Searching for Earths around other stars is one of the most urgent quests in science today. But to find out what conditions are necessary for these bodies to form, researchers must first solve the mystery of how our own Earth arose. Tiny, millimeter-size spheres of melted silicate rock called chondrules—the dominant constituent of meteorites— may hold the clue to this puzzle.

Steven Desch, a Carnegie Fellow at the Department of Terrestrial Magnetism and a member of NASA’s Astrobiology Institute, with Harold C. Connolly, Jr., of CUNY-Kingsborough College, published a new model in the March issue of Meteoritics and Planetary Science that has made huge strides toward understanding chondrule formation and thus understanding what went on in our early solar system. It also helps answer a series of problems that have plagued theoreticians for years. The model determines how chondrules melted as they passed through shock waves in the solar nebula gas. They changed from fluffy dust to round, compact spheres, which altered their aerodynamic properties, allowing them to grow larger. Since this process would have occurred early in the solar nebula's evolution, the results are consistent with the common idea that chondrule formation was a prerequisite to the formation of planets.

“This model may be the key that unlocks the secrets of the meteorites,”says Desch.“It is the first model detailed enough to be tested against the meteoritic data, and so far it has passed every test.”Based on the evidence, scientists know that at the time of formation chondrules reached peak temperatures of 1800 to 2100 K for several minutes; that they almost melted completely; and that they cooled through crystallization temperatures of 1400 to 1800 K at rates slower than 100 K/hr, which kept them hot for hours. To prevent the loss of iron from the silicate melt, pressures had to be high—greater than 0.001 atmospheres—which is orders of magnitude higher than pressures expected in the nebula.A few percent of the chondrules stuck together while they were still hot and plastic. These “compound chondrules” tend to be more completely melted and to have cooled faster than the average chondrule.

Satisfying all of the known conditions simultaneously has been a challenge to theorists. A variety of mechanisms have been

proposed over the years, but none of the ideas has been able to calculate cooling rates precisely enough to match what is known about meteorites.

The model proposed by Desch and Connolly exactly correlates the cooling rates of chondrules with physical conditions in the solar nebula and includes several previously ignored effects, such as dissociation of the hydrogen gas by the shock wave, the presence of dust, and especially a precise treatment of the transfer of radiation through the gas, dust, and chondrules. This transfer of radiation has to be calculated accurately, since the gas and chondrules cool only as fast as they can escape the intense infrared radiation coming from the shock front. With this effect included, typical cooling rates are 50 K/hr, which is exactly in line with what is known about the average chondrule. Moreover, Desch and Connolly predict a correlation with the density of chondrules: regions with more chondrules than average will produce chondrules that are more completely melted and cooled faster. This is because in dense regions radiation from the shock front cannot propagate as far before being absorbed, and therefore chondrules can escape the radiation from the shock front more rapidly. Compound chondrules are overwhelmingly produced in regions with high chondrule densities, so the extra heating and faster cooling of compound chondrules is easily explained by the model. Since the time a chondrule spends in a semimelted, plastic state is also calculated, even the frequency of compound chondrules can be determined— satisfying another key condition. Finally, shocks compress the gas to pressures orders of magnitude higher than the ambient pressure, which is another key requirement that has not been met before.

Desch and Connolly do not specify the source of the shock waves, but they do identify gravitational instabilities as a likely candidate, assuming the solar nebula protoplanetary disk was massive enough. And there are sound theoretical reasons for believing it was. Observations of other protoplanetary disks, in which planets are forming today, indicate that sufficiently massive disks may be common. If shock waves triggered by gravitational instabilities are taking place in other protoplanetary disks, then the odds of chondrules melting and planets forming—including Earths around other stars—are greatly increased.