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Water Discovered in Extrasolar, Rocky World

  • VOA News

This is an artist's impression of a rocky and water-rich asteroid being torn apart by the strong gravity of the white dwarf star GD 61. (NASA, ESA, M.A. Garlick (, University of Warwick, and University of Cambridge)

Water has been discovered for the first time on a piece of rocky debris 150 light years away from Earth.

The debris, which is orbiting the white dwarf GD 61, is a relic of a planetary system that survived the burnout of its parent star. Scientists think the system had the potential to contain Earth-like exoplanets.

"These water-rich building blocks, and the terrestrial planets they assemble, may in fact be common. 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," according to Jay Farihi of the University of Cambridge, United Kingdom. "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."

Using the Hubble Space Telescope, Farihi’s team was able to do a chemical analysis of the debris around GD 61.

"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."

The white dwarf GD 61 is a relic of a star that once burned hotter and brighter than our Sun. The star exhausted its fuel in just 1.5 billion years. (Our Sun will last roughly ten times as long.)

NASA's Far Ultraviolet Space Explorer (FUSE) first found an abundance of oxygen in the dwarf's atmosphere in 2008. The only way to obtain a more precise measurement of the amount of oxygen in the debris around GD 61 requires observations in the ultraviolet, which can only be carried out above Earth's atmosphere.

Combining their results with a previous study that used the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii, the team also detected magnesium, silicon, and iron, which, together with oxygen, are the main components of rocks. By counting the number of these elements relative to oxygen the researchers were able to predict how much oxygen should be in the atmosphere of the white dwarf. They found significantly more oxygen than should have been carried by rocky minerals 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 Gänsicke of the University of Warwick, in Coventry, United Kingdom. He added that the small amount of carbon seen in the white dwarf rules out comets as the source of water. Comets are rich in both water and carbon compounds.

Earth is essentially a "dry" planet, with only 0.02 percent of its mass as surface water. So oceans came long after it had formed, most likely when water-rich asteroids in the solar system crashed into our planet.

The new discovery shows that the same water "delivery system" could have occurred in this distant, dying star's solar system — as this latest evidence points to it containing a similar type of water-rich asteroid that would have first brought water to Earth.

Six billion years from now an alien astronomer measuring similar abundances in the atmosphere of our burned-out Sun may reach the same conclusion that terrestrial planets once circled our parent star. Though GD 61 was different from our Sun, nevertheless, "it's a look into our future," said Gänsicke.

The new research findings are reported in the journal Science.

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