Archaeology of Exo-Terrestrial Planetary Systems and a Search for Water

The research I propose will, uniquely, constrain the frequency and bulk chemical composition of terrestrial planets around other stars. Critically, my research program has the potential to detect the signature of water within rocky planetary bodies and thereby constrain the potential for oceans and habitats for life. This is made possible by stars at the end of their lives called white dwarfs. Over 95% of all stars in the Milky Way, including our Sun, will end their lives as white dwarfs, gently shedding their outer layers, then shrinking to Earth-sized, slowly cooling embers. By studying rocky planetary systems at white dwarfs, one sees a glimpse into the future of the Earth.

In the search for terrestrial planetary systems around other stars, white dwarfs offer a unique advantage. Owing to high gravities, heavy elements sink rapidly to the interior, leaving behind pure hydrogen or helium atmospheres. Those white dwarfs with rocky planetary systems can become contaminated by the infall of small, but detectable amounts of heavy elements such as silicon, magnesium, and iron (termed simply 'metals' by astronomers). Only recently, a number of white dwarfs polluted by rocky planetary debris have been discovered, and I have played a lead role in identifying and understanding these stars and their environments. These systems arose analogously to the rings of Saturn; a large asteroid or moon passing too close to the star was torn apart by gravity and now resides as a disk of material orbiting the white dwarf. This ring of debris gradually falls onto the star, contaminating its otherwise-pristine atmosphere with metals.

I will use white dwarfs polluted by metals to infer the masses, basic chemical compositions, and water content of their surviving rocky planetary systems. The proposed project has two parts: 1) to observe the rocky debris itself in orbit about the star, and 2) to measure the chemical content of this debris as it pollutes the star, broken down into its constituent elements.

The first part requires infrared and submillimeter observations to detect warm orbiting dust at these metal-polluted stars. The basic goal of this work is to unambiguously correlate orbiting rocky debris with the polluted stellar atmospheres, and firmly establish the material as planetary. The long-wavelength light from the warm dust provides a measure of the spatial distribution, temperature, size, and composition of the orbiting particles, thus constraining the nature of the now destroyed, rocky asteroid, moon, or planet. I will significantly increase the number of known white dwarfs with debris rings to gain a superior statistical understanding of the frequency of terrestrial planetary systems around other stars.

The second step is to use the star itself to determine the bulk chemical composition of the planetary debris falling onto and polluting its atmosphere. I will measure the relative elemental abundances of several to two dozen heavy elements in each of these stars using optical and ultraviolet observations. In doing so, it will be possible to establish the basic makeup of terrestrial planets around other stars, as well as catalog their diversity. Water can be identified by searching for oxygen in excess of that carried by rocks alone; a simple accounting of each detected metal as an oxide can reveal extra oxygen delivered as H2O. In this way I will determine the mass of water in rocky asteroids and moons, and compare these findings with water-rich Solar System bodies