The boundary between our solar system and interstellar space is defined by physics and the far-reaching influence of the Sun’s energy, not by planetary orbits. Space within our system, known as interplanetary space, is dominated by material originating from our star. Interstellar space, in contrast, is the vast expanse between star systems, defined by matter and magnetic fields that are not solar in origin. The transition between these two distinct environments is a dynamic, invisible boundary marking the edge of the Sun’s energetic domain.
Defining the Sun’s Influence
The Sun constantly emits a flood of charged particles, primarily protons and electrons, known as the solar wind. This supersonic outflow streams away from the star, carrying the Sun’s magnetic field with it. The region carved out by this expanding solar wind is called the heliosphere, a protective magnetic bubble that envelops all the planets.
The heliosphere’s outer boundary is determined by the pressure exerted by the surrounding galaxy’s gas and magnetic fields. As the solar wind travels outward, it encounters this external pressure and dramatically slows down. This deceleration happens at the termination shock, where the supersonic solar wind abruptly drops to subsonic speeds, similar to a sonic boom in reverse.
The termination shock is located approximately 75 to 90 astronomical units (AU) from the Sun, far beyond Neptune’s orbit. Past this shock, the solar wind becomes compressed, heated, and turbulent, entering a vast buffer zone. This transitional region, where the slowed solar wind is deflected toward the tail of the heliosphere, is known as the heliosheath.
The Heliopause: The Solar System’s Edge
The heliopause is the plasma boundary that serves as the gateway into the interstellar void. This is the precise point where the outward pressure of the solar wind plasma is balanced by the inward pressure of the charged particles and magnetic fields of the local interstellar medium. It is not a static shell but a constantly fluctuating and porous boundary, shifting in response to changes in the Sun’s activity and the galactic environment.
The distance to the heliopause varies, but on the side facing the Sun’s movement through the galaxy, it lies roughly between 119 and 123 AU from the Sun. At this boundary, the Sun’s magnetic field lines are thought to reconnect with those of the interstellar medium. This process creates a complex magnetic field structure that influences how charged particles move across the boundary.
The heliopause marks the final drop in the density of solar particles and the beginning of a region where galactic influences dominate. While the Sun’s gravitational pull extends far beyond this point, the heliopause signifies the termination of the star’s electromagnetic and particle control over its local environment.
The Interstellar Medium
Immediately beyond the heliopause lies the Interstellar Medium (ISM), the tenuous material that fills the space between star systems. This region is fundamentally different from interplanetary space, consisting primarily of matter not produced by our Sun. The ISM is made up of gas, dust, and cosmic rays, traveling through the galaxy in what is often called a galactic wind.
The composition of this medium is overwhelmingly simple, consisting of about 90% hydrogen and 9% helium atoms, with only trace amounts of heavier elements such as oxygen and neon. The density of particles is extremely low, averaging around 0.1 atoms per cubic centimeter in the local region of space. This is a thousand to a million times less dense than the interplanetary medium found closer to Earth.
The ISM is also characterized by the presence of high-energy galactic cosmic rays, which are charged particles accelerated by distant supernovae and other massive stellar events. The heliosphere acts as a shield, deflecting most of these particles, but once outside the heliopause, their intensity sharply increases. The temperature of the charged particles in the local ISM is high, estimated to be between 30,000 and 50,000 Kelvin, though the gas is so sparse that it would feel colder than absolute zero.
Reaching the Interstellar Void
The precise location of the heliopause has been confirmed by the journeys of the Voyager space probes. Voyager 1 became the first human-made object to cross into interstellar space on August 25, 2012, at a distance of about 121.7 AU from the Sun. The evidence for this crossing was a three-step change in the surrounding environment.
The spacecraft detected an abrupt drop in the number of particles originating from the Sun and a sudden spike in the intensity of galactic cosmic rays. Furthermore, the direction of the magnetic field surrounding the probe shifted, confirming the transition from the solar-dominated field to the galactic magnetic field.
Voyager 2 followed, crossing the boundary on November 5, 2018, at approximately 119 AU. Voyager 2 carried a functional plasma science instrument that Voyager 1 lacked, providing definitive measurements of the solar wind’s speed and density. The instrument recorded a complete cessation of the outward flow of solar wind plasma, replacing it with the denser, colder plasma of the interstellar medium. These measurements confirmed that the heliopause is the measurable boundary where the Sun’s energetic influence ends and the interstellar void begins.