What Is Zodiacal Light? Sunlight Scattering Off Dust Between the Planets
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A faint cone of light rising from the horizon before dawn or after dusk is not twilight and is not the Milky Way. It is sunlight scattered by dust particles spread through the inner solar system, and seeing it at all requires a genuinely dark sky.
What produces it
A cloud of fine dust occupies the inner solar system, concentrated towards the plane in which the planets orbit and thickening towards the sun, and sunlight scattering off those particles is what an observer sees. The particles are small, typically tens of micrometres, and the cloud is extremely tenuous, with the total mass roughly comparable to a modest asteroid despite filling an enormous volume. Because the dust lies in the orbital plane, the glow follows the ecliptic across the sky, which is the band the zodiac constellations occupy and which gives the phenomenon its name. It is brightest closest to the sun, which is why it is visible as a tapering cone extending upward from where the sun has set or will rise rather than as a general glow. A related and far fainter feature, the gegenschein, appears directly opposite the sun and is caused by dust particles scattering light back towards the observer, which requires exceptional conditions to see.
Where the dust comes from
The cloud cannot be primordial, since the particles are removed steadily and must be replenished:
- •Radiation pressure pushes the smallest particles outward and removes them from the system entirely
- •The Poynting-Robertson effect causes larger particles to spiral slowly inward and eventually into the sun, over timescales far shorter than the age of the solar system
- •Collisions between asteroids grind material into dust continuously, which was long assumed to be the main source
- •Comets shed dust each time they pass close to the sun, leaving trails along their orbits
- •Analysis of the composition and orbital characteristics of the dust has shifted the balance substantially towards comets, with Jupiter-family comets implicated as the dominant contributor
- •A separate proposal that a component originates from Mars has been argued from spacecraft measurements and remains contested
Seeing it
The requirements are demanding and the reward is a phenomenon most people have never noticed. A genuinely dark site is essential, since the glow is comparable in brightness to the Milky Way and is completely erased by light pollution. The moon must be absent. The best times are when the ecliptic stands steeply relative to the horizon, which for northern observers means the evenings of late winter and spring and the mornings of autumn, and which reverses in the southern hemisphere. Observation should begin well after astronomical twilight has ended, since the shape is easily mistaken for residual twilight, and the distinguishing feature is that it continues long after twilight should have finished and is tilted along the ecliptic rather than standing vertically. From tropical latitudes the ecliptic is steep year round and the phenomenon is correspondingly easier. Photographically it is straightforward to capture with a wide lens and a long exposure from a dark site.
Why it matters scientifically
The dust cloud is more than a curiosity. It is the dominant source of foreground contamination for infrared astronomy, since the particles are warmed by sunlight and glow in the infrared, which means every space telescope working at those wavelengths must model and subtract it, and characterising the cloud is a prerequisite for observing anything fainter. It is the local example of a debris disc, and comparable but far denser discs are observed around other stars and are a principal way of studying planetary systems too young or too faint to image directly. Its structure records the history of the bodies that supplied it, so mapping asymmetries and bands within it identifies the parent populations. And it delivers material to Earth continuously, with tens of thousands of tonnes of this dust arriving annually, which is the largest single input of extraterrestrial material to the planet and which is collected from deep sea sediments, polar ice and the upper atmosphere.
The takeaway
Sunlight scatters off fine dust concentrated in the plane of the planets, which is why the glow follows the ecliptic and tapers upward from where the sun is. The dust is removed within a fraction of the solar system's age and must be resupplied, with comets now thought to dominate over asteroid collisions. It is the main infrared foreground every space telescope has to subtract.