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Disks form in the rotating dark molecular clouds around protostars. Solids in the cloud fall to the midplane dust layer. Planetesimals form in the midplane dust layer and form planets within a few million years. The disk temperature varies with distance from the star, with inner disks often hot, dry, and devoid of gas and outer disks with thick and frozen gas atmospheres and midplane dust layers. The solar system has small inner rocky planets where the circumsolar disk should have been frozen and retained its water.
Figure 4‑7. Sequence of disk formation. Credit: Frank Shu, NASA.
When a very large dark molecular cloud collapses and forms a relatively small cloud core, the total angular momentum (spinning momentum) remains the same, but it is confined to a much smaller volume. This causes the smaller cloud core to spin much faster than the larger cloud. It is the same effect as a figure skater drawing in her arms and spinning faster.
The gravitational low points in the cloud are the star and the midplane. As the cloud spins, material in the cloud falls to the midplane and toward the star. Gravity continues to pull material in the disk toward the star, but centrifugal force pushes material away from the star.
Figure 4-7 shows three phases in disk formation. Solar radiation begins to blow away the cloud above and below the disk. Eventually, all of the gas above and below the midplane is blown away by solar radiation. This generally takes three million years. Solids from the disk atmosphere fall into the disk midplane by gravity. Due to viscosity in the disk, material in the disk loses energy and continues to move into the star from the disk. Finally, planets begin to sweep up materials in sections of the disk and form gaps in the disk.
Figure 4‑8. Particles in disk atmosphere fall to midplane, but gases in upper disk atmosphere are ionized and blown away into space.
Figure 4‑8 shows a side view of the disk. The disk atmosphere is the part of the disk above and below the midplane dust layer. Extreme ultraviolet radiation) eventually dissipates the disk atmosphere. Solids (ice and dust fall to the disk midplane, which is where planets form. Figure 4‑9 is an artists representation of a young disk.
Figure 4‑9. “Artist’s impression of the baby star TMC-1A and surrounding disk. Credit: NAOJ. Accessed at <http://www.almaobservatory.org/en/press-room/press-releases/930-alma-spots-baby-stars-growing-blanket>
Figure 4‑10. Circumstellar disk redirects material from molecular cloud envelope into protostar.
During the early phases of disk formation, disks expand outward as angular momentum from infalling material stretches the disk to hundreds of AU. An astronomical unit (AU) is the distance from the Earth to the sun. Much of the mass of a star is accumulated through the disk. First, material from the disk atmosphere falls into the disk. Then the disk material slowly spirals into the star through the disk, adding mass to the protostar (Figure 4‑10). After the protostar volatilizes the gas near the star, matter ceases to flow into the star.
The high energy UV radiation from the host star ionizes hydrogen, and other volatiles on the surface of protoplanetary disks. A temperature gradient forms in the disk in which the surface is hot and ionized, and the dusty midplane is cold (Figure 4‑11).
Figure 4‑11. Regions of protoplanetary disks. Credit Avi Mandell, NASA.
Scientists can observe different parts of disks at different wavelengths. Figure 4‑12 shows a side view of regions of young disks, and the wavelengths of light that impact (ionize) different sections. X-rays ionize the layers below the outer layer. Ionization of disk sections impacts how that section interacts with the rest of the disk. This is because ions have a charge and are thus influenced by (and cause) electromagnetic fields in the disk. If there are no ionized particles in a part of a disk, then the region is not influenced by the electromagnetic fields that are in disks. These regions are called dead zones. Scientists do not fully understand the effects of dead zones. However, they can extend out beyond 40 AU (larger than the solar system) and influence disk and planet formation processes. Dust in dead zones would be more likely to accrete and form large particles and planetesimals.
Figure 4‑12. Side view of young disk with chemistry of different regions within the disk. (credit Inga Kamp & Wing-Fai Thi). www.astrowing.eu/figures Creative commons.
Figure 4‑13. Deformed protoplanetary disks in the Orion Nebula due to stellar wind from large nearby stars. Credit. Hubble Space telescope.
In dense stellar nurseries, the many large stars emitting high energy radiation impact the formation of disks around other stars and cause disks to be misshapen. Dr. Ilse Cleeves studied Hubble NICMOS telescope images of disks in the Orion nebula (Figure 4‑13), and showed that many disks were obliterated by nearby stars in the cluster. Disks in the outer part of clusters are relatively safe and not distorted; however, stars cycle in and out of the center of the clusters, which leads to gravitational interactions with other stars that distort or even completely disrupt the orbits of planets. The circumsolar disk was impacted by the supernovae that triggered the formation of the protosun. The supernova emitted radioactive aluminum 26, which caused ionization in all parts of the disk and also led to melting of the early earth.
Scientists learn about the characteristics of the circumsolar disk by studying the planets and other objects in the solar system (Figure 4‑14). The inner planets in the solar system are mostly rock (terrestrial) while the outer planets (Jovian) are primarily lighter elements. Other objects that scientists use to determine the characteristics of the circumsolar disk include asteroids, comets, and meteoroids.
Figure 4‑14. Relative sizes and compositions of the planets. Credit: NASA Lunar and Planetary Laboratory.
Disks are hotter near the star, which explains why rocky planets without water (without ice) formed near the sun. In contrast, the four outer gas giants have cores of rock and ice, surrounded by mantles and atmospheres of water, methane, hydrogen, and other volatiles. Figure 4‑14 shows the distribution of planets and the inner structure of the outer gas and water giants. Scientists determine the structures of planets based on their densities, which are revealed by their gravitational interactions with other planets. The outer planets have solid cores surrounded by mantles and atmosphere. Once the cores formed, they attracted the gas in the disk in their orbital ranges to the planet. The cores of Jupiter and Saturn are surrounded by enormous hydrogen mantles and atmospheres (Figure 4‑14). The cores of the outer planets formed quickly, prior to the time that the gas in the disks dissipated. Scientists hypothesize that a phenomenon called pebble accretion allowed the cores of these planets to grow quickly.
Based on the distribution of rocky planets in the inner solar system, Avi Mandell proposed that the circumsolar disk was thin and dusty between the orbits of Mercury and Mars and thick, gaseous, and frozen between the orbits of Jupiter and Neptune (Figure 4‑15). The terrestrial planets would have gathered from the remaining midplane dust layer in the inner disk. The cores of the gas giants would have formed from the midplane dust layer encased in the thick disk atmosphere in the outer solar system and then attracted the gases in the outer disk by their gravity. A recent paper in the Journal Science, described in chapter 5, might indicate that the reason that the inner planets are dry is that they were dried out by aluminum 26, but that they formed in a wet part of the disk.
Figure 4‑15. Possible shape of circumsolar disk with inner dry zone. Credit: ESO/NASA/Avi Mandell.
Most people do not realize how small the planets, particularly the four inner planets, are in the solar system (Figure 4‑16). In the upper part of Figure 4‑16, the sun is a tiny dot, and the Earth is 100 times smaller than the dot that represents the sun. Even the sun is too small to be visible at the scale of the entire solar system (lower part of Figure 4‑16). Jupiter, Saturn, Neptune and Uranus contain 99.5 % of the planetary mass of the solar system, and the four inner planets contain 0.5% of the mass of planets in the solar system.
Figure 4‑16. Sizes and distances in the solar system.
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