Markus Landgraf of the European Space Agency and colleagues (*) have found
the first direct evidence that a bright disc of dust surrounds our Solar
System, starting beyond the orbit of Saturn. Remarkably, their discovery
gives astronomers a way to determine which other stars in the Galaxy are most
likely to harbour planets and allows mission planners to draw up a
‘short-list’ of stars to be observed by ESA’s future planet-search missions,
Eddington and Darwin.

The discovery of the Solar System’s dust ring strengthens the idea that such
features around mature stars are signposts to planetary systems. The reason
for this is that planetary systems are thought to condense from a cloud
of gas and dust. Planets form near the central star, where the material is
densest. However, at great distances from the star, the gas and dust is
sparse and can coalesce only into a vast band of small, icy bodies. In our
Solar System, they form the so-called Edgeworth-Kuiper belt that extends out
beyond the orbit of Neptune. Any remaining dust is lost to deep space.

Ordinarily, dust is either incorporated into larger celestial bodies or
ejected from the Solar System. For it still to be present today, means that
something is replenishing it.

“In order to sustain such a ring, 50 tonnes of dust have to be generated
every second,” says Landgraf.

He and his colleagues believe that collisions between the icy remnants of the
Edgeworth-Kuiper belt create the Solar System’s dust ring. If the same is
going on in other planetary systems, then those stars will also have dusty
rings around them.

“If you have a dust disc around a star that’s not particularly young, then
it’s extremely interesting because the dust has to come from somewhere. The
only explanation is that the star has planets, comets, asteroids or other
bodies that collide and generate the dust,” says Malcolm Fridlund, ESA’s
study scientist for Darwin, the mission under development to search for
life-supporting planets around other stars.

To trace the collisions in the Edgeworth-Kuiper Belt, Landgraf and colleagues
had to do some celestial detective work. They began by sifting through data
from the 1970s and early 1980s, when NASA space probes Pioneer 10 and 11
first found dust particles of unknown origin beyond Saturn’s orbit. The
hypothesis of dust coming from comets was discarded: in fact near the Earth,
comets give off dust; beyond Saturn, however, they freeze and shed little
material. So, no one knew whether the Pioneer dust grains were coming from
inside the Solar System ? from a source other than comets – or beyond it from
the interstellar space. Now, using data from ESA’s Ulysses spacecraft, which
has been orbiting the poles of the Sun for more than 10 years, Landgraf and
colleagues have been able to rule out an origin beyond the Solar System. The
Ulysses data shows that dust grains of interstellar origin are considerably
smaller than interplanetary dust grains, which originate in the Solar System.

The interstellar grains detected by Ulysses are typically ten to a hundred
times smaller than the smallest grain that could be detected by Pioneer.
Thus, the Pioneer grains have to be made somewhere within our Solar System.

So, by a process of elimination and computer simulations, the scientists came
to the conclusion that the only possible source of the dust is the collisions
between the small, icy objects in the Edgeworth-Kuiper belt.
Since these are the remnants of planet formation, the team believe that
planetary systems around other stars will also produce constantly
replenishing dust rings.

>From the number of dust particles detected by the Pioneers, Landgraf and
colleagues were able to calculate the density of dust in the ring. “There’s
only one dust particle every 50 cubic kilometres but it’s enough for a bright
dust ring like those we see around other stars,” says Landgraf.

Indeed, a number of such features have been observed shining brightly at
infrared wavelengths around stars such as Vega and Epsilon Eridani.
Future missions, such as ESA’s Herschel mission will search for many more and
take detailed pictures of them. As these images become available, astronomers
will be able to predict the sizes and orbits of giant planets within the
alien solar system.

“If we see a similar dust ring around a main sequence star (a mature star,
like the Sun), we’ll know it must have asteroids or comets. If we see gaps
in the dust ring, it will probably have planets which are sweeping away the
dust as they orbit,” says Landgraf.

The result slots into place another piece of the puzzle for those scientists
working on ESA’s missions that will search for extrasolar planets, as it will
allow them to draw up a well motivated list of target stars based upon
whether they are surrounded by dust rings. “This finding has exciting
implications for both missions,” confirms Fridlund.

The full details of Landgraf’s results will be published in a future issue of
The Astrophysical Journal.

Note to Editors

(*)Members of the team of scientists include:

Markus Landgraf, ESA’s Space Operations Centre (Darmstadt, Germany)

Eberhard GrUn, Max Planck Institute for Nuclear Physics, Heidelberg, Germany
Jer-Chyi Liou, Lockheed Martin Space Operations, United States of America
the late Herb Zook, NASA’s Johnson Space Centre, United States of America

The European Space Agency’s space science programme pools the scientific and
engineering skills of 15 countries, to develop innovative and exciting
missions to explore our Solar System, observe the wider Universe and probe
the secrets of fundamental physics.

Ulysses, a joint ESA/NASA mission, was launched in 1990 and is charting the
unknown reaches of space above and below the poles of the Sun.

Eddington will determine the precise chemical composition of the stars and
search for Earth-sized worlds around other planets.

The Darwin mission, envisaged for the next decade, will use a flotilla of
space telescopes working together to scan the nearby Universe, looking for
signs of life on Earth-like planets.

Herschel will be the largest space telescope by the time of its launch,
in 2007. One of its main goals will be to solve the question of how the
first stars and galaxies formed.

For more information please contact:

Clovis De Matos

ESA – Science Programme Communication Service

Tel: +31 71 5653460

Email: Clovis.De.Matos@esa.int

For further information on ESA’s science programme go to
http://sci.esa.int