15 November 2013
Venue: Queens EB2
Exo-planet science is rapidly growing from discovery to characterisation. Most of our current knowledge regarding the intrinsic properties of planets comes from transit studies: we now routinely measure the mean densities of planets, and gain insight into the composition of their atmospheres. Yet, it is impossible to determine the bulk elemental composition of extra-solar planets orbiting their host stars.
The study of evolved planetary systems around white dwarfs provides an orthogonal approach to this problem. The vast majority of all known planet host stars, including the Sun, will eventually evolve into red giants, devouring close-in planets, and finally end their lives as white dwarfs: extremely dense Earth-sized stellar embers. During their post main sequence evolution stars lose substantial amounts of mass. Consequently, the orbits of their minor and major planets will widen. In the solar system, Mars, the asteroid belt, and all the giant planets will escape evaporation, and the same is true for many of the known exo-planets. It is hence certain that a significant fraction of the known white dwarfs were once host stars to planets, and many of them still have remnants of planetary systems.
The detection of metals in the atmospheres of white dwarfs is the unmistakable signpost of such evolved planetary systems. The strong surface gravity of white dwarfs causes metals to sink out of the atmosphere on time-scales much shorter than their cooling ages, leading unavoidably to pristine H/He atmospheres. Therefore any metals detected in the atmosphere of a white dwarf imply recent or ongoing accretion of planetary debris. In fact, planetary debris is also detected as circumstellar dust and gas around a number of white dwarfs. These debris disks are formed from the tidal disruption of asteroids or Kuiper belt-like objects, stirred up by left-over planets, and are subsequently accreted onto the white dwarf, imprinting their abundance pattern into its atmosphere.
Determining the photospheric abundances of debris-polluted white dwarfs is hence entirely analogue to the use of meteorites, "rocks that fell from the sky", for measuring the abundances of planetary material in the solar system. Detailed model atmosphere analyses of about a dozen polluted white dwarfs demonstrate (a) that the debris is extremely volatile-depleted, confirming its rocky nature, (b) that the abundance patterns observed in the planetary debris follow trends (including the large diversity) observed among solar system asteroids/meteorites.
I will review our current insight into the elemental composition of rocky exo-planetary material that can guide our understanding of terrestrial planet formation.