Formation of the Sun and the Planets

The study of the formation of the universe, the galaxy, and the solar system is known as cosmology. Cosmology is a relatively new science: Until the last half of the 20th century there were little data to help along hypotheses.With the development of radiodating, the age of the solar system was determined, and with the development of better telescopes, other examples of young solar systems in this galaxy could be found. Before these pieces of evidence could be brought together, the science of solar system origins was similarly primitive. Some scientists though that planets were rare in the universe, and no one had a clear theory for their formation.

Thanks to technological developments, there is now a good theory for solar system formation. The solar system is thought to have formed from an interstellar cloud of dust and gas that began to collapse upon itself through gravity, and then to spin. Astronomers see many clouds of dust and gas at great distances from this solar system. Some of these clouds are large, quiet masses of gas that slowly coalesce into stars here and there throughout their interiors. These giant molecular clouds, as they are called, slowly evolve without much interaction with surrounding bodies. Other clouds, called HII regions, form in an intense radiational environment near a massive star. Smaller stars like the Sun condense from the HII regions, and later, when the nearby massive star dies in a supernova, they are sprayed with an isotope of iron that is only made in the center of massive stars. Evidence for this isotope of iron in primitive meteorites in this solar system indicates that it formed near such a massive star.

After the death of the massive nearby star, the dust cloud around the new small star can begin to condense into planets through gravitational attraction to its central star. As the cloud becomes denser in the center, that dense center attracts matter in from the edges of the cloud more strongly by gravitational attraction. If spinning material is brought closer to the axis of its spin, it spins faster, like a spinning ice skater when he or she pulls in his or her arms (this is caused by conservation of angular momentum). The cloud contracts, spins faster, and flattens into a disk, called the protoplanetary disk, since it is the last structure prior to formation of the planets (see the sidebar "Protoplanetary Disks," on page 9.)

The center of this disk accumulates the greatest amount of mass, through gravitational attraction, and that central mass eventually condenses to the point that nuclear fusion begins in its center and the Sun is born.The Sun is in the "yellow dwarf" star category, joined by other small stars in the universe.The majority of stars contain from 0.3 to three times the mass of the Sun, though giant stars as large as 27 solar masses are found occasionally. The initial collapse under the influence of gravity of primordial material to form the Sun is thought to have taken about 100,000 years, based on computer modeling. This collapse is very fast in geological terms, only about 1/50,000 of the age of the solar system.

Because such a protoplanetary disk would have its highest temperatures in the center, from the heat caused by collapse of material inward and then later by the early heat of the Sun, gaseous elements and molecules nearer the Sun would be heated to temperatures high enough to keep them as gases, rather than solid ices. At distances from the Sun less than two AU (one AU, or astronomical unit, is the equivalent of about 93 million miles, or 150 million kilometers), only metals and silica-based molecules and compounds (called "silicates") could condense and form planets. The inner, terrestrial planets are thought to have formed at temperatures up to almost 3,630°F (2,000°C). This is the reason that the terrestrial planets, which include Mercury,Venus, Earth, and Mars, are made primarily of silicates and metals. (Their material is also similar to the asteroids in the asteroid belt between Mars and Jupiter.) The material that makes up the inner planets is denser than the gases and ices that make up much of the outer planets. The relationship between density and distance from the Sun is shown in the figure on page 10.

The elements that make up silicates and metals, primarily oxygen, silica, iron, magnesium, calcium, nickel, chromium, manganese, sodi-

Protoplanetary Disks EE-very text about solar system formation mentions the "protoplanetary disk," and most simply state that the spinning cloud of dust flattens into a disk. But why should it? There is evidence that it does because the planets in this solar system all move in the same direction around the Sun (thought of as the result of forming out of a uniformly spinning cloud) and because their orbits are in or close to the plane of the Sun's equator (thought to be the result of forming out of a disk, rather than a sphere).

First, all the particles in the dust cloud are being attracted toward its center by gravity. Most of the particles are in orbits around a central spin axis for the cloud, and these orbits have a wide variety of radii: Some particles are close to the spin axis, and some are far away. The particles in orbits feel a centripetal force pulling them in and an opposing centrifugal force pushing them out. (These are the forces a person feels when spun fast on a fair ride or when sliding on the seat when a car corners fast.) They also feel a universal gravitational pull toward the center of the cloud. Their centripetal and centrifugal forces keep the particle in an orbit, not allowing it to go directly toward the cloud's center despite the gravitational force, and so the net result is that the gravitational force pulls the particles down to an equatorial plane without being able to pull them entirely into the center of gravity. There are also some particles that are on the spin axis, and since they have no centripetal or centrifugal forces, they simply move down or up the spin axis under the force of gravity, so the axis of spin flattens most easily and first.

In the equatorial plane, contraction is slowest because the orbits go out to the farthest distance from the gravitational center. When a particle is pulled in toward the gravitational center, its velocity increases (conservation of angular momentum again). Particles on the equatorial plane are pulled inward by gravity to the point that their velocity is high enough to keep them in orbit despite their gravitational attraction toward the center, and they are then at some stable orbit. This contributes to the assembly of dense elements in the inner solar system (their higher density makes their gravitational pull stronger and requires a smaller and faster orbit to balance it) and lighter elements in the outer solar system: This is part of the story of why the inner planets are rocky and the outer planets gaseous.

um, and potassium, are not the most common elements in the solar system.The most common elements in the solar system are hydrogen and helium, the major constituents of the Sun. Where, then, are

All Planets: Planetary Density

L_ Earth Mercury 300 (5,000) fl Venus

Density of Sun and All Planets

Neptune

Uranus

Saturn j_

Uranus

Saturn j_

AU from the Sun Density of Sun and Terrestrial Planets vi Pluto

AU from the Sun Density of Sun and Terrestrial Planets

AU from the Sun

The innermost planets, made of metals and silicates, are far denser than the outer planets, which consist mainly of ices and gases. The density gradient in the planets is a result of density gradients in the solar nebula.

hydrogen and helium found in the planets? The simplest explanation follows the understanding that different elements have different weights and condense at different temperatures, as described in the sidebar "Elements and Isotopes," on page 11.

