ESA Hubble finds dead stars 'polluted' with planetary debris

This illustration is an artist's impression of the thin, rocky debris disc discovered around the two Hyades white dwarfs. 

Rocky asteroids are thought to have been perturbed by planets within the system and diverted inwards towards the star, where they broke up, circled into a debris ring, and were then dragged onto the star itself. 

Credit: NASA, ESA, STScI, and G. Bacon (STScI)

The NASA/ESA Hubble Space Telescope has found signs of Earth-like planets in an unlikely place: the atmospheres of a pair of burnt-out stars in a nearby star cluster.

The white dwarf stars are being polluted by debris from asteroid-like objects falling onto them. This discovery suggests that rocky planet assembly is common in clusters, say researchers.

The stars, known as white dwarfs—small, dim remnants of stars once like the Sun—reside 150 light-years away in the Hyades star cluster, in the constellation of Taurus (The Bull).

The cluster is relatively young, at only 625 million years old.

Astronomers believe that all stars formed in clusters. However, searches for planets in these clusters have not been fruitful—of the roughly 800 exoplanets known, only four are known to orbit stars in clusters.

This scarcity may be due to the nature of the cluster stars, which are young and active, producing stellar flares and other outbursts that make it difficult to study them in detail.

Jay Farihi

A new study led by Jay Farihi of the University of Cambridge, UK, instead observed "retired" cluster stars to hunt for signs of planet formation.

Hubble's spectroscopic observations identified silicon in the atmospheres of two white dwarfs, a major ingredient of the rocky material that forms Earth and other terrestrial planets in the Solar System.

This silicon may have come from asteroids that were shredded by the white dwarfs' gravity when they veered too close to the stars.

The rocky debris likely formed a ring around the dead stars, which then funnelled the material inwards.

The debris detected whirling around the white dwarfs suggests that terrestrial planets formed when these stars were born.

After the stars collapsed to form white dwarfs, surviving gas giant planets may have gravitationally nudged members of any leftover asteroid belts into star-grazing orbits.

"We have identified chemical evidence for the building blocks of rocky planets," says Farihi. "When these stars were born, they built planets, and there's a good chance that they currently retain some of them. The signs of rocky debris we are seeing are evidence of this—it is at least as rocky as the most primitive terrestrial bodies in our Solar System."