Podcaster: Avivah Yamani

Title: What is the boundary of the Solar System?

Organization: 365 Days Of Astronomy

Link :

The limits of Our Solar System:

Chasing the edge of the solar system

Description: Today’s question  What is the boundary of the Solar System?

Bio: Avivah is a project director of 365 Days Of Astronomy and astronomy communicator from Indonesia.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2015, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at


Hi! Welcome to the Q&A of 365 Days of Astronomy. I’m Avivah your host today. Today’s question

What is the boundary of the Solar System?

Hmm… good question. I wonder about it myself.

Back in my childhood at least, I knew that the boundary of my home and the neighbours was the fence. And at school I also learned about the boundaries of many countries, which are mostly invisible on the ground, but at least we know which cities are in each country.

But how exact are these boundaries? Not like a football field or a table where we know exactly their boundaries.

Now when we think about our solar system, the question arises. Where is the edge of the solar system? Where is the exact place we can really say this is the boundary of the solar system?

The first thing to do is to define the boundary itself because the edge of the solar system can be defined in many ways.

As we know, the Sun contains 99,8% mass of the solar system and dominates it gravitationally. This means that the solar system extends as far as the influence of the Sun as is is the main component we should consider to define the boundary.

So what’s the limit for our beloved solar system? It could mean that the boundary relies on the influence of the Sun’s light, or the influence of the Sun’s gravity, or the influence of the Sun’s magnetic field and solar wind.

Let’s check these one by one.

If we said the solar system boundary relies on the influence of the Sun’s light, then we have to consider that the light from the Sun gets fainter as we move farther away. But where does the sunlight stop or suddenly get weaker? None can answer that.

What we know is that the inverse square law states that the farther we are from the Sun then the fainter is the sunlight. We can see this in the Kuiper belt where the sunlight is weaker and the temperature decreases as well. That’s the place for icy bodies.

So next is gravity.

If we said that the Sun’s gravity is the limit then we must define where the Sun’s gravitational force stops working. Just like light, the influence of the Sun’s gravity extends without limit, although it gets weaker farther away from the Sun. So faraway objects will be weakly bound to the Sun.

Even though we can’t really define where the Sun’s gravity stops working, we can say that the Oort cloud is the edge of the Sun’s orb of physical, gravitational, or dynamical influence.

The Oort cloud is a spherical shell of icy bodies surrounding the solar system. Basically it is the comet reservoir at the outer edge of solar system. Here objects are weakly bound to the Sun and passing stars and other forces can readily change their orbits, sending them into the inner solar system or out to interstellar space.

But still there is not a hard boundary at which it stops.

That’s about the light and gravity of the Sun.  How about the influence of the Sun’s magnetic field and solar wind?

What’s the solar wind?

Our Sun continuously emits a “wind” of material outward in all directions, typically at speeds of about 1.6 million kilometers per hour. The solar wind is composed mostly of charged particles, such as electrons and protons. It also carries the Sun’s magnetic field.

As the solar wind streams away from the Sun, it races out past all the planets, past Pluto and all Kuiper Belt objects, and toward the space between the stars more than 16 billion kilometers away. This is not empty space. It is filled with matter that we call the interstellar medium, the material between the stars in our galaxy.

The interstellar medium is not uniform, but is composed of many clouds with different densities, temperatures, magnetic field strengths and directions, and flow speeds and directions. it’s a very thin mix of cold hydrogen gas, dust, ionized gas, and traces of other material, that comes from exploded stars and the stellar wind of other stars.

Once the solar wind reaches the interstellar medium, it will encounter the interstellar clouds. When the magnetic fields of the solar wind hit the magnetic fields of the interstellar medium, they do not intermix.

The expanding solar wind pushes against the interstellar medium, clearing out a bubble-like region in interstellar space. This bubble that surrounds the Sun and the solar system is called the heliosphere.

The boundary of that bubble is where the solar wind’s strength exactly matches the pressure of the interstellar medium. We call it the heliopause, and it’s often considered to be the very outer edge of our solar system.

Thank you for listening to the 365 Days of Astronomy Podcast.

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365 Days of Astronomy

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