posted February 14, 1997

Solar System Formation

Comets are important scientifically because they represent the remnants from early in the era of the formation of our Solar System, and may provide clues to the physical and chemical conditions within the nebula out of which our Solar System formed. Comets are potentially our only direct samples of this early epoch in our history, and like the archaeologist who uses relics to piece together the chronology and structure of an ancient civilization, astronomers hope that by understanding the composition and physical structure of comets that we can constrain the models which describe the process of planet formation.

Our Solar System was formed approximately 4.55 billion years ago out of a molecular cloud - a large concentration of gas (roughly 75% hydrogen and 21-24% helium with trace amounts of other molecules) and dust grains. In addition to hydrogen (H) and helium (He), the interstellar medium (ISM) gas consists of a rich array of organic and inorganic molecules, some of which include: H₂O, CO, NH₃, CN, CO₂, OH, HCN, CH₃OH, H₂CO, CH₃C₂H, HNCO, CH₃CN etc. The dust, which comprises roughly 1% of the ISM, is made of refractory cores (silicates and carbon) coated with organics and ice. Molecular clouds are large, with diameters between 50-200 light years, and extremely cold temperatures (10-50K). Although dense compared to the surrounding ISM, their densities (10⁴ to 10⁵ particles per cubic cm) are about 100 million times lower than the best vacuums we can create in the laboratory.

The Coalsack nebula, of Barnard 86, is what one of these dark molecular clouds might look like as the star formation process is just beginning.

The process of star and planet formation (depicted in the graphic to the right) begins with the collapse of the molecular cloud (1), which can be triggered by factors such as a nearby supernova explosion, or collision with another cloud. As the cloud begins to collapse, material gets drawn at an ever-increasing rate toward the center of the cloud, where it begins to heat up. At the same time, the decreasing size of the cloud causes it to spin faster (due to the conservation of angular momentum). Near the center of the cloud, where the density is the greatest, collisions between dust and gas remove energy until the particles no longer collide. This causes a general flattening of the system where the particles orbit the center in the same direction, (2). This flattening occurs first near the center where the densities are the highest. During the collapse, the gravitational energy is converted to heat. At first the heat can easily escape, but as the density increases in the center, the heat cannot easily radiate away, and the temperature starts to rise. Temperatures will eventually rise high enough to break apart the molecules and vaporize of the dust in the central regions. As more and more material settles to the core and midplane of the young nebula, enough material will eventually accumulate to shield the outer regions from the growing temperatures in the central region. At this point, as the gases cool, some of the vaporized materials will re-condense into micron-sized grains, (3). Closest to the central regions, to the young protostar, only high temperature refractory materials can condense, but farther from the star volatiles can also condense. Since the volatiles are the most abundant species in the original interstellar medium, they will overwhelm the refractory material farther out in the nebula. This provides a natural explanation for the difference in composition between the terrestrial planets (rocky) close to the sun, and the Giant planets (volatile-rich) farther out.

The condensing material will slowly start to clump together. Eventually, some clumps grow larger than others, becoming planetesimals, and these begin to sweep up all the other debris along their orbits about the central protostar. As the planetesimals get larger, the velocities get higher and the collisions get more violent, (4). In the vicinity of the giant planets, some of the volatile-rich planetesimals get thrown into the inner solar system, bringing volatiles to the inner planets, and some get thrown out into a vast spherical swarm of small bodies surrounding the planets. The icy planetesimals which are thrown to vast distances (near 50,000 AU - the Oort Cloud) are then stored unaltered from this early epoch in the history of the Solar System, and are what we call comets today. Once the center of the nebula becomes hot enough that thermo-nuclear reactions can begin, the collapse of the nebula stops, and any remaining material (gas and dust) is blown out of the system, and the era of planet formation has ended, (5). The entire process occurs relatively quickly, taking only a few million years.

The image on the left, above, of the Eagle Nebula (M16) shows a star forming region 2 million years old. The hydrogen gas is emitting light from atoms excited by newly formed hot stars. In a Hubble Space Telescope close up of the same region (at right), we get a high resolution view of the small dense regions which are embryonic stars in the process of contraction.

The early models of the Solar System formation assumed that the comets (e.g. the icy planetesimals) formed in the vicinity of the giant planets, and that they may have contained some pristine unaltered material from the ISM. After being thrown into the Oort cloud, perturbations by nearby passing stars could alter their orbits and send them toward the inner Solar System where they would be observed as a long-period comet or captured into a low-inclination short-period cometary orbit after a close passage to Jupiter. It was believed that comets could not form too far out in the Solar System because the density of material would be insufficient for planetesimal growth. However, this scenario has changed significantly in the past few years:

• New technology, particularly in the infrared wavelengths, has permitted a detailed study of the processes of star formation. Observations of young stellar objects show that 20-50% of these objects have extended disks out to a few hundred AU associated with them. This suggests that the disks associated with the star / planet formation process are common and that they are much more massive that previously believed.

• New models of the collapse of the Solar Nebula are suggesting that because of these larger disks, comets may form much farther out in the nebula. Likewise, it may not be likely that totally pristine, unaltered interstellar grains survive the infall process. As they settle through the nebula to the mid-plane, frictional heating may vaporize the icy coating, which can later recondense on the cold grain cores at the midplane.

  

  
  

• Laboratory experimentation has shown that when ice condenses below temperatures of 100K, it does not develop the regular crystal structure, rather adopts an amorphous irregular structure (because there is not enough energy for the ordered crystal structure). In the amorphous phase, the water-ice can trap a large number of other molecules in the voids. The lower the temperature of condensation, the more gas can be trapped. This is shown in the graph (Owen and Bar-Nun, 1995), where it can be seen that the difference in the amount of trapped gases varies by 6 orders of magnitude between 20 and 100K. As the ice is heated, trapped gases are released as voids close up, and the release of gas may be observable as activity on the comet.

• The recent discovery of objects in the Kuiper Belt, a region between 35-50 AU (beyond the orbit of Pluto) with inclinations ranging between 0-30 degrees, has prompted a detailed investigation of the dynamics of small bodies in the outer Solar System. Dynamical models show that the short period comets, which unlike the long period comets have a flattened inclination distribution, cannot have a source in the Oort cloud, but most likely come from the vicinity of the Kuiper Belt.

Known Extra-Solar Planets

Within the past few years, we are not only developing a new understanding of the process of planet formation from both new observational techniques and theories, but we have recently detected direct evidence of extra solar planetary systems. The first extra-solar planets detected were objects around pulsars, the ultra-dense remnants of a supernova explosion (see the first 3 listings in the table below; Wolszczan and Frail, 1992). However, during the past year, astronomers have finally discovered planets orbiting around solar-type stars (i.e. star systems which would be capable of supporting life). Many of the characteristics of the planets (shown in the table and figure below, from Beichman, 1996; Marcy & Butler, 1996) are challening our ideas of planet formation. The combination of new technology and high- quality observations with new theories is converging on a new more sophisticated understanding of the process of planet formation and evolution. It is believed that the detailed study of the most primitive samples we have from the era of our Solar System's formation, the comets, will contribute greatly towards our understanding of these processes.

Figure Credit: NASA ExNPS report, updated by the San Francisco State University Astronomy Department.

References

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