For a long time, Earth was the weirdest planet we knew about. In our little corner of the universe, where Mercury is the hot one, Jupiter is the protective bigger brother, and Pluto is the one we kicked out of the club for breaking the rules, Earth is the crazy cat lady, hoarding billions of life forms.
The shattered remains of planets that bear a striking resemblance to our own Earth have been found around white dwarf stars – offering a vision of what will one day happen to our planet.
White dwarfs are the final stage of life of stars like our Sun – once the thermonuclear furnace inside a star ‘burns out’.
Using the Hubble Space Telescope to carry out the biggest survey to date of the chemical composition of the atmospheres of white dwarf stars, the researchers found that the most frequently occurring elements in the dust around these four white dwarfs were oxygen, magnesium, iron and silicon — the four elements that make up roughly 93 percent of the Earth.
At least one of the stars is in the process of sucking in the planet’s core – rich in iron, nickel and sulphur – at a rate of around a million kilos a second.
Professor Boris Gänsicke of the Department of Physics at the University of Warwick, who led the study, said the destructive process which caused the discs of dust around these distant white dwarfs is likely to one day play out in our own solar system.
‘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.
‘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.
‘During the transformation of the Sun into a white dwarf, it will lose a large amount of mass, and all the planets will move further out. This may destabilize the orbits and lead to collisions between planetary bodies as happened in the unstable early days of our 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.
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.
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 exoplanetary bodies which have now been destroyed, the observations of one particular white dwarf, PG0843+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 center 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.
The University of Warwick led team surveyed more than 80 white dwarfs within a few hundred light-years of the Sun, using the Cosmic Origins Spectrograph onboard the Hubble Space Telescope.