What is zodiacal light?
If you go to an area with low light pollution, you may notice that the western sky in the evening appears bright even after twilight. The dim light appears to be distributed from the ecliptic plane and lingers until late evening. A similar phenomenon occurs in the morning towards the east. This is zodiacal light. It is a phenomenon where dust particles near Earth are illuminated by sunshine and the light is reflected towards ground-based observers.
Astronomers have debated for over a century about the origin of the dust particles. As is well known, comets – small icy bodies from the outer Solar System – eject dust and gas when they approach the Sun. Asteroids also emit dust due to collisions or rapid rotation. Some asteroids in the outer asteroid belt – between the orbits of Mars and Jupiter – may contain ice underneath and eject dust together with gas. These small bodies of the Solar System are significant dust sources that become observable as zodiacal light.
The contribution of dust particles from beyond the Solar System used to be discussed, but was conjectured to be ignorable. However, motivated by the recent discoveries of interstellar comets such as ‘Oumuamua, astronomers may consider the contribution of interstellar dust once again.
Before modernisation, it should have been possible to observe zodiacal light from almost every part of the world, with records of it being visible from the centre of Tokyo, Japan, during World War II. However, due to rapid industrial growth and the increase of artificial light around us, zodiacal light has been obscured behind severe haze and light pollution. The night sky illuminated by artificial light is indistinguishable from zodiacal light and, for this reason, it is now challenging to observe this phenomena in most areas of the world.
Although it does give off a rare celestial glow, zodiacal light and the Milky Way are not comparable. The Milky Way glows due to the shining of millions of stars, whereas zodiacal light is reflecting the Sun’s light via debris.