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article imageFormer water world discovered in deep space

By Robert Myles     Oct 12, 2013 in Science
Warwick - Astrophysicists from the universities of Warwick and Cambridge in the UK have found evidence of what was once a water-rich rocky planet outside our solar system, by analysing the atmosphere of white dwarf star GD 61.
Previously, evidence of water on planets outside the solar system — exoplanets, as they are known — has only been found in the atmosphere of gas giants, planets similar to Jupiter and Saturn, orbiting other suns.
The latest discovery is the first time water has been associated with a rocky planetary body, like Earth or Mars, within our solar system.
The new study, which was published Science, analyzed dust and debris orbiting the white dwarf star GD 61, some 170 light years distant from us. Using observations obtained from both the Hubble Space Telescope and the large telescopes of the W. M. Keck Observatory, the UK researchers looked at the shattered remnants of what was once a rocky planet circling star GD 61.
Their observations found an excess of oxygen in the debris. Oxygen’s chemical signature indicated the orbiting debris had once been a constituent of a larger planetary body composed of 26 percent water by mass. By comparison, Earth’s mass comprises a miniscule 0.023 percent water.
Although the discovery points to there being, at one time, an exoplanet very different in composition to our Earth, there are comparators in our own solar system. Ceres, now classed as a minor planet and the largest body in the Asteroid Belt, situated between the orbits of Mars and Jupiter, contains ice buried beneath a solid crust.
Another parallel may be one of Jupiter’s moons, Europa. In March 2013, research from NASA/JPL found large parts of Europa’s surface were laden with salts. The NASA scientists speculated that the salts were bubbling up from huge liquid oceans below Europa’s surface.
Scientists believe bodies like Ceres may have been the source for most of the water found on Earth. Like Ceres, the scientists believe GD 61’s former planet had massive quantities of water below the former planet’s surface.
In the case of the GD 61 oxygen signature, the researchers suggest this emanates from a long-shattered minor planet at least 90 kilometers across, but possibly significantly bigger, that at one time orbited its parent star before star GD 61 became a white dwarf.
Dependent on a star’s original size, white dwarf stars are formed when a star begins to exhaust its supply of hydrogen, fuelling fusion reactions and converting the hydrogen to helium. The star then first expands before collapsing in on itself, re-establishing an equilibrium as a white dwarf.
The discovery of water once existing in large measure on a rocky exoplanets is a significant step in understanding how habitable planets, and, with that, possible extra-terrestrial life, may have evolved, say the research team.
Some 200 million years ago, star GD 61, originally a star bigger than our sun, began its death throes that would change it to a white dwarf. Despite this massive solar event, parts of GD 61’s former solar system survived. The minor planet believed to have given rise to the oxygen signature now discovered was blasted to smithereens by the irresistible power of GD 61’s gravitational forces, after being knocked out of its original orbit and pulled closer to the parent star.
Commenting on the research, Professor Boris Gänsicke, of the Department of Physics at the University of Warwick said, “At this stage in its existence, all that remains of this rocky body is simply dust and debris that has been pulled into the orbit of its dying parent star. However this planetary graveyard swirling around the embers of its parent star is a rich source of information about its former life.”
He continued. “In these remnants lie chemical clues which point towards a previous existence as a water-rich terrestrial body. Those two ingredients – a rocky surface and water – are keys in the hunt for habitable planets outside our solar system so it’s very exciting to find them together for the first time outside our solar system.”
And on the GD 61 findings, lead author Jay Farihi from Cambridge’s Institute of Astronomy said, “The finding of water in a large asteroid means the building blocks of habitable planets existed – and maybe still exist – in the GD 61 system, and likely also around substantial number of similar parent stars,” adding, “These water-rich building blocks, and the terrestrial planets they build, 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.”
The UK research once again demonstrated the worth of the orbiting Hubble Space Telescope, able to view distant objects in other solar systems free from the blocking effects of Earth’s atmosphere on ultra-violet light. Supplementary observations from both the 10-meter telescopes of the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii, were also used.
The two telescopes allowed the Warwick and Cambridge researchers to identify different chemical elements present in the outer layers of the white dwarf. From a sophisticated computer model of the white dwarf atmosphere, developed by Detlev Koester from the University of Kiel in Germany, they could infer the chemical composition of the shredded minor planet. The scientists detected a range of “elemental abundance” in the white dwarf’s contaminated atmosphere. Elements such as magnesium, silicon and iron were found. These, together with oxygen, are the main constituents of rocks.
Knowing how much oxygen ought to have been a constituent part of what was once a rocky planet, researchers could predict how much oxygen should be in the white dwarf’s atmosphere. But they found “significantly” more oxygen than would have been the case if rocks were the sole source.
Quoted on the University of Cambridge website, Gänsicke said, “This oxygen excess can be carried by either water or carbon, and in this star there is virtually no carbon – indicating there must have been substantial water. This also rules out comets, which are rich in both water and carbon compounds, so we knew we were looking at a rocky asteroid with substantial water content – perhaps in the form of subsurface ice – like the asteroids we know in our solar system such as Ceres.”
Previously scientists at various research establishments had observed 12 destroyed exoplanets orbiting white dwarves. The GD 61 research is the first time the signature of water has been found.
It’s another tantalizing sign that there may be life out there. As Farihi put it, “Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system.”
More about GD 61, Astronomy, Astrophysics, Hubble space telescope, Exoplanets
 
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