Cosmic dust, also called extraterrestrial dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles are between a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust (such as in the zodiacal cloud) and circumplanetary dust (such as in a planetary ring).
In the Solar System, interplanetary dust causes the zodiacal light. Solar System dust includes comet dust, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. Thousands of tons of cosmic dust are estimated to reach the Earth's surface every year, with each grain having a mass between 10−16 kg (0.1 pg) and 10−4 kg (100 mg). The density of the dust cloud through which the Earth is traveling is approximately 10−6 dust grains/m3.
Cosmic dust contains some complex organic compounds (amorphous organic solids with a mixed aromatic–aliphatic structure) that could be created naturally, and rapidly, by stars. A smaller fraction of dust in space is "stardust" consisting of larger refractory minerals that condensed as matter left by stars.
Interstellar dust particles were collected by the Stardust spacecraft and samples were returned to Earth in 2006.