively, there’s barely any mass whatsoever past the gas giants, so it can be considered debris.

Carl Saga even once referred to Neptune as the “frontier of the solar system.”

From a practical perspective, it makes some sense, because although there is scientific merit in studying asteroids, we’re far more interested in the planets and their moons. However, cutting out the Kuiper Belt from the solar system seems unwarranted. Despite its relatively small amount of mass, it’s still home to many dwarf planets, including pluto.

The Kuiper belt is home to the last verified satellite in orbit around our Sun, and it’s also massive, with a width that extends from 30 to 50 Astronomical Units.

One Astronomical Unit is roughly the distance between the Sun and the Earth.

Many of the closer ranged comets come from the Kuiper Belt, and it could be the reason that life exist on Earth in the first place. Life itself could have been seed by a stray asteroid, and it’s thought that water arrived on Earth due to massive frozen comets and asteroids crashing into Earth’s surface.

The Kuiper Belt’s importance in the solar system means it arguably deserves to be included. Of course, if we define the solar system as everything captured by the Sun’s gravitational pull, we can extend it all the way out to the Oort Cloud.

The Sun’s pull is sometimes called the Hill or Roche Sphere and is the region around a celestial body within which it’s able to trap satellites. Adding in the Oort cloud would massively increase the size of our solar system.

The Oort cloud’s dimensions are unknown, but estimate put its outer edge at up to 200,000 Astronomical Units from the Sun. Home to trillions of objects, it’s distinct from the Kuiper belt in that it’s a cloud, rather than a belt, extending in a giant sphere or bubble around the entire solar system.

This also gives a handy cutoff point for the Solar System, marking the last challenge or obstacle that travelers looking to exit our solar system would have to pass through.

Beyond the Oort cloud, there isn’t much of anything besides gas and dust for a long long way. Then again, we could also define the limit of the Solar System in terms of the Sun’s heliosphere — placing it between the Kuiper Belt and the Oort Cloud.

The heliosphere is a giant bubble of charged particles emitted by the Sun, known as the solar wind. In 2012, Voyager 1 became the first man-made object to pass the edge of the heliosphere. Located about 120 AU away, the heliopause is the divide between the solar wind and the interstellar medium and marks a clear dropoff in the Sun’s influence.

Voyager 1 was widely described in news outlets as having “departed the solar system” despite being a long way still from the Oort Cloud. In 2018, Voyager 2 followed suit, and was also said to have “left the solar system.”

So, which definition is right?

Well, the Oort cloud remains theoretical, but technically, it does better fit standard definitions of the “solar system”. According to the Merriam Webster dictionary, the “solar system” is “the sun together with the group of celestial bodies that are held by its attraction and revolve around it.”

NASA describes it as “our star the Sun, and everything bound to it by gravity.” The heliopause marks the end of the Sun’s stream of charged particles, but NOT its gravitational pull. In the end, there are several different ways to define the solar system, and marking the exact edge comes down to what criteria you use. And that’s where the solar system ends.