The outcome of the first stage of planetary formation, which is characterized by ballistic agglomeration of preplanetary dust grains due to Brownian motion in the free molecular flow regime of the solar nebula, is still somewhat speculative. We performed a microgravity experiment flown onboard the space shuttle in which we simulated, for the first time, the onset of free preplanetary dust accumulation and revealed the structures and growth rates of the first dust agglomerates in the young solar system. We find that a thermally aggregating swarm of dust particles evolves very rapidly and forms unexpected open-structured agglomerates. PACS numbers: 96.35.Cp, 61.43.Hv, 81.10.Mx It is now widely accepted that planets form from the nebula of gas and dust that comprises nascent solar systems. Inelastic, adhesive collisions between these dust particles eventually form kilometer-sized bodies, called planetesimals, which then collide under the influence of their mutual gravity to form planets [1][2][3][4][5][6]. After condensation of the micron-sized dust grains in the cooling gas, these initially collide with each other due to thermal (Brownian) motion, and, by adhesion due to van der Waals forces, form aggregates. The agglomeration rate of freshly condensed [7,8] preplanetary dust grains is determined by three factors: the collision cross section, the collision velocity, and the sticking probability of the dust particles, which are mutually interdependent. Laboratory experiments with micron-sized solid particles and dust agglomerates thereof have shown that, for moderate collision velocities y c # 1 m s 21 , the sticking probability is always unity [9][10][11]. The collision cross section and the collision velocity strongly depend on the morphology of the interacting preplanetary dust aggregates. Open-structured, fluffy particles generally have a larger cross section than compact grains, but couple also much better to the gas motion, so that relative velocities between fluffy agglomerates are suppressed. The gas-grain interaction is best described by the dust particles' response time to the gas motion, t f . In the free molecular flow regime, t f~m s a , where m and s a are the mass and the geometrical cross section (i.e., the projected area) of the dust aggregate. Aggregation models [1,12] for the Brownian motion-driven dust growth predict a scenario in which dust clusters of similar mass predominantly contribute to the agglomeration process. This leads to the evolution of a quasimonodisperse distribution of aggregate masses and to a relation between aggregate mass and size s ofwith an exponent ("fractal dimension") in the range of D f ഠ 1.8 2.1. For quasimonodisperse systems, the mean aggregate mass grows by a power law in time, m͑t͒~t z , and is related to the mass dependence of the collision cross section s c~m m and the collision velocity y c~m n of the aggregating particles through m 1 n z21 z (e.g., see the review in [13]). Here, z, m, and n are the respective exponents of the assumed power law functions for ...
