White dwarfs and exoplanets

White dwarf stars are the final evolutionary state of the vast majority of stars. Over 97% of stars end their lives in this form and studying them gives us a multitude of useful data on star evolution in general and the history of the stellar formation of the galaxy. Although the analysis of white dwarfs deserves great attention all by itself, the Institute’s researchers are primarily interested in their surroundings.

We recently found that it is possible to analyze the overall chemical composition of extrasolar rocky bodies with great precision, thanks to white dwarfs. After losing mass for a short time during the red giant phase, stars like our Sun exhaust all their available thermonuclear “fuel” and their cores collapse under their own weight. It is this kind of extremely dense stars (typically 60% of the Sun’s mass concentrated in a volume comparable to that of Earth) that we call “white dwarfs.”

Since the surface gravity of white dwarfs is extremely high (on average more than 100,000 that on the Earth’s surface), light elements rapidly float to the surface. The very high purity of hydrogen or helium observed at the surface of most white dwarfs comes from the fact that heavier elements very quickly sink beneath the surface.

Elements that shouldn’t be there!

NASA's Spitzer Space Telescope set its infrared eyes upon the dusty remains of shredded asteroids around several dead stars.

Artist’s impression of an asteroid broken up by the tidal forces of a white dwarf. Source: NASA/JPL-Caltech

Oddly enough, heavy elements like calcium, iron and magnesium are occasionally observed in the spectra of some white dwarfs. Since these elements cannot have remained at the surface since the origin of the star, their presence necessarily implies that they were very recently accreted there.

It is only in recent years, however, that the source of these elements at the surface of white dwarfs has been identified: they come from rocky bodies like planets, asteroids and comets that appear to have survived the final evolutionary states of their parent star before falling to the surface as a result of gravitational disturbances.

Consequently, it is now recognized that the heavy elements observed by spectroscopy at the surface of certain white dwarfs result from the accretion of planet or asteroid dust, from a debris field orbiting around the star. This is a unique opportunity to study the chemical composition of the rocky bodies that once orbited a distant star.

High-resolution spectroscopic observations of the white dwarf the most polluted by heavy elements known at the time, GD 362 (a star discovered by our team in 2004), revealed no fewer than 15 elements. Analysis of these quantities showed a remarkable similarity with the chemical composition of Earth, suggesting that an Earth-like planet had probably landed on the star’s surface.

Exo-asteroids!

An artist’s impression of a massive asteroid belt in orbit around a star.  The new work with SDSS data shows that similar rubble around many white dwarfs contaminates these stars with rocky material and water. Credit: NASA-JPL / Caltech / T. Pyle (SSC).

Artist’s impression of a debris disk and an exoplanet around a white dwarf. Source: NASA-JPL/Caltech/T. Pyle (SSC)

Following the lead of the pioneering analysis by Zuckerman et al., high-resolution spectroscopic observations were done for a half-dozen polluted white dwarfs (e.g. Zuckerman et al. 2010 and Klein et al. 2011). This work showed that the bodies responsible for the pollution of these white dwarfs consisted of over 90% oxygen, magnesium, silicon and iron (in comparison, 94% of Earth’s mass consists of these 4 elements). The total mass of the heavy elements found on these white dwarfs ranges from 1019 to 1023 g, similar to the typical mass of many of the asteroids in our solar system.

On the other hand, there are significant variations in the Fe/Si ratio, which could indicate the presence of differentiation (core-mantle-crust) to varying degrees from one rocky body to another. The proportions between volatile and refractive elements also vary, which could be a signature of the process of formation at different distances from the star. This kind of diversity is predicted, in fact, by estimations of the overall composition of terrestrial planets based on dynamic and chemical simulations of protoplanetary disks.

The study of white dwarfs polluted by heavy elements gives us some essential tools for validating different models describing the formation and evolution of terrestrial-type planets that could support life as we know it.