Our solar system consists of the Sun and all of the objects in orbit around it. It includes the planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—as well as smaller objects such as dwarf planets, asteroids, and comets. But where does the solar system end?

The Sun's light gets weaker as you get farther away, but it doesn't have a boundary where it suddenly stops. Similarly, the strength of the Sun's gravitational force decreases with distance but there is no border where it simply ends. Some people consider the Oort cloud—a spherical cloud of icy objects that surrounds all the other objects in the solar system—to be the effective outer limit of the Sun's gravitational influence and the edge of the solar system. The Oort cloud is so far away that there have not been direct observations of it, but it is thought to contain trillions of objects and be the source of some comets.

Another type of boundary is where the solar wind—the flow of charged particles that streams out from the Sun—interacts with the interstellar medium (the trace amounts of gas and dust that exist in the space between stars). The Sun is one of hundreds of billions of stars in the Milky Way galaxy; it is located in the disk of the galaxy and orbits at a speed of about 220 km/s. The solar wind and the interstellar medium are both very low density, but they still have a pressure associated with them (similar to air pressure). When the solar wind pushes against the material of the interstellar medium, it creates a bubble-like boundary called the heliosphere.

The inner edge of the heliosphere, the termination shock, occurs where the solar wind first collides with the interstellar medium and it slows down to less than the speed of sound. Beyond this boundary, the solar wind interacts with interstellar matter in the area called the heliosheath. At the outer boundary of the heliosphere, the heliopause, the pressure from the interstellar medium is strong enough to stop the flow of the solar wind. The shape of the heliosphere is influenced by the movement of the solar system relative to the galaxy as well as by the flow of the interstellar medium itself.

NASA has supported a number of missions to explore to the outer bounds of the solar system. Voyager 1 and Voyager 2, launched in 1977, studied the gas giant planets and then continued traveling outward (in different directions); they made direct measurements of the termination shock in 2004 and 2007, respectively. The Interstellar Boundary Explorer (IBEX), launched in 2008 into orbit around Earth, collects particles coming from the boundary region to create an all-sky view of the boundary. Scientists are studying the dynamic conditions of the heliosphere and are exploring the consequences of its variations. For example, the heliosphere acts as a barrier to galactic cosmic rays (high energy particles), which can affect Earth's atmospheric chemistry and damage DNA, which would have implications for the evolution of life on Earth. IBEX also provides information about the chemical composition of the space beyond our solar system, which can provide insight into the history of the universe and what may happen as the solar system moves through the galaxy into a different environment.

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