III.D Signs of Planets

The large-scale structure of the zodiacal cloud is determined by planetary perturbations. The plane of symmetry of the cloud is inclined to the ecliptic and slightly warped, and the Sun is offset from the cloud's center of rotational symmetry (Dermott et al. 1996b). These offsets might be discernible in some circumstances in other systems and allow inference of the existence of planets.

COBE observations (Figure 3) have recently confirmed that the zodiacal cloud near Earth has a marked trailing/leading asymmetry due to the trapping of dust particles with small orbit eccentricities into orbital resonances with Earth (Figure 4) (Dermott et al. 1994; Reach et al. 1995). There is a ring of dust that co-revolves with Earth around the Sun; Earth resides in a cavity in this ring and a cloud of enhanced dust density permanently trails the Earth in its orbit. The dust number density in the trailing feature peaks about 0.2 AU behind the Earth and is estimated to add between 10% and 40% (Dermott et al. 1994 and Reach et al. 1995, respectively) to the local smooth cloud density over an ecliptic plane area several x 0.1 AU diameter.

Figure 3
COBE 25 µm surface brightness of the zodiacal dust at ecliptic latitude 0o as a function of Earth's orbital longitude. The upper and lower curves represent observations pointing directly behind and in front of the Earth along its orbit, respectively. The clear difference reveals the presence of a "wake" of dust trapped in orbital resonance behind the earth. The sinusoid pattern around the year results from Earth's vertical motion relative to the symmetry plane of the cloud which is not quite identical to the ecliptic.

Figure 4
Model of the motion of a particle trapped in a resonance with the Earth, relative to a coordinate frame revolving with Earth. The axis units are AU. The resonance holds the particle in a ring at ~ 1 AU, temporarily interrupting its journey via PR drag from the asteroid belt into the Sun.

  Additional work is needed to better define the true size of the wake signals, but the Earth's wake has an IR signal at least 0.1x that of Earth. Each planet probably has such a wake. The width and relative enhancement of a wake is determined by the orbital period of the planet, the typical grain PR drift velocity, and the mass of the planet. The absolute wake density probably remains proportional to the smooth cloud density, within some limits, as the latter varies with fluctuations in intensity of grain injection processes. Wakes in exozodi clouds could be mistaken for planets, depending on spatial resolution, but it is important to note that a wake can be a prominent indicator of the presence of a planetary mass. The wakes should be distinguishable from planets via spectroscopy.