Planets as small as Earth are hard to spot orbiting other stars; obtaining good data about their chemical composition is well beyond the abilities of our current instruments. However, a new study of four white dwarfs provides hints about the fate of planets like our own. It's not an especially happy one, as the astronomers found that the chemical composition of debris on the white dwarfs closely matches that of Earth.

These observations, which will be published in the Monthly Notices of the Royal Astronomical Society, focused on four "polluted" white dwarfs, which have traces of elements not usually seen in this type of star. B. T. Gänsicke et al. studied their ultraviolet spectrum using the Hubble Space Telescope, and determined they contained excessive amounts of silicon, aluminum, iron, and other elements, with abundances similar to those found on Earth. These results indicate that rocky debris has crashed onto the white dwarfs—debris that may possibly be from the destruction of a planet similar in composition to Earth.

White dwarfs are the cores of lower-mass stars that have run out of nuclear fuel and shed their outer layers. Our Sun falls into this mass category and will eventually experience that fate, so white dwarf systems provide potentially interesting glimpses into the evolution of the Solar System. White dwarfs are very compact: while their masses are still comparable to the Sun's, their sizes are closer to Earth's.

The combination of small size and relatively large mass means the gravitational field is strong. Thus, any heavy elements (including oxygen and metals like iron) will not collect in the atmosphere. Instead, they tend to sink out of sight into the white dwarf's interior, leaving an atmosphere of nearly pure hydrogen and helium. However, this sinking process takes enough time that astronomers can spot it happening, and any white dwarfs "polluted" with heavier elements can reveal aspects of the star system.

Gänsicke et al. examined the ultraviolet spectra of four polluted white dwarfs (euphoniously named PG0843+516, PG1015+161, SDSS1228+1040, and GALEX1931+0117). They looked at the specific signature of absorption by heavier elements, and compared the abundances of these elements both to typical white dwarfs and to other astronomical objects. They found elevated levels of silicon—an element common in terrestrial planets, the Moon, and some asteroids.

PG0843+516 also exhibited noticeable amounts of iron and nickel: the materials that dominate Earth's rocky interior. Not only that, the abundances of silicon and these other elements are consistent with those on Earth. At minimum, that suggests rocky asteroids were torn apart and accreted by the white dwarfs. However, the presence of sulfur and chromium hints that the destroyed object may have had a differentiated interior: a core and layers outside it, like Earth's mantle. (The terrestrial planets, the Moon, and the asteroid Vesta are known to be differentiated.)

Based on these data, Gänsicke et al. argue the chemical abundances are closer to terrestrial planet cores than to meteorites or other rocky debris. If this is true, then it is possible these white dwarfs devoured planets, and the "pollutants" are the remains of their cores.

To date, no white dwarfs are known to have planets. But given the large number of exoplanet systems, it seems only a matter of time before such a system is found. This study shows that some white dwarf systems may at least have the right elements in the right proportions, showing that they either maintain the environment needed for planet formation, or caused the death of a terrestrial planet.

MNRAS, 2012. Preprint: astro-ph.EP:1205.0167. This paper has been accepted for publication, though it has not yet appeared.