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White dwarf stars caught 'eating' Earth-like planets

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Rocky material in orbit around a white dwarf star (centre). Collisions turn larger material into dust, some of which then rains down on to the white dwarf. Image Credit: Mark A. Garlick / space-art.co.uk / University of Warwick

LONDON (BNS): Stumbling upon a weird phenomenon, astrophysicists have caught four white dwarf stars, in the dead end of their lives, consuming Earth-like exoplanets which once revolved around them.

Using the Hubble Space Telescope to carry out the biggest ever survey of chemical composition of the atmospheres of white dwarf stars, University of Warwick astrophysicists have found four such stars surrounded by dust from shattered planetary bodies which once bore striking similarities to the composition of the Earth.

White dwarfs are the final stage of life of stars like our Sun, the residual cores of material left behind after their available fuel for nuclear reactions has been exhausted. The atmosphere of such dying stars is made up of lighter elements like hydrogen and/or helium.

However, the astronomers studying the white dwarfs found that the most frequently occurring elements in the dust around the four white dwarfs were oxygen, magnesium, iron and silicon - the four heavy elements that make up roughly 93 per cent of the Earth.

They also observed, for the first time, the presence of very low amount of carbon which matched quite closely that of the Earth and other rocky planets orbiting closest to our own Sun.

These findings confirmed that the four white dwarf stars once had at least one rocky exoplanet which they have now destroyed.

The heavy elements from the rocky planets got dragged downwards to the dying stars' core and out of sight within a matter of days by the dwarfs' high gravity.

One particular interesting finding was dwarf star PG0843+516. The dust found in its atmosphere contained overabundance of the elements iron, nickel and sulphur.

Iron and nickel are found in the cores of terrestrial planets, as they sink to the centre owing to the pull of gravity during planetary formation, and so does sulphur thanks to its chemical affinity to iron.

Therefore, researchers believe they are observing white dwarf PG0843+516 in the very act of swallowing up material from the core of a rocky planet that was large enough to undergo differentiation, similar to the process that separated the core and the mantle of the Earth.

A similar fate could be waiting for our Earth in distant future when the Sun, towards the fag-end of its life, could gobble up its inner planets.

"What we are seeing today in these white dwarfs several hundred light years away could well be a snapshot of the very distant future of the Earth. As stars like our Sun reach the end of their life, they expand to become red giants when the nuclear fuel in their cores is depleted.

"When this happens in our own Solar System, billions of years from now, the Sun will engulf the inner planets Mercury and Venus.

"It's unclear whether the Earth will also be swallowed up by the Sun in its red giant phase - but even if it survives, its surface will be roasted," Professor Boris Gänsicke of the Department of Physics at the University of Warwick, who led the study, said.

As the Sun will transform into a white dwarf, it will lose large amount of mass, all the planets will move further out. This could destabilise the orbits and lead to collisions between planetary bodies as happened in the unstable early days of the Solar System.

This may even shatter entire terrestrial planets, forming large amounts of asteroids, some of which will have chemical compositions similar to those of the planetary core.

In our Solar System, Jupiter will survive the late evolution of the Sun unscathed, and scatter asteroids, new or old, towards the white dwarf, Gänsicke said.

The researchers studied more than 80 white dwarfs within a few hundred light years of the Sun and report their findings in the Monthly Notices of the Royal Astronomical Society journal.

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