In fact, interplanetary space is not at all empty. The average particle density is about 2 - 5 atoms per cubic centimeter, and most of this is gas blown off from the surface/corona of the Sun as the solar wind. Eventually, this wind runs up against the interstellar medium which has a density of about 0.1 to 0.05 atoms per cubic centimeter, and astronomers expect that this 'bowshock' region will be discovered about 100 - 200 astronomical units from the Sun. That's about 3-5 times the distance to the planet Pluto.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.