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The building blocks of a water-rich world?

Artist's impression of a rocky and water-rich asteroid being torn apart by the strong gravity of the white dwarf star GD 61. Similar objects in the solar system likely delivered the bulk of water on Earth and represent the building blocks of the terrestrial planets.
[Image © Mark A. Garlick, space-art.co.uk, University of Warwick and University of Cambridge]

One day in the very distant future, our Sun will burn out and collapse. And whenever that happens, there's a fair chance that it will evolve into a white dwarf star—a small but incredibly dense burned-out star. As a white dwarf star with tremendous gravity, our Sun would begin stripping all of the elements away from the solar system's inner planets, sucking their mass onto itself like a stellar vacuum cleaner. That's how white dwarfs grow.

Now, though, Jay Farihi and colleagues say that a distant white dwarf star known as GD 61 has destroyed a rocky, water-rich planetary body in this very fashion. Before the exoplanet—or possibly, asteroid—was reduced to a ring of dust by the white dwarf, the researchers suggest that it contained about 26% water by mass. This discovery represents the first time that a white dwarf star has been shown to collect water and rocky materials—two ingredients that are needed to make a livable planet—from a nearby planetary body.

Farihi and his colleagues used instruments onboard the Hubble Space Telescope to analyze the light coming from GD 61 and calculate the amount of rock-forming elements—oxygen, magnesium, aluminum, silicon, calcium and iron—that were present around the star. They observed an abundance of oxygen—26 to 28% of which could not be easily explained—and they suggest that it came from water.

According to the researchers, this amount of water is comparable to what is contained within some asteroids in our solar system's main belt. Their findings suggest that GD 61 may have had Earth-like planets orbiting it at one point in the past.