Munich

Results from the first experiments to be carried out aboard the International Space Station are being analysed — and have already sparked debate among plasma physicists about the processes behind the formation of planets.

The experiments, run by a German–Russian collaboration, involved the study of plasma crystals. Plasma is the most disordered form of matter. But order — in the form of crystallization — can be produced by injecting microspheres made of melamine formaldehyde into the system. These become negatively charged amid the electrons and ions that form the plasma, and their electrostatic interactions then cause them to arrange themselves in an ordered way.

Because this effect of the microspheres is disturbed by gravity, scientists designed a series of gravity-free experiments on the space station (see Nature 405, 7; 2000).

Greg Morfill celebrates the mistake that offered him fresh insight into how planets form. Credit: ALISON ABBOTT

At a workshop held in Garching, near Munich, last month, the researchers reported a result obtained purely by chance. On one occasion, a cosmonaut forgot to make the plasma before injecting the microspheres. Without a plasma, the microspheres should remain uncharged and just float about randomly, they say. But instead, the microspheres coagulated almost immediately.

“One hundred thousand particles coagulated in a matter of seconds,” says Greg Morfill, a director of the Max Planck Institute for Extraterrestrial Physics in Garching, and joint chief investigator of the experiments. “We couldn't believe what we were seeing at first.”

Uncharged particles would take a day to aggregate through a chance encounter, he says. Further experiments showed that the injected microspheres did in fact carry opposite, attractive, charges, and this produced a millionfold acceleration in the coagulation.

Morfill's team proposes that, in the absence of plasma, a negative charge is generated on some of the microspheres through the transfer of electrons from the metal of the grid through which the microspheres are injected. These negatively charged microspheres could then cause a redistribution of charge on newly injected, neutral spheres. For a split second, the side of the neutral microsphere closest to the approaching, negatively charged sphere would become positive. Then the particles would collide and shatter, leaving the resulting fragments differentially charged.

Morfill, a theoretical astrophysicist, suggests that this mechanism might help to explain an enduring mystery about how planets form. Scientists believe that planets emerge from the disks of gas and dust that surround new stars, as the swirling dust particles coagulate into ever-bigger particles. But as they aggregate they tend to be accelerated towards the star. The problem for theoreticians is how the particles ever reach the critical size of a metre or so that can resist the gravitational pull of the star. Calculations indicate that aggregation would occur too slowly to overcome the star's drawing power.

Morfill theorizes that the particles could become differentially charged by the same charge-redistribution mechanism and shattering that were seen in the failed plasma-crystal experiment. This could allow sufficiently fast coagulation to the critical size, he says.

The idea is likely to prove controversial among planetary scientists. Jürgen Blum, of the University of Jena in Germany, is principal investigator of another experiment planned for the space station, ICAPS (Interactions in Cosmic and Atmospheric Particle Systems), which will study the aggregation of dust particles in space. “I'm not sure that there could be an abundance of charge on particles this size,” he says. “But any new experiments that indicate how planet precursors can be formed are very helpful.”

Morfill's team will have a chance to explore his theory further in another batch of experiments scheduled to take place on the space station early next year.