Boundaries of solar system

A planetary system is a general term used for describing the system of planets and a host star around which these planets orbit. Our Sun is named ‘Sol’ from the Latin word: solis and anything related to it is ‘solar’, Hence the: Solar System. 

Our Planetary system consists of our host star, the Sun, and planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. It also has a number of dwarf planets like pluto, many moons, and millions of asteroids and meteoroids.

The question is: how big is our planetary system? Where can we plot the boundaries and declare that outside is not in our solar system? How far the Voyager 1 and Voyager 2 have to travel to finally be outside the solar system?

Solar Winds

The Sun generates a wind of ionized particles and magnetic field that stream millions of miles into space. This is known as ‘Solar Wind’ which defines the boundaries of the solar system. This can be imagined as a bubble-like region of space created and inflated by our Sun’s plasma(Heliosphere). 

Termination shock

The particles from the sun are emitted at about 400 km/hour. But, as we move farther away from the Sun, the pressure of the solar wind drops as a square of that distance. And eventually, the solar wind can no longer maintain the supersonic flow against the interstellar medium. Here, the wind slows down to subsonic speeds causing compression, heating, and change in the magnetic field known as ‘Termination shock’. It is believed to be 75 to 90 Astronomical units from our Sun.  

Heliosheath

The wind flow is not yet stopped but slowed down due to interaction with the interstellar medium. Outside of the termination shock is the region called ‘heliosheath’ where the flow of ionized particles is turbulent. This region is believed to be at approximately 80 to 100 astronomical units from the Sun.

Heliopause

The outer edge of the heliosheath is known as ‘Heliopause’ where the solar wind does not possess enough strength to counteract the cosmic galactic wind. The protons from the Sun become rare, approaching the Heliopause. This is the theoretical boundary of the solar system where stellar winds from the surrounding stars dominate the region of space. 

Fun Facts

• Voyager 1 has crossed the Heliopause on August 25, 2012, at a distance of 121 AU(18 billion km) from the Sun. 

• Because we are revolving around the center of our galaxy, the Solar system grows a ‘Heliotail’ where solar wind ultimately escapes the Heliosphere. 

• All the planets including Earth, are partly shielded from the galactic cosmic rays due to the presence of the Heliosphere.