ESA Science & Technology - INFO 21-1999: Sunlight keeps star dust at bay

Landgraf and colleagues made the discovery when looking at the mass distribution of dust particles collected by the detector between February 1992 and April 1996 as Ulysses swept along its solar polar orbit. They were interested in interstellar dust, which can be distinguished from dust originating in the solar system by its speed and direction of travel. Interstellar dust travels very fast, and can be found outside the plane of the planets' orbits, whereas solar system dust tends to travel more slowly in the planets' plane. "When I got the printout on my desk, it was obvious immediately that there was something unusual about the mass distribution. It was a big surprise," says Landgraf. Interstellar dust grains of a particular size - not too big and not too small - were missing from the volume of space between two and four times the distance of the Earth from the Sun.

After eliminating all other possible explanations, the team concluded that the pressure of sunlight was keeping the dust grains out of this region of space. The particles were just the right size for sunlight to have this effect. "If the particles are very small, light doesn't see them because they are smaller than the wavelength of light. If they are very big, they absorb and reflect light, but the light doesn't push them away because they're too heavy. The particles being pushed away are large enough to absorb and reflect light, but they don't have enough inertia to stay still," explains Landgraf.

As grains of this particular size travel towards the Sun, they are repelled constantly by the sunlight. This causes them to lose speed until eventually they come to halt at a distance of four times the Earth-Sun distance. At distances closer than twice the Earth-Sun distance, the findings are unreliable because interstellar and solar system dust are more difficult to tell apart, says Landgraf.

The discovery says something about the optical properties of the banished dust grains. So Landgraf and colleagues decided to consult tables that list the strength of radiation pressure needed to deflect same-sized grains of different types of material. The best match, they found, was with a mixture of silicates detected in interstellar clouds elsewhere in the Milky Way, suggesting that we are moving through an identical cloud. "We were very excited when we found that the composition fits," says Landgraf.

The notion that we are travelling through an interstellar cloud is not new. First a cloud of gas was discovered moving through the solar system. Then by 1995, Ulysses had detected enough galactic dust grains to see that they travel through the solar system in the same direction as the gas, suggesting that the cloud consists of dust as well as gas.

The composition of dust grains in distant interstellar clouds can be measured with telescopes from the way in which light is absorbed and scattered by the clouds. But dust grains in our local interstellar cloud are too sparse and close to us for such measurements. Landgraf and colleagues' findings, however, show that there are indirect ways and means of deducing their composition.

The evidence is stacking up for our own local interstellar dust and gas cloud. But where did the cloud come from? "We don't know the whole history of these grains, which is why they're so intriguing. They could have originated in supernovae explosions, or they could be the outflow of old stars which give out star dust when they get old, much like a candle produces soot when it becomes too cold," speculates Landgraf.