I. Executive Summary
The zodiacal dust cloud is the most prominent sign of the presence of planetesimal-sized objects in our solar system. Although its total mass is relatively tiny, the thermal emission and scattered light from a portion of the zodiacal cloud a few x 0.1 AU across is comparable to that from a terrestrial planet.
Our best knowledge of the zodiacal cloud is of the material near Earth's orbit, at heliocentric radii from about 0.9 to 2 AU. The detailed structure of the cloud is only now being explored. The relative contributions of various dust creation and destruction mechanisms are not yet well understood. What can be said at present is that most of the dust near 1 AU is from main belt asteroid collisions and erosion, comets are an important secondary source, and Poynting-Robertson (PR) radiation drag inducing drift toward the Sun is the most important local removal mechanism.
Simple back-of-the-envelope arguments presented below indicate that the observed zodiacal dust density is probably about the norm (within a factor of ~2) for the past Gyr. This is quite uncertain, however. Individual asteroid collisions, giant comets, and comet showers may produce large enhancements lasting 107 yr or more.
We cannot easily derive the density of the local zodiacal cloud in the past because the main process (aside from collisions) that removes the dust's planetesimal parent bodies is the "chaotic" influence of planetary perturbations. Early in the history of the solar system the zodiacal cloud could have been 106 times as dense and bright as at present, based on an estimate of the maximum plausible original mass in the asteroid belt. Models of the construction of a planetary system indicate, however, that the presence of an asteroid belt is not guaranteed. Thus, there is apparently a large possible range in mass of original asteroid populations in planetary systems otherwise resembling ours, along with an unpredictable rate of removal of asteroids and comets and the likelihood of occasional substantial transient increases in dust injection rate.
The above concerns lead to the main conclusion of this report: it is impossible to predict the density of dust at terrestrial temperatures in extrasolar planetary systems, even assuming the arrangement and characteristics of major planets resemble those in our system. The necessary presence of comets might imply a very uncertain general lower limit of 1/10 the solar zodiacal dust density, and IRAS measurements indicate few cases among nearby main sequence stars of more than several x 100-zodi clouds of warm dust. Thus, our solar system's zodiacal cloud, which would pose some problem for external detection of the Earth, is probably near the low end of the possible range of system densities although the distribution within that range is not even guessable at this point.
It can be stated safely that the average zodiacal density should steadily decrease over Gyr time scales in each system due to a decrease in the mass of dust parent body populations. This implies that planet-search instruments should be directed first at older systems.
A planet-search interferometer should have precursor studies to identify those systems with the least dust emission. Dust emission at 1- to 100-zodi levels is not easily detected by unresolved photometry because the corresponding fractional dust excess over stellar photospheres would be only 10-4 to 10-2, smaller than the uncertainty in stellar mid-IR spectral energy distributions. Thus, high spatial resolution is necessary to separate exozodiacal emission from stellar brightness. The optical and IR brightness contrasts between planets and dust are comparable because the albedo and temperature of the dust are like those of planets. Finding planets and finding exozodiacal clouds will be of similar difficulty for spatial resolutions of order 0.1 AU and dust densities like those in the solar system's zodiacal cloud.
Possible instruments to carry out the necessary preliminary exozodiacal dust surveys include the Keck, Magellan, LBT, and VLT interferometers working on the ground in the thermal-IR, and HST plus coronagraph in the visual and near-IR. NGST at visual and near-IR wavelengths with coronagraph and "slow" adaptive optics to compensate for its modest optical smoothness could also be sensitive to exozodiacal clouds. Ground-based IR interferometers can probably reach the 100- or 10-zodi level for systems within 10 pc using simple and already-understood Bracewell (chromatic) interferometry. Levels down to 10- to 1-zodi appear feasible with implementation of achromatic interferometry in a dual-nuller configuration.
A space-based exozodiacal mapper would be a useful intermediate step between exozodiacal detection from the ground and planet detection from space. Maps of exozodiacal emission could exploit expected dynamical effects of extrasolar planets on their surrounding dust clouds to infer the presence, orbit locations and orientation, and even masses of the planets.
Last updated March-06-1998