Hubble Finds White Dwarfs Surrounded by Planet Debris

Hubble Space Telescope has discovered two burnt-out stars called white dwarfs surrounded by clouds made of silicon and low levels of carbon. Silicon is a major constituent of rocky materials that the Earth and other solid planets are made of. This discovery suggests that these star systems had rocky planets in the past or possibly still have them and is the result of the white dwarf pollution technique which is used for discovery of extrasolar planets.

In the constellation Taurus, in the star cluster Hyades 150 light-years from Earth, two white dwarfs have been discovered, which have asteroid-like debris surounding them and falling onto them. The star cluster these dead stars are located in is only 625 million years old and the stars are "polluted" by clouds made of silicon and to lesser extent carbon. The discovery was made as a part of a new study led by Jay Farihi of the University of Cambridge in England.

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

The study suggests that white dwarfs' strong gravitational forces tore apart asteroids less than 200 kilometers wide, consisting of same materials as terrestrial planets, most notably silicon. "It's difficult to imagine another mechanism than gravity that causes material to get close enough to rain down onto the star," Farihi said. There is also a possibility of Earth-sized, rocky planets in the same system.

Most exoplanets discovered by other exoplanet discovery methods, like Doppler or transit method, are Jupiter-like gas giants. Farihi considers that using Hubble's Cosmic Origins Spectrograph to analyze the clouds surrounding white dwarfs is the best way for finding the signs of solid planet chemistry and concluding their composition.

"Normally, white dwarfs are like blank pieces of paper, containing only the light elements hydrogen and helium,"Farihi said. "Heavy elements like silicon and carbon sink to the core. The one thing the white dwarf pollution technique gives us that we just won't get with any other planet-detection technique is the chemistry of solid planets."

Over 100 white dwarfs with planetary debris around them are being researched in an effort to not only identify the rocks' composition, but to also find exoplanets that have not yet been torn apart by burnt-out stars' gravitational force.



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