Where’s the Edge?

Article September 12, 2025

Today we’re going right to the edge! Or edges, actually. Or, well, where the edges of things should be. Or might be. Or…okay, you know what, it turns out defining the edges of things out in space is notoriously hard.

So today we’re going to explore the boundaries of things: where one thing ends and another thing begins. And because a lot of this is very fuzzily defined if it is defined at all, there are times when we might get a little pedantic or even argumentative. Fair warning.

To the edge, y’all!

Earth

We’ll start with something easy: where does Earth end? Duh, it ends with the ground! Except…wait, the air is definitely a part of Earth and the ground and atmosphere are an undeniably integrated system. So no, Earth doesn’t end with the ground, it has to end with the edge of the atmosphere.

And here’s our first fuzzy boundary—there is no universally agreed upon point on which Earth ends and space begins. There is something called the Kármán line which is frequently used, but it’s somewhat arbitrary. The Kármán line is 62 miles (or 100 km) above sea level. While there is no universally accepted definition for the beginning of space, most folks will agree that if you cross the Kármán line you’ve made it.

Earth’s atmosphere has many layers and whether you choose to use the Kármán line to define the edge or not, space is usually reckoned to begin before Earth’s atmosphere officially ends. Credit: NOAA

But you won’t be above Earth’s atmosphere, not all of it at least. You’d be in the section called the thermosphere. Air is so thin up here that by all sensible definitions this is space. It’s where you’d find the International Space Station hanging out. But given time you will still feel atmospheric drag which is why the ISS needs to have its orbit boosted from time to time. So it’s definitely space, but it’s still a little of Earth too.

And there’s an argument to be made that “space” starts before you reach the Kármán line, an argument that came to a head with the first launches of Blue Origin’s New Shepard rocket and Virgin Galactic’s VSS Unity with passengers, which happened within weeks of each other in 2021. New Shepard crossed the Kármán line, but Unity didn’t.

Unity did, however, get above 50 miles which for the US Air Force is defined as being in space. And it’s high enough that Unity was required to maneuver the way a spacecraft does, with jets, rather than with airfoils like an airplane. Virgin Galactic’s argument (and the Air Force’s, apparently) is that if you have to fly like a spacecraft you’re in space. It’s a fairly solid argument in my personal opinion.

So what is the boundary between Earth and space? Eh, nobody can say for dead certain. But, you know, if you go 100 km up, there’s a decent chance you’ve crossed it.

The Solar System

If you’ve ready some of my other blog posts, then you already know we don’t have a definition for where the solar system ends. As with the boundary between Earth and space, there are a couple of different definitions and I have my personal favorite.

Here’s a great place to get pedantic, because we do have a definition for the edge of interstellar space! It is everything outside the heliosphere, which is the area of space where the solar wind you’d feel most would be that coming from the Sun, not other stars. When you’re feeling other stars more, you’re in interstellar space. So there we are, an edge to the solar system, right?

The edge of the heliosphere is not, in fact, a sphere, but represents the area of space dominated by solar winds from the Sun rather than the interstellar medium. Credit: NASA/JPL-Caltech

Well…here’s the thing. The heliosphere isn’t actually a sphere. It’s very unevenly-shaped, because the Sun is moving through interstellar space as it goes around the Milky Way. This means that the distance to the edge of the heliosphere is very short if you look in the direction the Sun is moving, like the sharp front edge of a comet, caused by the solar wind smacking directly into the incoming interstellar medium. And then behind the Sun, just like a comet’s tail, the heliosphere is wide and long.

You may have heard that humanity’s farthest spacecraft, Voyager 1 and Voyager 2, have left the solar system. What they’ve done is left the heliosphere, and that’s only because they were moving in the direction where the heliosphere is narrowest, almost directly in the direction the Sun is moving. If they had happened to be moving in the opposite direction, they’d be within the heliosphere for many, many years.

Oort Cloud objects are the farthest-out objects controlled by the Sun’s gravity, which is a point in favor of marking the edge of the Oort Cloud as the edge of the solar system. Credit: NASA

And then there’s the fact that there are things controlled by the Sun’s gravity that are outside of that front section of the heliosphere. We don’t know for sure how far out from the Sun the Oort Cloud goes, but it could be up to a couple of light years. That would put part of it outside the heliosphere and part of it inside of it. And those Oort Cloud Objects are orbiting the Sun, which means they’re controlled, however weakly, by the Sun’s gravity.

