GD 61: A White Dwarf 170 Light Years Away, Formed at the Triassic-Jurassic Boundary and Tore Apart a Minor Planet with Water

Astrophysicists have found the first evidence of a water-rich rocky planetary body outside our solar system in its shattered remains orbiting a white dwarf.

A new study by scientists at the Universities of Warwick and Cambridge published in the journal Science analysed the dust and debris surrounding the white dwarf star GD61 170 light years away.

Using observations obtained with the Hubble Space Telescope and the large Keck telescope on Hawaii, they found an excess of oxygen – a chemical signature that indicates that the debris had once been part of a bigger body originally composed of 26 per cent water by mass. By contrast, only approximately 0.023 per cent of the Earth's mass is water.

Evidence for water outside our solar system has previously been found in the atmosphere of gas giants, but this study marks the first time it has been pinpointed in a rocky body, making it of significant interest in our understanding of the formation and evolution of habitable planets and life.

We know from our own solar system that the dwarf planet Ceres contains ice buried beneath an outer crust, and the researchers draw a parallel between the two bodies. Scientists believe that bodies like Ceres were the source of the bulk of our own water on Earth.

The researchers suggest it is most likely that the water detected around the white dwarf GD 61 came from a minor planet at least 90 km in diameter but potentially much bigger, that once orbited the parent star before it became a white dwarf.

Like Ceres, the water was most likely in the form of ice below the planet's surface. From the amount of rocks and water detected in the outer envelope of the white dwarf, the researchers estimate that the disrupted planetary body had a diameter of at least 90km.

However, because their observations can only detect what is being accreted in recent history, the estimate of its mass is on the conservative side.

It is likely that the object was as large as Vesta, the largest minor planet in the solar system. In its former life, GD 61 was a star somewhat bigger than our Sun, and host to a planetary system.

About 200 million years ago, GD 61 entered its death throes and became a white dwarf, yet, parts of its planetary system survived. The water-rich minor planet was knocked out of its regular orbit and plunged into a very close orbit, where it was shredded by the star's gravitational force. The researchers believe that destabilising the orbit of the minor planet requires a so far unseen, much larger planet going around the white dwarf.

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