Hubble discovers hoard of water-rich rocks

The rocks were discovered orbiting around white dwarf star GD 61

Credit: NASA, ESA, M.A. Garlick (, Uni. of Warwick, Uni. Of Cambridge

White dwarf star GD 61 is surrounded by water-rich rocky debris

By Kieron Allen

Astronomers using the Hubble Space Telescope have discovered planetary material containing substantial amounts of water around a dying star 150 lightyears from Earth.

The team used Hubble’s Cosmic Origins Spectrograph (COS) to analyse rocky debris orbiting white dwarf star GD 61, with the results suggesting that the distant stellar system could once have contained Earth-like exoplanets.

Jay Farihi of the University of Cambridge was part of the research team, "These water-rich building blocks, and the terrestrial planets they assemble, may in fact be common,” he said.

“A system cannot create things as big as asteroids and avoid building planets, and GD 61 had the ingredients to deliver lots of water to their surfaces.

"Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system. The system almost certainly had (and possibly still has) planets, and it had the ingredients to deliver lots of water to their surfaces."

Fragments of long extinct planets are hurtling towards GD 61 giving astronomers a unique opportunity to analyse the debris.

"The only feasible way to see what a distant planet is made of is to take it apart, and nature does this for us using the strong gravitational tidal forces of white dwarf stars," said Farihi. "This technique allows us to look at the chemistry that builds rocky planets, and is a completely independent method from other types of exoplanet observations."

Although NASA had found evidence of these fragmented planets back in 2008, the only way to precisely measure the levels of oxygen in the debris was through ultraviolet observations carried out above Earth's atmosphere – cue Hubble’s COS.

Along with oxygen the team also discovered magnesium, silicon, and iron – the main components of rocks. By counting the number of these elements relative to oxygen, the team could predict how much oxygen should be in the atmosphere of GD 61.

Far more oxygen was detected than should have been present in the rocks alone. "The oxygen excess can be carried by either water or carbon mono- or dioxide. In this star there is virtually no carbon, indicating there must have been substantial water," said Boris Gnsicke of the University of Warwick.




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