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NASA illustration: Our solar system (not to scale).

Interplanetary space is that part of outer space between planets in a solar system and its local star(s), many of which are binaries. Around any one planet, "interplanetary" space begins in the broad region where any amosphere, magnetic field and moons end, ceding dominance to the local star; in our case, the Sun. The diffuse outer boundary of our interplanetary space is characterized by the heliopause, where the solar wind meets the interstellar medium, and by the Oort cloud, the region where comets originate.

Earth is the third planet from the Sun, after Mercury and Venus. Next out, Mars is the last of the rocky planets with definite surfaces. Beyond it lie the four "gas giants": Jupiter, Saturn, Uranus and Neptune. Beyond Neptune is the Kuiper belt, containing planetoids and the dwarf planets Pluto and Eris. Our solar system has an asteroid belt between the orbits of Mars and Jupiter. It generates meteors, whose impact craters cover the surface of our Moon, and some other moons and planets, with more than 150 also found on Earth. (Some Earth meteorites originate from material ejected from prior impacts on the Moon, and even Mars.)

Since planets have been discovered orbiting other stars in recent decades, we now know that there are other planetary systems, each with its own interplanetary space, a term which may be used even if there is only one planet. (See Stars with planets.) Each system presumably evolved from a primitive solar nebula. Most stars, and presumably most planetary systems, belong to galaxies, between which there is intergalactic space.

The space separating all astronomical bodies is characterized by a vacuum emptier than any experimentally produced on Earth. There is also more energetic electromagnetic radiation coming from the hot stars (and other objects), including infrared, visible, and ultraviolet light, and X-rays and gamma rays. Permeating everywhere is the tenuous interstellar medium, consisting of gas, plasma (ionized gas), and dust; and cosmic rays, which include atomic nuclei and various subatomic particles. Radiation is a hazard to astronauts, and to delicate electronics, even on robotic spacecraft.