Four white
dwarf stars caught in the act of consuming ‘earth-like’ exoplanets
Royal
Astronomical Society press release (forwarded from the University of Warwick)
RAS PR
12/38
3 May 2012
For
immediate release
Four white
dwarf stars caught in the act of consuming ‘earth-like’ exoplanets (RAS PR
12/38)
University
of Warwick astrophysicists have pinpointed four white dwarf stars surrounded by
dust from shattered planetary bodies which once bore striking similarities to
the composition of the Earth. The scientists publish their results in a paper
in the journal Monthly Notices of the Royal Astronomical Society.
White
dwarfs are the final stage of life of stars like our Sun, the residual core of
material left behind after the available fuel for nuclear reactions has been
exhausted. 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 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 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 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.
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. 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.
‘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 destabilise the
orbits and lead to collisions between planetary bodies as happened in the
unstable early days of our solar systems.
‘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.
‘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.”
The
University of Warwick led team surveyed more than 80 white dwarfs within a few
hundred light years of the Sun, using the Cosmic Origin Spectrograph onboard
the Hubble Space Telescope.
Images and
captions
The
following high resolution artist impressions are available. They were all
created for the University of Warwick by the space artist Mark A. Garlick and
are free for use by media or the University but are otherwise copyright as
follows: “© Mark A. Garlick / space-art.co.uk / University of Warwick”
First
Artist’s impression by Mark A. Garlick
The inner
region of an exo-planetary system where four terrestrial planets orbit a
solar-like star.
Second
artist’s impression by Mark A. Garlick
The host
star is running out of hydrogen in the core, swells up, and its surface becomes
cooler. It is also losing mass, which causes the planets to move further out.
The perturbation of the orbits may lead to collisions that will generate large
amounts of rocky debris.
Third
artist’s impression by Mark A. Garlick
This
depicts what the researchers are now observing. A white dwarf sits in the
centre 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.
Image that
brings together all three artist’s impressions by Mark A. Garlick together in
one sequence
Science
contact
Professor
Boris Gänsicke, Department of Physics
University
of Warwick
Tel:
+44 (0)2476 574741
Mob: +44
(0) 7904 444830
Email:
boris.gaensicke(at)warwick.ac.uk
Media
contacts
Anna
Blackaby
University
of Warwick Science Press Officer
Tel: +44
(0)2476 575910
Mob: +44
(0) 7785 433155
Email: a.blackaby@warwick.ac.uk
Robert
Massey
Royal
Astronomical Society
Tel: +44
(0)20 7734 3307 x214
Mob: +44
(0)794 124 8035
Email: rm@ras.org.uk
Further
information
The new work is published in “The chemical diversity of exo-terrestrial planetary debris around white dwarfs”, B. T. Gänsicke, D. Koester, J. Farihi, J. Girven, S.G.Parsons, E. Breedt, Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper is available at http://arxiv.org/abs/1205.0167
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