THE FORMATION AND DYNAMICS OF PLANETARY SYSTEMS

Gregory Laughlin

In the past year, progress was made in a number of areas bearing on the overall problem of planetary systems formation and evolution. Specific topics of research have ranged from the earliest stages of star formation through the long term fate of the Earth, and are described in four peer-reviewed research papers.

In the present-day solar system, the sun contains 99.9% of the mass, whereas the planets contain the bulk of the system angular momentum. The clouds of gas and dust which collapse to form star-planet systems, however, are essentially in uniform rotation. One of the major unsolved puzzles in the theory of star and planet formation thus involves the detailed mechanism by which mass is transported inwards onto the protostar while angular momentum is simultaneously pushed outwards. It is believed that spiral gravitational instabilities play a key role in eliciting angular momentum transport, but a full description of how spirals grow and operate on a global scale (i.e. throughout the entire protoplanetary disk) is not understood. Considerable theoretical progress was made in this area by performing a stability analysis of idealized singular isothermal disks. This research, carried out and published in collaboration with researchers at UC Berkeley, Arcetri (Italy), and UNAM (Mexico), has clearly explained the role of the co-rotation amplifier in allowing spiral waves to grow. This in turn gives us a clearer theoretical picture of the very earliest stages of star and planet formation.

A second line of inquiry has developed a way to constrain the conditions under which our own solar system formed. The outer giant planets in our solar system all have nearly coplanar, circular orbits. This orderly configuration indicates that the Sun and the planets have always existed in relative isolation. Had another stellar system passed within several hundred astronomical units of the Sun, gravitational perturbations would have scattered the outer planets (particularly Neptune) into highly eccentric, inclined orbits. An extensive set of Monte-Carlo star-planet scattering calculations has shown that the solar system likely formed in an aggregate containing fewer than 1500 stars, and thus was not born in a dense stellar cluster (resembling, say, the Trapezium region in Orion). Primitive meteorites, however, contain daughter products of extinct radioactive elements which have half lifes of one million years or less. In order to explain the presence of such short-lived isotopes in meteorites, it has been proposed that either (1) the pre-solar nebula was enriched by a nearby supernova explosion, or alternately that (2) X-ray flares associatied with the nascent sun were able to create radioactive atoms via processes such as spallation. The new research strongly favors scenario (2), since the presence of a nearby supernova would imply that the sun formed in a very massive aggregate of stars, and this possibility is effectively ruled out by the Monte-Çarlo calculations.

A third focus of the research effort examined the emerging correlation between high stellar metallicity and the detected presence of an extrasolar planet. Now that more than 70 extrasolar planets have been found, it is possible to evaluate the emergence of statistical trends. An analysis of volume-limited samples of stars in the solar neighborhood demonstrated that stars with metal content >50% higher than solar are 10 times more likely to harbor a short-period planet than the average star in the solar neighborhood. This finding can be exploited to find extrasolar planets with less effort, thus saving large amounts of time on instruments such as the Keck Telescope. A catalog of 200 highly metal-rich stars was compiled, and within 6 months, 5 planets have veen detected in this catalog. Two were found by the Marcy group, two were found by Swiss researchers, and one was found by Ames researchers (HD 20675b, to be confirmed and announced in Fall, 2001).