Interplanetary space is not empty.

Interplanetary space, or the space between the planets, is not empty but is filled with the solar wind, and the Interplanetary Magnetic Field (IMF), as well as cosmic rays, and dust.

Another name for interplanetary space is the Heliosphere, because the space between the planets is the total space which is under the influence of the sun. Just how big the heliosphere is (in AU's), nobody knows.

The space between the planets is different from the space between the stars, or Interstellar Space.

Particles of the solar wind travel through interplanetary space at supersonic speeds. A bow shock helps the solar wind to go around obstacles in interplanetary space such as comets and planetary magnetospheres.