Astronomers have made the first direct detection of water in a rocky body that once orbited a sun-like star.

The object is the remnant of a rocky planet or a large asteroid that was shredded by its burned-out star, a white dwarf known as GD 61 and located about 150 light-years from Earth.

The fingerprints of water emerged after a tally of oxygen molecules littering the star’s otherwise pristine hydrogen and helium surface. (Heavier elements like silicon and carbon sink into the dead star’s core.)

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Astronomers determined the object was about 25 percent water, similar to Ceres, the largest object in our solar system’s main asteroid belt.

Earth, by comparison, is a paltry 0.02 percent water -- essentially a dry planet. Scientists believe Earth was too warm to retain any water it had during its formation and that water-rich asteroids later crashed into the planet, delivering its oceans.

“The Earth was mostly finished and then you hit it with a couple of water-rich guys and then you’re done. That’s the final bit in the recipe,” astronomer Jay Farihi, with the University of Cambridge, told Discovery News.

“Because this object had a lot of water, we know it formed in a region that is pretty similar to our main asteroid belt’s outer region,” Farihi said.

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Whether the object was a large asteroid or a piece of a planet is immaterial, he added.

“Because we have rocks this big, we know that (this system) had rocky planets. You just simply cannot get to something that big without going all the way to planets. It’s a runaway process,” Farihi said.

“This is the first time we’ve seen the kind of building blocks and puzzle pieces that are necessary for habitable, water-rich planets,” he added.

Farihi and colleagues used a light-splitting spectrograph on the Hubble Space Telescope to chemically analyze GD 61 after other astronomers discovered it contained huge amounts of oxygen.

“Where the oxygen will go will determine if you’ve got water,” said Torrence Johnson, a senior scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

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Johnson presented a separate study this week about how carbon-rich systems could deprive otherwise habitable planets of water, which is believed to be a key ingredient for life.

After accounting for GD 61 oxygen molecules chemically tied with silicon, magnesium, iron, calcium, aluminum and other elements, Farihi and colleagues realized there were far too many oxygen molecules left over.

The excess oxygen could have been chemically bonded with carbon to make carbon dioxide gas, but GD 61 turned out to be very carbon-poor.

“The only other chemically viable candidate is water,” Farihi said. “Water is very abundant in the universe. Oxygen combines with hydrogen pretty much wherever it can. Chemically speaking, there’s just no other alternative for this much oxygen.”

Astronomers are looking for other white dwarf stars littered with water-rich debris, but so far they haven’t been able to make a good measurement. Primarily, the stars’ temperatures are too high for currently available instruments on Hubble and other observatories to detect oxygen molecules for a headcount.

“GD 61 happens to sit in a bit of a sweet spot where it shows its oxygen very well. These other stars may have even more, but they just don’t show it to us,” Farihi said.

The research appears in this week’s journal Science.