Strategy for the Detection and Study of Other Planetary Systems and Extrasolar Planetary Materials 1990-2000 (1990) / Chapter Skim
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3. Present Understanding of the Origin of Planetary Systems
3. Present Understanding of the Origin of Planetary Systems
Pages 21-33
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From page 21... ...
Collisions of the dust particles and their accretion into subplanetary objects, as well as gravitational and electromagnetic interactions in a many-body system, also must be incorporated. 'Ib understand the evolution of the central star in a planetary system requires the addition of nuclear physics to the above processes.
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From page 22... ...
The masses and compositions of the inner planets as compared with those of the outer planets, as well as the existence of the asteroid belt and the composition and orbital configurations of comets, provide important clues to the nature of the solar nebula and the planetary formation process. The ordered variation in the properties of planets and satellites with radial distance from the Sun is consistent with the interpretation that they are spatially separated samples of an ongmal continuous nebula, although accretion of each body probably occurred over a range of radial distances and the temporal sequence of formation of the planets may not coincide with their present spatial ordering.
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From page 23... ...
STAR FORMATION The observational evidence indicates strongly that most if not all star formation takes place in molecular clouds (mean density 10-2~ g cm~3) , and probably in the cores of such clouds, where densities are approximately 10-~9 g cm~3 and temperatures about 10 K The basic condition that has to be satisfied for gravitational collapse to occur is the Jeans criterion the requirement that the thermal energy of a volume of gas be less than the absolute value of the gravitational energy.
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From page 26... ...
observed during the ~105 to 106 yr of the ensuing contraction phase are now widely believed to occur simultaneously with infall of material from the protostellar cloud to the circumstellar disk and inflow of material to the central mass through the dish There is growing evidence that these winds require the presence of an accretion disk and that the mass inflow rate through the disk and the mass outflow rate in the wind are related. It is important to realize that the sequence of events outlined above is simply a sketch and that only a few aspects of protostellar evolution have been calculated in detail, usually with restrictive assumptions.
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From page 27... ...
Given an infinite time for evolution, almost all of the nebula would spiral into the central star. However, time scales for circumstellar disk evolution of only a few million years are inferred from theoretical calculations, as well as from recent observations of excess infrared radiation arising from dust embedded in the disks with large infrared excesses at stellar ages of <3 million yr, but fewer than 10 percent of such stars still display this signature by the tune they reach ages of 10 million yr.
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From page 29... ...
Some of the infrared objects that exhibit bipolar flow also may be interpreted as having associated disks or tori, with the plane of the disk perpendicular to the flow. As noted above, outflow from the boundary layer and ionization by ultraviolet photons may control the dissipation of the dish Third, the tidal influence of the star plays an important role in the planetary formation process by limiting the region of gravitational influence of the protoplanets.
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From page 30... ...
In the inner solar system both the heating and tidal ejects of the Sun, as well as the smaller amount of condensable material, apparently prevented the buildup of cores to the Critical mass where significant gas accretion was possible. The starting point is the nebular disk composed of gas mixed with about 1 percent by mass of dust, at temperatures in the range of 100 to 2000 K The dust particles have essentially interstellar characteristics, with typical particle sizes of 10-5 to 10-4 cm (0.1 to 1 ~m)
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From page 31... ...
Monte Carlo simulations of the accumulation process suggest that it proceeds rapidly during the first few million years, forming objects up to ~25 percent of the Earth's present mass in the terrestrial planet zone; subsequent collisional growth occurs more slows because Secreting objects decline in number and become orbitally more isolated from the protoplanets. Estunates for the total time required to fully accrete the terrestrial planets range from 107 to 108 yr.
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From page 33... ...
Many theoretical and computational initiatives will require expansion of currently available computer capacity and unproved numerical techniques for solving coupled systems of differential equations. New and detailed observational data on chemical compositions, present physical states, and dynamical behavior both within and outside our own solar system, from spacecraft instruments and from ground and Earth-orbital facilities, are central to this effort.
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