Dust Accretion onto Exoplanets

Abstract

Accretion of interplanetary dust onto gas giant exoplanets is considered. Poynting-Robertson drag causes dust particles from distant reservoirs to slowly inspiral toward the star. Orbital simulations for the three-body system of the star, planet, and dust particle show that a significant fraction of the dust may accrete onto massive planets in close orbits. The deceleration of the supersonic dust in the planet's atmosphere is modeled, including ablation by thermal evaporation and sputtering. The fraction of the accreted dust mass deposited as gas-phase atoms is found to be large for close-in orbits and massive planets. If mass outflow and vertical mixing are sufficiently weak, the accreted dust produces a constant mixing ratio of atoms and remnant dust grains below the stopping layer. When vertical mixing is included along with settling, the solutions interpolate between the mixing ratio due to the meteoric source above the homopause, and that of the well-mixed deeper atmosphere below the homopause. The line opacity from atoms and continuum opacity from remnant dust may be observable in transmission spectra for sufficiently large dust accretion rates, a grain size distribution tilted toward the blowout size, and sufficiently weak vertical mixing. If mixing is strong, the meteoric source may still act to augment heavy elements mixed up from the deep atmosphere as well as provide nucleation sites for the formation of larger particles. The possible role of the Lorentz drag force in limiting the flow speeds and mixing coefficient for pressures $\la 1\, \rm mbar$ is discussed.