At the Edge of the Solar System – Part I – The Final Frontier

What is beyond the outermost planets? How is it out there?

We know that the sun emits plasma a.k.a. coronal gas or solar winds. This plasma forms and maintains a bubble that encompasses a large area that extends well beyond Neptune.

This bubble is called the Heliosphere. (This bubble of the other stars is called an astrosphere).

At the Edge of the Solar System – The Final Frontier


Picture Credit: Adler Planetarium IBEX

For a moment let us step into the Milky Way galaxy.

We know that the sun travels around the center of the Milky Way galaxy at a tremendous speed.

Due to this, like the hair blowing backwards when we run, the heliosphere also streams backwards like a wind-sock.

While the sun is travelling around the Milky Way Galaxy all the other stars are also travelling.

Therefore we now have other astrospheres (plasma) streaming behind.

Adding to the forces of the astrospheres are leftover dust and electromagnetic material from dead stars that are also moving along.

The combined forces of the moving astrospheres and leftover dust and electromagnetic material are referred to as interstellar medium and those near the sun are referred to as the Local Inter Stellar Medium (LISM). It is believed that the main LISM for our solar system is from our closest neighbor Alpha-Centauri.

Going back to our Heliosphere…


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The following are parts of the Heliosphere.

Termination shock

The solar wind moves very fast, at supersonic speed from the Sun in all directions and abruptly slows down at a point. This creates a shock wave known as the termination shock.

At the termination shock the fast winds start gathering and form a sort of demarcation point.

After this point the Solar winds become subsonic and some LISM also mingle with the Solar winds.

The Solar System termination shock area is believed to be 75 to 90 astronomical units from the Sun (15 AU wide)

We can also observe termination shock in the below picture where water is turned on the sink.


The ring formed where the water flow slows down is the termination shock which is what takes place in the Solar system and other stars also.


Beyond the termination shock, once the Solar wind slows down, it gets compressed and becomes very turbulent because the interstellar winds start to interact here.

This area is about 20AU wide and is about 80AUs from the Sun


The Heliopause is the boundary where the Sun’s solar wind is completely stopped and is not strong enough to push back the interstellar winds of the surrounding stars.


The heliotail is caused by the streaming of the solar winds against the forward movement of the sun in the Milky Way galaxy and eventually escaping into the interstellar medium. Like the wind sock.

Bow shock and bow wave

What is a Bow shock?

When the speed of the object is faster than sound (supersonic) we can actually hear and see the bow shock also called as sonic boom.

Here are some bow shock images.

The white cloud we see surrounding the rocket and the plane is the bow shock.


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Picture courtesy:

Have you seen the curve of water that flows on the sides of a ship on water, a duck or a swimmer moving on water? Even if we run our hand on water we see this ripple which is actually a bow wave.

Below is a bow wave picture:


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It was named thus because of the wave that was created by the bow of a ship when it moves through water.

Initially it was believed that interstellar medium was moving supersonically and was creating a “Bow shock”.

However, in 2012 the Interstellar Boundary Explorer (IBEX) took measurements and found that it was just a gentle “wave” because the speed of the solar winds have already diminished to the “sub-sonic” level and the speed of the LISM was also not great enough to form a “bow shock”.

However, there are other stars that do create bow shocks which have been captured by various telescopes.

Below: Bowshock on Betelgeuse



Bow shock on another star:


Picture courtesy:apod.nasa

We have now come to the end of the Solar System.
But wait…humans have traveled only to the moon. Other than observations through telescopes, humans have sent numerous spacecraft to explore the planets and areas in between.

Of these spacecraft, only two have travelled to the edge of the Solar system and one has already crossed the Heliopause and has entered interstellar region.

They are Voyagers I and II.

We’ll read about them next.

Information courtesy: NASA, Universetoday,, & other sources.

(This post was written by Mrs. Kay)


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