Gegenschein is a faintly bright spot in the night sky centered opposite the position of the sun. The backscatter of sunlight by interplanetary dust causes this optical phenomenon, also called counterglow.
Like zodiacal light, gegenschein is sunlight scattered by interplanetary dust. Most of this dust orbits the Sun near the ecliptic plane, which is the plane of Earth's orbit around the sun.
The interplanetary dust in the Solar System collectively forms a thick, pancake-shaped cloud called the zodiacal cloud, which straddles the ecliptic plane. Over 85 percent of the dust is attributed to occasional fragmentations of Jupiter-family comets that are nearly dormant. Jupiter-family comets have orbital periods of less than 20 years and are considered dormant when not actively outgassing, but may do so in the future. The first fully dynamical model of the zodiacal cloud demonstrated that only if the dust was released in orbits that approach Jupiter, is it stirred up enough to explain the thickness of the zodiacal dust cloud. The dust in meteoroid streams is much larger, 300 to 10,000 micrometres in diameter, and falls apart into smaller zodiacal dust grains over time.
The Poynting–Robertson effect forces the dust into more circular (but still elongated) orbits, while spiralling slowly into the Sun. Hence a continuous source of new particles is needed to maintain the zodiacal cloud. Cometary dust and dust generated by collisions among the asteroids are believed to be mostly responsible for the maintenance of the dust cloud producing the zodiacal light.
Particles can be reduced in size by collisions or by space weathering. When ground down to sizes less than 10 micrometres, the grains are removed from the inner Solar System by solar radiation pressure. The dust is then replenished by the infall from comets.
In 2015, new results from the secondary ion dust spectrometer COSIMA on board the ESA/Rosetta orbiter confirmed that the parent bodies of interplanetary dust are most probably Jupiter-family comets.
Gegenschein is distinguished from zodiacal light by its high angle of reflection of the incident sunlight on the dust particles. It forms a slightly brighter elliptical spot directly opposite the Sun within the dimmer band of zodiacal light. The intensity of the gegenschein is relatively enhanced because each dust particle is seen at full phase.
The gegenschein is not visible in most inhabited regions of the world due to light pollution.