The Zodiacal dust cloud is visible as a diffuse glow, known as the zodiacal light, that stretches along the zodiac, and is best seen just after sunset and before sunrise in spring and autumn when the zodiac is at a steep angle to the horizon.
The glow was first correctly identified as being due to scattered sunlight from dust particles in the solar system by Joshua Childrey in 1661. The dust forms a thick pancake-shaped cloud in the Solar System collectively known as the zodiacal cloud, which occupies the same plane as the ecliptic. The dust particles are between 10 and 300 micrometres in diameter, with most mass around 150 micrometres.[1]
The zodiacal cloud can be observed by the naked eye at certain times of the year and appears as a second "Milky Way" along the ecliptic. This phenomenon is called the zodiacal light.[2]
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The source of that dust was long debated. Until recently, it was thought that the dust originated from the tails of active comets and from collisions between asteroids in the asteroid belt.[3] Peter Jenniskens had previously recognized that many of our meteor showers have no known active comet parent bodies. In a 2010 article in the Astrophysical Journal, David Nesvorny and Peter Jenniskens attributed over 85 percent of the dust to occasional fragmentations of Jupiter Family Comets that are nearly dormant.[4] Jupiter Family Comets have orbital periods of less than 20 years[5] and are considered dormant when not actively outgassing, but may do so in the future.[6] Nesvorny and Jenniskens' 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 in smaller zodiacal dust grains over time.
Small particles of dust that orbit the Sun are steadily forced into more circular (but still elongated) orbits by the Poynting-Robertson effect. According to Nesvorny and Jenniskens, when the dust grains are as small as about 150 micrometres in size, they will hit the Earth at an average speed of 14.5 km/s, many as slowly as 12 km/s. If so, they pointed out, this comet dust can survive entry in partially molten form, accounting for the unusual attributes of the micrometeorites collected in Antarctica, which do not resemble the larger meteorites known to originate from asteroids.
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.
Zodiacal dust around nearby stars is called exozodiacal dust; it is a potentially important source of noise for directly imaging extrasolar planets. Nesvorny and Je nniskens have pointed out that this exozodiacal dust, or hot debris disks, can also help find planets, as planets tend to scatter the comets to the inner solar system.
In August 2007, Brian May, lead guitarist with the band Queen, handed in his PhD thesis Radial Velocities in the Zodiacal Dust Cloud 36 years after starting it and then abandoning it in favour of a musical career. That he was able to submit it was possible only because of the minimal amount of research on the topic that had been carried out in the intervening years. May describes the subject as being one that became "trendy" again in the 2000s.[7]