Boundary of the Solar System- Beyond the 8 Planets?


Whenever we imagine the solar system as a whole, our image is somewhat like that which is given in figure 1. For most people, the picture ends at the orbit of Pluto, and for those who love to study the objects in our solar system, the picture might end at the “Eris” (the farthest dwarf planet in the Solar system).

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So, is it the boundary of our solar system?

The answer is no. Then what is the boundary of the solar system? Well, to locate the boundary of a particular place on Earth, we either use rivers, roads, valleys, or borders to mark the boundary of a region. But in an empty space, how can we mark a boundary? Or is it that we haven’t even seen the outskirts of the solar system?

Well, it’s not that we haven’t seen the outskirts of the solar system. In fact, scientists even launched two space probes named Voyager 1 and Voyager 2 in 1977. Not only that, but these probes have reached the outermost edge of the solar system and will travel even further and operate for an extended mission till 2025.

So, what have these space probes seen there and what information have we received from them till now?

Before talking about the actual boundary of the solar system we must understand a few important terms.

1) Solar wind: It is the supersonic flow of tenuous plasma originating in the solar corona that fills up a certain volume of space around the sun. Due to its interaction with the magnetosphere, it carries the solar magnetic field during its outward flow.

Image credit: World Atlas

2) Local Interstellar medium (LISM): It is the matter consisting of ionic rays, cosmic rays, dust and gas in atomic or molecular form, and radiation that fills the space between any two-star system. Our Solar System is traveling around the galactic core in this medium.

Structure of Outskirts of Solar System

The spherical region around the sun filled with solar winds consists of electrons and protons and solar magnetic fields that protect us from the cosmic rays coming from outer space, just like our earth’s magnetic field protects us from solar wind and energetic particles and ions. It all starts from the corona in the sun itself. As we know, even our sun travels around the galactic core at a speed of 250 km/s, and its magnetic field creates this cavity, called the heliosphere, in the interstellar medium, which is beyond the orbits of the planets in our solar system. Basically, it is the volume of space within which our sun’s influence predominates. The size and shape of the heliosphere vary as per the strength and level of the solar wind. However, the shape of the heliosphere is not the same in all directions around the sun like an egg-shaped, as the sun, while moving, drags the solar wind into the medium just like a boat dragging the water along with it in the ocean.

Termination shock
As we go further deep into the heliosphere, gradually we will realize that the supersonic solar wind starts to lose its speed due to the interstellar medium penetrating within the heliosphere. This region where the solar wind starts to decrease its speed is called a termination shock, where the solar wind for the first time encounters the interstellar medium. A termination shock can be called the beginning of the end of the heliosphere.

Further ahead, the solar wind continues to drop its speed due to the interstellar medium acting as a barrier. The highly charged particles of the solar wind are compressed together here to form the Heliosheath, a region where solar winds still dominate over interstellar winds. However, the neutral atoms in the heliosheath form a “ribbon” as they get deflected back into the solar system due to the magnetic field of the interstellar medium.

The outermost edge of the heliosheath and end of the heliosphere is what we call the “heliopause”, which can also be called the outermost edge or the boundary of our solar system. Here, the solar wind pressure almost balances the outward pressure of interstellar winds, which means they are in equilibrium. However, it is a theoretical boundary and crossing it signals a sharp drop in the temperature of charged particles. The heliopause’s size is constantly changing as the heliosphere contracts and expands, followed by the solar cycle (periodic flipping of magnetic poles). However, not only does the solar cycle affect the shape of the heliosphere but also the interstellar medium that it drags along with it, which also has dramatic effects on the heliosphere. Still, for the idea, it is about 123 AU (or 18 billion km) away from the sun. Voyager 1 discovered the location of the Heliopause when it observed an increase in cosmic-ray particles and detected that radio emissions are generated when material thrown off by the Sun crosses it.

Where are Voyagers now?
Both Voyager 1 and Voyager 2 have reached “ interstellar space”. They are currently in the constellation of Ophiuchus and will continue their journey through the universe.



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