NASA Artist’s depiction of a tidally-disrupted asteroid and resulting circumstellar dust disk accreting onto its host white dwarf.

With data from the Keck Telescope and the Hubble Space Telescope, we measure the compositions of planetesimals (building blocks of rocky planets) in other solar systems. In the graph at left, we compare an observed exoplanet composition measurement with the compositions of objects in our own solar system. The minor planet accreted by the white dwarf, GD 40, has a mass composition that most closely resembles Bulk Earth. As for other white dwarf systems, from the modest set of currently well-studied sources so far, we find as a first approximation, extrasolar planetesimals are measured to be more than 85% composed of the four main terrestrial elements: oxygen, magnesium, silicon, and iron. In other words, extrasolar planetary systems clearly do produce rocky bodies that are compositionally similar to Bulk Earth.  

Thus, planets with an Earth-like composition apparently do form elsewhere in the galaxy.

Compositions of extrasolar planetesimals Derived from Externally-polluted white dwarf stars

In this process, solid bodies are (conveniently!) ground down by the tidal forces of the high-gravity white dwarf stars. The elemental constituents of the parent bodies are accreted onto otherwise pure white dwarf atmospheres.  Compared to main sequence stars, white dwarf atmospheres are nearly blank slates for displaying even minuscule amounts of “high-Z” (atomic number > 2) material that enters from the outside.  Therefore, we have the unique opportunity to measure the bulk compositions of accreted planetary parent bodies at an extraordinary level of detail and precision. 

From Klein et al. (2011), Keck/HIRES spectrum of an externally-polluted white dwarf, HS2253+8023, displaying accreted elements, iron, magnesium and silicon, which came from an asteroid-sized planetesimal.

Comparison of bulk compositions between the accreted planetesimal at the polluted white dwarf, GD 40, and those of Solar System objects, from Klein et al. (2010). The accreted planetesimal is most similar in composition to Bulk Earth.

The most common end-state of stellar evolution is a white dwarf star along with the surviving components of its planetary system. These systems often include planets, as well as collections of smaller planetary bodies, such as minor planets, asteroids (also called planetesimals, i.e. the building blocks of rocky planets), and comets. Over the past decade or so of research, it has become evident that a significant population of white dwarfs are swallowing tidally-disrupted planetary bodies from their perturbed planetary systems. 

Nature has provided us with a back door for studying exoplanets. High-resolution, high-sensitivity spectroscopy of externally polluted white dwarfs enables us to measure the relative elemental compositions of rocky bodies in ancient planetary systems; it provides unique and detailed insight into the results of planetary formation and evolution in our galaxy.

Earth-sized planets can theoretically exist with a wide range of compositions, including: Earth-like, iron-dominated (Mercury-like), refractory-dominated, carbon-dominated, and ice-dominated (Pluto-like). What is the actual composition of planetesimals in exoplanetary systems?