Young-earth "proof" #7: Space dust would be vacuumed out of our solar system by the Poynting-Robertson effect in a few thousand years. Since that is not the case, the earth is very young.The Poynting-Robertson effect is an effect that sunlight has on small dust particles orbiting the sun. The continuing absorption of sunlight robs the dust particle of more and more of its angular momentum, giving it a tendency to slowly spiral into the sun as its orbit shrinks.
Based on the Poynting-Robertson effect alone, particles 0.001 cm in diameter located at a distance equal to that of the earth's distance from the sun (one AU) would spiral into the sun in about 19,000 years; particles 0.0001 cm in diameter would require less than 2,000 years.
Slusher, in his book Age of the Cosmos (a 1980 ICR technical monograph), argued that the presence of such fine dust in our solar system limits its age to less than 10,000 years. However, Slusher has overlooked several things:
Reflected sunlight (as versus absorbed light for the Poynting-Robertson effect) applies an outward force on dust particles. As a particle gets nearer to the sun, this outward radiation pressure increases faster than the force of gravity pulling the particle in (Strahler, 1987, p.145). Observe that the tail of a comet points away from the sun. As many comets have tails of dust as well as gas, we have dramatic proof of the above fact! We have a case of dust moving away from the sun! Slusher didn't tell us about that little complication.
Another point overlooked by Slusher is the gravitational effect the planets would have on dust spiraling in. Many dust particles would be kicked into elliptical orbits which would greatly lengthen their time in space. A different gravitational effect, recently confirmed with the aid of a supercomputer at the University of Florida, is responsible for a huge dust ring which is associated with the earth's orbit. This diffuse ring is about 30 million miles wide from its inner to its outer edge and about 200,000 miles thick (Discover, Nov. 1994, page 31). Al Jackson and Herb Zook of the Johnson Space Center did the initial work, which was confirmed in much greater detail (and certainty) by Stanley Dermott, Bo Gustafson, and their colleagues at the University of Florida. The details of this ring, which only a supercomputer could work out, explain for the first time why the zodiacal light is 1-2% brighter in the direction trailing Earth than in the forward direction of the earth's orbit. (At certain times of the year, just after sunset or before dawn, one can see a faint glow in the sky due to sunlight being reflected from space dust--the zodiacal light.) We might reasonably suppose that Mars, Venus, and maybe even Mercury have dust rings associated with their orbits. Thus, we have yet another source of long-lasting dust which Slusher has (understandably) overlooked.
Still another effect "...overlooked by Slusher is trapping of particles by gravitational resonances with the larger planets (Alfven and Arrhenius, 1976, p. 81). So trapped, particles could remain in stable orbits indefinitely." (Strahler, 1987, p.145).
What about those comets which sweep through our inner solar system every few years? Comets usually have two different kinds of tails, one of gas and one of dust, and those tails often extend many tens of millions of miles across space. As they near the sun comets are constantly outgassing material. Comets contribute a fair amount of new dust (Dutch, 1982, p.31; Discover, Nov. 1994, page 31).
Even major asteroid impacts on the smaller planets or moons would occasionally contribute some dust to the interplanetary spaces. We now know, for example, that a few of the meteorites collected on Earth actually came from Mars! No doubt some dust also escaped into space during the largest of those impacts.
Some of the dust continually created by collisions within the asteroid belt would, by the very Poynting-Robertson effect under discussion, gravitate to the inner solar system at about 2000 miles per year (Discover, Nov. 1994, page 31). That is, the radius of the dust particles' orbits would initially shrink at that rate. In less than 40,000 years, roughly speaking, some of that spiralling-in dust from the asteroid belt would reach the vicinity of Earth.
To summarize, new dust is constantly being added to the inner solar system. As it spirals in after leaving its initial source, much of it is trapped in various ways by gravity for perhaps millions of years or more or else kicked into new orbits which may greatly increase its time in space. The solar wind actually blows some of this dust away from the sun, should it get too close. Consequently, it is not surprising that the inner solar system is still a dusty place after all these billions of years--despite the Poynting-Robertson effect.