As one example, prior to detection, the stellar light is passed through an iodine gas-filled absorption cell to imprint a velocity reference on the stellar spectrum. However, after search techniques had been honed in this way for years to detect a Jupiter-like world in other solar systems, a surprising result has emerged: a much greater diversity of planetary systems than was expected. According to Najita et al., searches have revealed planets with a wide range of masses, including planets much more massive than Jupiter; planets with a wide range of orbital distances, including planets much closer to their suns than Jupiter is to our Sun; and planets with a wide range of eccentricities, including some with much more eccentric orbits than those of the planets in our solar system.

These results were essentially unanticipated by theory and reveal the diversity of possible outcomes of the planet-formation process, an important fact that was not apparent from the single example of our own solar system. This diversity is believed to result from the intricate interplay among the many physical processes that govern the formation and evolution of planetary systems, processes such as grain sticking and planetesimal accumulation, runaway gas accretion, gap formation, disk-driven eccentricity changes, orbital migration, and dynamical scattering with other planets, companion stars, or passing stars. Thus far, what has changed is not so much our understanding of the relevant physical processes but, rather, how these processes fit together, i.e., our understanding of their relative importance and role in the eventual outcome of the planet formation process.

Femtochemistry

The essence of the chemical industry and indeed of life is the making and breaking of molecular bonds. The elementary steps in bond making and breaking occur on the time scale of molecular vibrations and rotations, the fastest period of which is 10 femtoseconds. Chemical reactions are, therefore, ultrafast processes, and the study of these elementary chemical steps has been termed “femtochemistry.” According to Tanimura et al., a primary aim of this field is to develop an understanding of chemical reaction pathways at the molecular level.23 With such information, one can better conceive of new methods to control the outcome of a chemical reaction. Because chemical reaction pathways for all but the simplest of reactions are complex, this field poses both theoretical and experimental challenges. Nevertheless, much progress is being made, and systems as complex as biomolecules can now be investigated in great detail.

Ultrafast dynamics of molecules have long been studied theoretically by integrating a relevant equation of motion. The time-dependent wave packet ap-

23  

Frontiers of Science/1998. Yoshitaka Tanimura, Koichi Yamashita, and Philip A. Anfinrud, at <http://www.pnas.org/cgi/content/full/96/16/8823>.



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