Implications for planet formation

The discovery of exo-solar planets more than a decade ago made understanding of the connections between disks (both primordial and secondary/debris) and planets more critical than ever. The near ubiquity of circumstellar dust and gas disks around very young stars has been advocated for decades, but has been uniformly accepted by the astronomical community only within the past decade. The turning point was the availability of spatially resolved images of young gaseous and dusty disks at millimeter, sub-millimeter, infrared, and optical wavelengths. Beyond evidence for disks, the detailed information provided by 1) such images, 2) spectral energy distributions sampled over more than four decades in wavelength, and 3) dust and gas spectroscopy, is increasing our understanding of the initial conditions for planet formation. This review has concentrated on dust disk diagnostics.

However, detailed understanding of the processes of star and planet formation requires statistical assessment of global properties and evolutionary trends, in addition to the study of individual objects. Despite the large amount of data presently available, we are only now beginning to achieve the observational sensitivity needed to probe the full range of disk conditions. For the assembly of statistics, we still need to rely on traditional photometric and spectroscopic techniques, rather than well-sampled spectral energy distributions plus spatially resolved imaging at multiple well-separated wavelengths, which are available in relatively few cases.

With the statistics available at present, there are constraints on disk dissipation time-scales, though they are limited in terms of the detail needed to constrain theories. Evidence for decreasing trends with age in the disk fraction, the mean disk accretion rate, and the mean disk mass are apparent. There are also signs in individual young disks of evolution from interstellar grain parameters. What may be most interesting, however, in all of these trends, is the large dispersion about the mean at any given age. This, in particular, speaks to the frequency distribution of paths for solar system formation and evolution.

Over the next decade, we will take the first step in understanding the possibilities for planetary formation by establishing the decay with time of primordial dust via near- and mid-infrared excess around stars of different mass. Studies to determine the timescales for dust disk dissipation should be followed by those aiming to similarly quantify timescales for gas disk dissipation. Fully constraining the time period over which the raw materials needed for planetary formation are available ultimately means following the evolution of disk surface density as a function of the radius from the central star. One outstanding problem in planning for this kind of statistically robust future is that we do not have adequate samples of stars in the 5-50 Myr age range, a critical time in planet formation and early solar system evolution.

Various theories of dust settling, planet formation, and planetary migration within disks are discussed elsewhere in these proceedings. The limited constraints from theory are consistent with the equally vague precision with which disk lifetimes can be inferred from observations of potential planetary systems now in the making. Thus the interpretation of observations is not—yet—the limiting step in solidifying our understanding of planet formation.

When, where, and how frequently do planets form in circumstellar disks? How do forming planetary systems evolve dynamically? What is the range in diversity of stable planetary system architectures? How frequent are habitable planets? How unique is our solar system? These are fairly sophisticated questions to be asking, especially given our rough knowledge of the planet formation process in our own solar system. Meteoritic evidence concerning survival time of the solar nebula suggests "several Myr" as the relevant evolutionary time scale. Studies, especially those concerning extinct radionuclides, support this time span for initial accretion, differentiation, and core formation (see e.g., the review by Wadhwa & Russell 2000). It should be emphasized that although dispersal of the solar nebula may occur quickly, the total duration over which inner planet formation was completed, in fact, approached 30-100 Myr.

An overarching goal of these pursuits is to connect what is observed elsewhere with the history of our own solar system, and hence enhance our appreciation of the uniqueness— or lack thereof—of it, our Earth, and in some respects the human circumstance.

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