The space between the sun and the planets within the solar system. The interplanetary matter that occupies this region of space mostly originates in the sun, from which it flows as the solar wind. The solar wind consists primarily of protons emerging from the sun at a rate of about 10⁹ kilograms per second. At the earth’s distance from the sun the particle density has fallen to a few particles per cm³. Apart from this very tenuous gas, there are also dust particles in interplanetary space, largely believed to originate in the belt of asteroids. Particles weighing about 1 g produce visible meteors in the earth’s atmosphere; micrometeorites as small as 1 nanogram can be detected by their impact on spacecraft.