Kinematics, Galactic Age, Chemistry and Water Fraction of Asteroid-Polluted White Dwarfs from the Sloan Digital Sky Survey

Of the nearly 800 extrasolar planets confirmed around other stars, only a small number of these are thought to be solid planets and thus similar in theory to Mercury, Venus, Earth, and Mars. These planets have been identified almost exclusively through NASA's Kepler mission, which detects the transit of the planet as it passes in front of the host star. The transit provides scientists with the size of the planet, and radial velocity observations from the ground can provide the mass in some case, thus giving the density and indicating a likely solid composition. However, the actual composition of these planets is unknown and no current or future means exist to make such measurements.

However, stars at the end of their lives -- called white dwarfs -- offer a unique advantage in the study of terrestrial exoplanetary systems. Over 95% of all stars in the Milky Way, including our Sun, will end their lives as white dwarf stars, gently shedding their outer layers, then shrinking to Earth-sized, slowly cooling embers. Owing to high gravities, heavy elements sink rapidly to the interior, leaving behind pure hydrogen or helium atmospheres in these stars. Those white dwarfs with rocky planetary systems can become chemically-enriched by small, but detectable amounts of heavy elements such as silicon, magnesium, and iron. In the last several years, astronomers have discovered white dwarfs ''polluted'' by rocky debris from asteroids, and been able to obtain the basic asteroid composition from the elements polluting the stellar atmosphere. In the Solar System, asteroids are the leftover building blocks of the terrestrial planets, and thus extrasolar asteroids at white dwarfs indicate the basic composition of their rocky exoplanets.

There have been three major epochs of star (and planet) formation in the Galaxy. The first stars formed 11-15 billion years ago and now lie in a spherical halo that encircles the disk of the Galaxy. The next stars formed 7-12 billion years ago and appear as a thick disk about the Galactic plane, while stars like our Sun formed 2-8 billion years ago and occupy a thin disk we recognize as the 'Milky Way' in the night sky. Each of these three star populations are chemically distinct as the material available was at first deficient in heavy elements, and then later enriched after one or more stages of nuclear processing within the interior of stars. The individual chemistry of these populations implies that the planets they built may differ substantially from those in our Solar System, and by studying their asteroids we can learn about the composition of solid planets built by the older halo and thick disk stars, as compared to younger, thin disk stars like our Sun. Such rocky planetary systems will be among the oldest in the Universe and represent the first and second waves of planet formation (and life, if present) in the Galaxy.

This project will identify hundreds of asteroid-polluted white dwarfs via the exploitation of large existing databases. The collected data will allow us to distinguish between thin disk, thick disk, and halo white dwarfs with remnant planetary systems by the stars' positions and motions in the sky. We can then assess the frequency of remnant, solid planetary systems as a function of Galactic age and study the asteroid composition within these stellar populations. Importantly, by focusing on white dwarfs with helium-dominated atmospheres and remnant planetary systems, we can assess the water content of their asteroids by studying the trace hydrogen in the stellar atmosphere. This in turn will give scientists a good handle on how much water is available for the construction of water-rich, habitable planets around other stars.

The following non-academic entities will benefit from the proposed research.

The Postdoctoral researcher supported on the grant through career development and training.

The UK economy via the provision of highly trained Ph.D. students and postdoctoral researchers associated with the research undertaken as part of the grant.

The wider UK public through the active Outreach programme undertaken at the Insititute of Astronomy.