The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move., magnetic and electric) that pervade this region of space. So what do we call. T he space between the planets is far from empty. the solar wind; cosmic rays; microscopic dust particles; and magnetic fields (primarily the Sun's). The four inner terrestrial planets are Mercury, Venus, Earth, and Mars, all of which consist mainly of rock. Pluto was considered. Solar system, assemblage consisting of the Sun —an average star in the Milky Way Galaxy —and those bodies orbiting around it: 8 (formerly 9) planets with about 170 known planetary satellites (moons); countless asteroids, some with their own satellites; comets and other icy bodies; and vast reaches.