Viewed from afar our Solar System would have a bright disc surrounding the Sun
Traces of the disc surrounding our Solar System
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.
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.
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