This project is an observational program to determine the abundances of select white dwarfs as a method of measuring the bulk compositions of extrasolar minor planets. It is motivated by a recent analysis of Keck I spectra of the white dwarf GD 362 whose photospheric abundances are naturally understood as resulting from the destruction of a minor planet similar in composition to the Earth/Moon system which is depleted in volatiles like carbon and sodium by more than a factor of 10. This suggests that white dwarfs with an infrared excess, such as GD 362, have accreted debris from asteroids that strayed within the star's tidal radius and that the atmospheres of these white dwarfs thus provide a powerful and currently unique opportunity to measure the bulk compositions of extrasolar minor planets.
Here, Professor Jura will use the Keck I telescope and HIRES spectrograph to obtain spectra of additional white dwarfs with an infrared excess (presumably the result of a dust disk inside the tidal radius) and determine the abundances of externally contaminated elements (including volatile versus refractory elements). White dwarfs without infrared excesses will also be observed to evaluate the role/possibility of interstellar accretion. The resulting data may well enable a more comprehensive understanding of the existence and frequency of extrasolar Earth-like planets.
This project will directly support science education at UCLA and the education/training of a graduate student whose thesis will be based on these data.
White dwarf stars occasionally accrete one of their own minor planets. Before the accreted matter sinks into the interior of the star, we have the opportunity to study the atmospheric "pollution" of the white dwarf to measure the elemental composition of the minor planet. This procedure provides a uniquely powerful tool to compare our Earth with extrasolar planets. We have combined observations fromthe Keck telescopes in Hawii, the Hubble Space Telescope and the Spitzer Space Telescope to accumulate data for these polluted white dwarfs. We have developed theoretical models to interpret the results. We have learned that at least 30% of white dwarfs are polluted by their own planetary sytems. Armed with this result, we have found that extrasolar planets are largely composed of oxgyen, magnesium, silicon and iron, the same four elements that make up most of our Earth. Remarkably, although carbon is essential for life as we know it and is very abundant in the Sun, carbon has a relatively low abundance both in bulk Earth and extrasolar planetesimals. Similarly, although liquid water is of great interest, most extrasolar planetesimals are largely composed of rock rather than ice. Bulk Earth also is largely rock with only a relatively small fraction of water. Our overall conclusion is that our planet Earth appears to be relatively "normal".