In my opinion, it’s hard to argue that only the part of the Oort Cloud within the heliosphere is in the solar system, while the other part is not. I say any definition of the solar system’s edge should either fully include or fully exclude the entire Oort Cloud, and it feels like stuff gravitationally controlled by the Sun belongs to the Sun. I’m not the only who feels this way and prefers to think of the edge of the Oort Cloud as the edge of the solar system, but that’s the definition I like best (although that only sort of clears things up since, as I said, we don’t actually know how far out the Oort Cloud goes…but we wouldn’t want things to be too easy, would we?).

Milky Way

Figuring out the edge of our home galaxy has two big huge whopping obstacles right from the get-go. For one thing, the Milky Way isn’t a solid object, but a conglomeration of stars, gas clouds, dust clouds, black holes, planets, and a whole bunch of other random stuff. So that was always going to be tricky.

Then there’s, you know, the fact that we’re stuck inside of it. Imagine trying to find the edge of a cloud that you are in the middle of. It ain’t easy, so the fact that we have any sort of idea where the Milky Way might end is a testament to the precision of our telescopes and astronomers.

But also, and stop me if you’ve heard this one before, we don’t really know where the Milky Way ends. You will usually hear the Milky Way described as a flat spiraling disk somewhere between 100,000 and 150,000 light years across, and about a thousand light years thick. These are all rounded to the nearest thousand or so light years to keep the numbers easy to picture and therefore make no attempt to be very precise about the edge of the Milky Way. As a rough description, this does fine—as long as you’re only worried about the part of the galaxy we can see.

Our Milky Way, as well as all other galaxies, is thought to be surrounded by an invisible halo of dark matter. Credit: NASA-ESA-A. Feild

Because here’s the catch: we think most galaxies, including the Milky Way, are surrounded by extended halos of dark matter. The way the Milky Way rotates suggests that the mass of it we can see cannot be all the mass of it there is—it would be rotating differently otherwise. Of course, notoriously we cannot see dark matter. So if you consider this halo to be part of the Milky Way, it means we are physically unable to see the edge of the galaxy and have to make a guess at its size based on the way its mass effects the visible parts of the galaxy. Easy peasy.

Our best guesses at this point suggest that the Milky Way’s dark matter halo may extend over a million light years out, and perhaps as much as two million. This complicates things even more because that means several of the other members of the Local Group of galaxies, themselves undeniably distinct entities, are within the Milky Way’s halo and our halo may or may not be already mixed up with Andromeda’s. There’s a reason why folks usually only stick to the visible part of the galaxies when describing their edges.

The Universe

Oh boy. We have no idea if the universe has an edge. It might not. Or it might. We don’t know and if you want to start an argument among a group of cosmologists go ahead and ask them this question. What we do have is an edge to the part of the universe we can see.

Even our best telescopes, from Hubble To Webb to Rubin, have a limit on how far out into the cosmos they are able to see. This is partially a limitation on the resolving power of our instruments and partly because light takes time to travel and the universe has only been around so long and has been making itself bigger that whole time. So if there is an edge to the universe, we’ll never be able to see it.

It’s impossible to put the actual observable universe all into a single image, but this artist’s rendition of it is certainly pretty to look at. Credit: Pablo Carlos Budassi/Wikipedia

But the edge of what we’re able to see is the edge of what we call the visible or observable universe. It’s 92 billion light years across. I can hear you asking how we can see farther than 13.8 billion light years when the universe is only 13.8 billion years old which means that the maximum amount of time light has had to travel.

Remember that whole universe expansion thing. We’re seeing things where they were when the light we’re observing left them, which may be where they were over 13 billion years ago in some cases. But they’re not there anymore, they’ve been riding an expanding universe for all that time. That means that things we’re seeing at the edge of the visible universe have, in the intervening eras, moved over 30 billion light years. That means we’re seeing things that are currently up to 46 billion light years away from us and the whole shebang is 92 billion light years across.

And that’s still not the edge. But it’s what we’ve got to work with so we’ll just have to be satisfied with that.