ObserveTheSky › Explanations › Where does the solar system end?

Where does the solar system end?

Ask three astronomers and you get three different answers. It depends what you mean by end. Do you mean the last planet? The edge of the bubble the Sun blows? Or the last lump of ice still going round it? Those three edges sit at 30.07, 121.6 and perhaps 100,000 times the Earth’s distance from the Sun.

The first answer 30.07 AU · the last planet

Neptune first, and after it the Kuiper belt. That is a ring of icy leftovers, from 30 out to about 50 times the Earth’s distance. Pluto is one of them. Most maps of the solar system stop here, and there is a good reason. It is the last part anybody can draw as a picture of worlds.

It is also the answer that stops too early. Past 50 AU there is almost nothing left big enough to draw. There is still plenty of solar system.

The second answer 121.6 AU · the heliopause

The Sun does not only shine. It also blows. A thin, fast wind of particles streams out in every direction, at hundreds of kilometres a second. That wind does not fade away politely. It keeps pushing outwards, and the thin gas between the stars pushes back. Far out, the two shoves balance. There the Sun’s wind stops — and it stops at a surface, not at a point. That surface is the heliopause. Everything inside it is the Sun’s own bubble.

Two spacecraft have crossed it, the only two things we have ever sent that far. Voyager 1 went through on 25 August 2012, at 121.6 times the Earth’s distance from the Sun. Voyager 2 followed on 5 November 2018, at 119 times, and in a different direction. Both had been flying for thirty-five years and more to get there.

How anyone knows nobody saw a wall

There is nothing to look at out there. No line, no shimmer, nothing a camera could show you. The crossing was not seen. It was counted.

Voyager 1 carries detectors that count particles. Near the edge, two of those counts changed at once. The count of particles coming from the Sun fell away almost to nothing. The count of particles arriving from the rest of the galaxy jumped up. The bubble had been holding most of those off, and now it was not. Two counters moving opposite ways at the same moment is a much better sign than either one alone.

The measurement that settled it is the odd one. Voyager 1 carries an instrument for measuring how thick the gas is. It had broken in 1980, thirty-two years before anybody needed it. So the answer came by listening instead. In April 2013 a burst from the Sun caught up with the spacecraft. It set the gas around it ringing, like a struck bell. How high a gas rings depends on how tightly packed it is. The note that came back said the gas outside was about forty times thicker than the gas inside. Thicker, colder, calmer: the gas between the stars, not the Sun’s.

Six years later Voyager 2 crossed, in a different direction and with a working instrument. It measured the same jump directly. One measurement is an argument. Two, from two places, is a fact.

At the crossing	Inside the bubble	Outside
Particles from the Sun	many	almost none
Particles from the galaxy	held off	the full count
How thick the gas is	thin	~40× thicker
Anything you could photograph	no	no

The third answer up to 100,000 AU · the Oort cloud

Being outside the Sun’s wind is not the same as being free of the Sun. Far past the heliopause sits the Oort cloud. It is a huge swarm of icy lumps, in a shell that may reach a hundred thousand times the Earth’s distance. They are still going round the Sun. They are still held by its gravity. That distance is a third of the way to the nearest other star.

Nobody has ever seen an object while it is out there in it. We know it is there the way you know there is a wasps’ nest in the garden: by where the wasps come from. Comets on very long orbits arrive from every part of the sky. They do not stick to the flat plane where the planets go. Trace their paths back and they run out to roughly that distance. So it is a shell rather than a ring. It is also the only part of the solar system we have never had a picture of.

Voyager 1 crossed the whole bubble in thirty-five years. On the same course it needs several hundred more just to reach the inner edge of the Oort cloud. Coming out the far side may take thirty thousand. Its power runs out some time in the 2030s. It will do all of that as a cold, silent piece of metal.

So which answer is right?

All three:

If the solar system means	Then it ends at	Distance
a set of worlds	Neptune and the Kuiper belt	30–50 AU
a bubble blown by the Sun	the heliopause	119–121.6 AU
everything the Sun still holds	the outer Oort cloud	~100,000 AU

The gap between the first answer and the last is a factor of about three thousand.

The bubble is not a fixed wall and not a sphere

The Sun’s wind gets stronger and weaker over an eleven-year cycle. The bubble breathes in and out with it, by several AU. The whole bubble is also drifting through the gas around it, at about 25.4 kilometres a second. So it is not round. It has a blunt nose where it ploughs forward, and probably a long tail behind — the shape water makes around a stone in a stream. That is part of why Voyager 1 and Voyager 2 crossed at 121.6 and 119 AU: they left in quite different directions.

Try it yourself no telescope needed

The tap

Turn a kitchen tap on gently, so the water lands on the flat bottom of the sink. It spreads out in a smooth, fast, glassy disc. Then, at a certain distance from the middle, it changes all at once. It humps up into a ragged ring and slows down. That ring is the same kind of edge as the place, just inside the heliopause, where the Sun’s wind suddenly slows down. Fast stuff flows outwards. Slower stuff sits there and refuses to move. The edge forms exactly where the two shoves balance. Turn the tap up and the ring moves out. Turn it down and it comes in. That is the Sun’s eleven-year cycle in a sink.

Walk to the edge

Count one pace for each AU. Neptune is thirty paces away, about the width of a playground. The heliopause is a hundred and twenty-one paces: a long corridor, or a short street. The far side of the Oort cloud is a hundred thousand paces. That is about seventy-five kilometres, or two long days of walking. And even there you would not have left the Sun behind. On the same scale, the nearest other star is two hundred kilometres from where you started.

Everything out to where the Voyagers are now has been visited. Everything past that has only been reasoned about. See also what is left to explain.