European astrophysicists using the Hubble Space Telescope have discovered four white dwarfs surrounded by dust from shattered planetary bodies which once bore striking similarities to the composition of the Earth.
Artwork depicts what the researchers are now observing: a white dwarf sits in the center of the remnant of a planetary system; asteroid sized debris is scattered inwards by interaction with the remaining planets and is tidally disrupted as it approaches the white dwarf forming a disc of dust some of which is raining down onto the star. The researchers have found that the composition of the debris that has just fallen onto the four white dwarfs matches the composition of Earth-like rocky worlds (Mark A. Garlick)
The new study, which will be published in the Monthly Notices of the Royal Astronomical Society, shows that the most frequently occurring elements in the dust around four white dwarf stars, known as PG 0843+516, PG 1015+161, SDSS 1228+1040, and GALEX 1931+0117, are oxygen, magnesium, iron and silicon – the four elements that make up roughly 93 per cent of the Earth.
However an even more significant observation was that this material also contained an extremely low proportion of carbon, which matched very closely that of the Earth and the other rocky planets orbiting closest to our own Sun.
This is the first time that such low proportions of carbon have been measured in the atmospheres of white dwarf stars polluted by debris. Not only is this clear evidence that these stars once had at least one rocky exoplanet which they have now destroyed, the observations must also pinpoint the last phase of the death of these worlds.
The atmosphere of a white dwarf is made up of hydrogen and/or helium, so any heavy elements that come into their atmosphere are dragged downwards to their core and out of sight within a matter of days by the dwarf’s high gravity.
Given this, the astronomers must literally be observing the final phase of the death of these worlds as the material rains down on the stars at rates of up to 1 million kilograms every second.
Not only is this clear evidence that these stars once had rocky exoplanets which have now been destroyed, the observations of one particular white dwarf, PG 0843+516, may also tell the story of the destruction of these worlds.
This star stood out from the rest owing to the relative overabundance of the elements iron, nickel and sulphur in the dust found in its atmosphere. 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, astrophysicists believe they are observing PG 0843+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.
“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,” said lead author Prof. Boris Gänsicke of the Department of Physics at the University of Warwick. “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.”
“In our Solar system, Jupiter will survive the late evolution of the Sun unscathed, and scatter asteroids, new or old, towards the white dwarf,” Prof. Gänsicke concluded. “It is entirely feasible that in PG 0843+516 we see the accretion of such fragments made from the core material of what was once a terrestrial exoplanet.”
