Science: Water and Rocky Minerals Glimpsed in a White Dwarf

Artist's depiction of a rocky and water-rich asteroid being torn apart by the strong gravity of the white dwarf star GD 61. [Mark A. Garlick/University of Warwick and University of Cambridge/space-art.co.uk]

One day in the distant future the sun will burn out and collapse. And when that happens, it will likely evolve into a white dwarf — the tired ember of a once-bright star. In such a scenario, the white dwarf’s tremendous gravity would begin stripping all the elements from the solar system’s inner planets, sucking their mass onto its own. It’s nothing spiteful; it’s simply how white dwarfs grow.

Now, Jay Farihi from the Institute of Astronomy at Cambridge University and colleagues in the United Kingdom and Germany say that a white dwarf known as GD 61 has destroyed a rocky, water-rich planetary body in this very fashion. Before it was reduced to a dusty, circumstellar disk, this particular asteroid (or possibly, piece of an exoplanet) contained about 26 percent water by mass, according to the researchers.

This discovery represents the first time that a white dwarf star has been caught catching water and rocky materials, both of which are building blocks for a habitable planet. It also implies that water can survive the incredibly hot evolution of a Sun-like star into a dense, white dwarf, and that GD 61 may have hosted an Earth-like planet in its orbit at some point.

The researchers report their data in the 11 October issue of the journal Science.

“On Earth, the acceleration of gravity is about 10 meters per second squared,” explained Farihi. “On a white dwarf, gravity is a hundred thousand times stronger, and that extreme gravitational pull on any nearby planets causes all their elements to separate.” So, when researchers observe a “polluted” white dwarf, they know that elements have either fallen onto it recently or that they are continually falling onto it.

Farihi and his colleagues used the Cosmic Origins Spectrograph onboard the Hubble Space Telescope to analyze the light emitted by GD 61. In doing so, they were able to constrain the amount of rock-forming minerals (oxygen, magnesium, aluminum, silicon, calcium and iron) surrounding the star.

“White dwarfs are like fly traps,” said Farihi. “If they accumulate elements, we can see them…And the diversity of the elements we’ve seen in GD 61 is similar to that which is found in our solar system.”
Artist's depiction of a rocky and water-rich asteroid being torn apart by the strong gravity of the white dwarf star GD 61. [Mark A. Garlick/University of Warwick and University of Cambridge/space-art.co.uk]

Upon reviewing their results, the researchers observed an abundance of oxygen near the surface of GD 61, 26 to 28 percent of which could not be accounted for by the formation of minerals. Thus, they propose that the excess oxygen originated in water.

“The universe, or at least our solar system, makes rocks in a very certain way,” said Farihi. “Even though there’s a great diversity of rocks on Earth, when you break them down into their constituent elements — with the exception of iron — there will always be an exact number of oxygen atoms accompanying every atom of a rock-forming element.”

When the researchers analyzed the spectrum of GD 61, however, they found an excess of oxygen atoms that could only be explained by carbon dioxide or water. “And the white dwarf really doesn’t have much carbon,” according to Farihi.

This particular amount of water is comparable to the amount contained within some asteroids in the solar system’s main belt. And since most astronomers believe that asteroids delivered water to a dry, rocky Earth eons ago, Farihi and his team acknowledge that GD 61 may have disrupted some similar water delivery processes.

“The system seems to have had all the tools,” concluded Farihi. “It was throwing around asteroids that seemingly had the potential to create oceans like the ones on Earth.”

There’s no way to tell what kind of system GD 61, which is approximately 150 light years away, once hosted. But the pollution that surrounds the white dwarf hints at some strangely familiar territory.

Read the abstract, “Evidence for Water in the Rocky Debris of a Disrupted Extrasolar Minor Planet,” by Jay Farihi and colleagues.