Institute of Astronomy

The Frequency and Compositions of Rocky Exoplanet Precursors via Polluted White Dwarfs

SpeakerTalk DateTalk Series
Ben Zuckerman1 August 2014Across HR 2014 Talks


The 2014 consensus view is that the presence of elements heavier than helium in the atmosphere of an isolated white dwarf is a signpost for a planetary system in orbit about that star. This interpretation was a long time in coming. The first identification of a white dwarf atmosphere “polluted” with heavy elements occurred in 1917; at that time little was understood about the nature and physics of white dwarfs. Subsequently, other polluted white dwarf atmospheres were identified, but until 1983 always of the helium-rich variety. Since the time required for heavy elements to settle out of the atmosphere of a (cool) He-atmosphere white dwarf is quite long, accretion of interstellar matter, perhaps long ago, was the most analyzed and perhaps the most plausible mechanism to explain the pollution.

Because of the short time frame associated with the settling of heavy elements out of hydrogen-rich white dwarf atmospheres, the 1983 discovery of calcium in the atmosphere of one such white dwarf stimulated astronomers to consider seriously alternatives to the interstellar accretion model. But the field advanced slowly, and it was not until 20 years later (2003) that the current consensus model began to take serious shape.

By now the evidence is overwhelming that white dwarf pollution from tidally disrupted planetesimals is common; we can utilize the heavy element abundances in these otherwise-pristine stellar atmospheres to obtain the bulk composition of these erstwhile planetesimals. For the roughly one dozen stars where detailed relative abundances of 8 to 15 heavy elements have been measured, the pattern is unmistakably refractory- rich, volatile-poor, and broadly consistent with Solar System rocky materials (i.e. meteorites and bulk Earth).

I will highlight important results in the journey to the present picture, and will discuss the inferred masses and bulk chemical properties of these large and rocky, extrasolar planetesimals. In particular, using the Solar System as a model, we can deduce a past history of differentiation and collisions for some of these rocky exoplanet precursors.