These dwarf stars consume exoplanets

Astrophysicists at the University of Warwick have identified four white dwarfs surrounded by dust from shattered planetary bodies which once bore striking similarities to the composition of the Earth.

Using NASA’s Hubble Space Telescope, researchers determined that the most frequently occurring elements in the dust around the four white dwarfs were oxygen, magnesium, iron and silicon – the four elements which make up approximately 93% of the Earth.

Interestingly enough, the material also contained an extremely low proportion of carbon, which closely matched that of the Earth and other rocky planets orbiting closest to our own Sun.

According to Professor Boris Gänsicke of the University of Warwick, 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 the stars once had at least one rocky exoplanet which they have now destroyed, says Gänsicke, but the observations must also pinpoint the last phase of the death of these worlds. 

Indeed, the atmosphere of a white dwarf is comprised 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. 

Essentially, 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 per second. Clearly, explains Gänsicke, the 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. To be sure, this star stood out from the rest due to the relative overabundance of the elements iron, nickel and sulphur in the dust discovered 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.

”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,” Gänsicke explained.

“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.”

Gänsicke also noted that the transformation 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. [As such, it is entirely feasible that in PG0843+516 we see the accretion of such fragments made from the core material of what was once a terrestrial exoplanet,” he added.