Metal Ring Round White Dwarf Solves Missing Planets Puzzle (Forwarded)

Press and Media Relations
University of Warwick
Coventry, UK

For further information please contact:

Dr Boris Gänsicke
Department of Physics, University of Warwick
Tel: 02476 574741

Richard Fern, Press Officer
University of Warwick
Tel: 024 76 574255

Peter Dunn, Press and Media Relations Manager
Communications Office, University of Warwick
Tel: 024 76 523708

PR82 PJD

18th December 2006

Metal Ring Round White Dwarf Solves Missing Planets Puzzle

Astrophysicists at the University of Warwick have found an unusual ring of
metal-rich gas orbiting very close around a white dwarf star. The presence
of the ring helps solve a problem for astronomers who, up till now, have
been puzzled by the apparent absence of planets around white dwarf stars.
Their research is published today in the Friday December 22nd edition of
the journal Science.

The research team led by Dr Boris Gänsicke and Professor Tom Marsh from
the University of Warwick's Department of Physics found this unusual gas
disc around a relatively young white dwarf star called SDSS1228+1040. It
is located in the constellation Virgo and it is around 463 light years
distant from our solar system. The star became a white dwarf around 100
million years ago, and is still fairly hot with a surface temperature
around 22,000 degrees.

The team observed double-peaked emission lines superimposed on the white
dwarf's starlight caused by iron, magnesium and calcium from material in
the vicinity of the star. This indicated that they were dealing with a
disc of metal-gas orbiting close around the star (around 1.2 solar radii
or roughly half a million miles). The observations also show that we are
looking nearly edge-on to the ring around the white dwarf.

The likely origin of the disc is an asteroid, of at least 50 kilometres in
size, which approached close enough to the star to be broken up by tides
generated from the gravitational forces of the white dwarf. Those
disrupted remains then entered a close orbit around the star and is
evaporated by the radiation from the white dwarf.

White dwarfs begin as a star similar to our sun (or a star up to 8 times
bigger than our sun). Late in the star's life it swells into a red giant
probably destroying any inner planets at orbits such as those of Mercury
and Venus and pushing out other planets and asteroids to a more distant
orbit than before.
Here is a link to some simple diagrams explaining this:
http://deneb.astro.warwick.ac.uk/phs...SDSS1228+1040/

In the evolution of what is today a white dwarf, the progenitor of
SDSS1228+1040 will have destroyed all planetary material out to a distance
of 1000 solar radii (roughly 500 million miles), but asteroids still exist
today at larger distances. To destabilise an asteroid from an orbit that
far out, it needs the gravitational force of a larger object, such as a
relatively massive planetesimal, or a genuine planet. While the presence
of asteroids around white dwarf has been hypothesized before, the case of
SDSS1228+1040 provides the first clear proof of the debris of a planetary
disc around a white dwarf, and provides an example of what our own Solar
system may look like in around 5 to 8 billion years.

This "metal" disc around SDSS1228+1040 appears to be relatively rare.
Before their study, three white dwarfs, out of a study of a few hundred,
were suggested to be surrounded by planetary debris material. However, in
none of those three cases could a definite proof of an asteroid origin be
made due to the lack of information on the geometry and the chemical
abundance of the material found in the vicinity of these stars. As part of
their study, the Warwick team investigated data for 500 additional white
dwarfs without finding conclusive evidence for another system harbouring
such a disc. The rarity of such a ring made from a disrupted asteroid
tells us that the majority of planetary systems may look quite different
from our own Solar system. They may not have asteroid belts at all, or not
as far out as it is the case in the Solar system, or they may not have
planets at such great distances as Mars or Jupiter. This conclusion is
consistent with the current knowledge on extrasolar planets found around
others stars similar to the Sun, where the vast majority of the
exo-planets are in very close orbits around their host stars.

Notes for editors

1. A Podcast interview with Dr Gänsicke and Professor Marsh is available
he
http://www2.warwick.ac.uk/newsandeve...hite_dwarf.mp3

2. Science have told us that copies of the full embargoed Science paper
can only be distributed only by Science's AAAS Office of Public Programs
on 1-202-326-6440 or

3. The research was based on observations obtained on the 4.2m William
Herschel Telescope on La Palma.

IMAGE CAPTION:
[http://mms.warwick.ac.uk/mms/getMedi...FB1169F8B7.jpg
(3.97MB)]
Ring around white dwarf. Credits: Mark A. Garlick