At distances farther from the early Sun, temperatures in the proto-planetary disk were cool enough that the most common molecules in the solar system could condense and form planets (condensation temperatures are shown in the table on page 13). Molecules such as carbon dioxide (CO), water (H O), methane (CH ), and ammonia (NH3) condensed into ices and were abundant enough that, along

Elements and Isotopes A ll the materials in the solar system are made of atoms or of parts of atoms. A family of atoms that all have the same number of positively charged particles in their nuclei (the center of the atom) is called an element: Oxygen and iron are elements, as are aluminum, helium, carbon, silicon, platinum, gold, hydrogen, and well over 200 others. Every single atom of oxygen has eight positively charged particles, called protons, in its nucleus. The number of protons in an atom's nucleus is called its atomic number: All oxygen atoms have an atomic number of 8, and that is what makes them all oxygen atoms.

Naturally occurring nonradioactive oxygen, however, can have either eight, nine, or 10 uncharged particles, called neutrons, in its nucleus, as well. Different weights of the same element caused by addition of neutrons are called isotopes. The sum of the protons and neutrons in an atom's nucleus is called its mass number. Oxygen can have mass numbers of 16 (eight positively charged particles and eight uncharged particles), 17 (eight protons and nine neutrons), or 18 (eight protons and 10 neutrons). These isotopes are written as 16O, 17O, and 18O. The first, 16O, is by far the most common of the three isotopes of oxygen.

Atoms, regardless of their isotope, combine together to make molecules and compounds. For example, carbon (C) and hydrogen (H) molecules combine to make methane, a common gas constituent of the outer planets. Methane consists of one carbon atom and four hydrogen atoms and is shown symbolically as CH . Whenever a subscript is placed by the symbol of an element, it indicates how many of those atoms go into the makeup of that molecule or compound.

Quantities of elements in the various planets and moons, and ratios of isotopes, are important ways to determine whether the planets and moons formed from the same material or different materials. Oxygen again is a good example. If quantities of each of the oxygen isotopes are measured in every rock on Earth and a graph is made of the ratios of 17O/16O versus 18O/16O, the points on the graph will form a line with a certain slope (the slope is 1/2, in fact). The fact that the data forms a line means that the material that formed the Earth was homogeneous; beyond rocks, the oxygen isotopes in every living thing and in the atmosphere also lie on this slope. The materials on the Moon also show this same slope. By measuring oxygen isotopes in many different kinds of solar system materials, it has now been shown that the slope of the plot 17O/16O versus 18O/16O is one-half for every object, but each object's line is offset from the others by some amount. Each solar system object lies along a different parallel line.

(continues)

Elements and Isotopes (continued) At first it was thought that the distribution of oxygen isotopes in the solar system was determined by their mass: The more massive isotopes stayed closer to the huge gravitational force of the Sun, and the lighter isotopes strayed farther out into the solar system. Studies of very primitive meteorites called chondrites, thought to be the most primitive, early material in the solar system, showed to the contrary that they have heterogeneous oxygen isotope ratios, and therefore oxygen isotopes were not evenly spread in the early solar system. Scientists then recognized that temperature also affects oxygen isotopic ratios: At different temperatures, different ratios of oxygen isotopes condense. As material in the early solar system cooled, it is thought that first aluminum oxide condensed, at a temperature of about 2,440°F (1,340°C), and then calcium-titanium oxide (CaTiO ), at a temperature of about 2,300°F (1,260°C), and then a calcium-aluminum-

silicon-oxide (Ca2Al2SiO ), at a temperature of about 2,200°F (1,210°C), and so on through other compounds down to iron-nickel alloy at 1,800°F (990°C) and water, at -165°F (-110°C) (this low temperature for the condensation of water is caused by the very low pressure of space). Since oxygen isotopic ratios vary with temperature, each of these oxides would have a slightly different isotopic ratio, even if they came from the same place in the solar system.

The key process that determines the oxygen isotopes available at different points in the early solar system nebula seems to be that simple compounds created with 18O are relatively stable at high temperatures, while those made with the other two isotopes break down more easily and at lower temperatures. Some scientists therefore think that l7O and l8O were concentrated in the middle of the nebular cloud, and l6O was more common at the edge. Despite these details, though, the basic fact remains true: Each solar system body has its own slope on the graph of oxygen isotope ratios.

Most atoms are stable. A carbon-12 atom, for example, remains a carbon-12 atom forever, and an oxygen-16 atom remains an oxygen-16 atom forever, but certain atoms eventually disintegrate into a totally new atom. These atoms are said to be "unstable" or "radioactive." An unstable atom has excess internal energy, with the result that the nucleus can undergo a spontaneous change toward a more stable form. This is called "radioactive decay." Unstable isotopes (radioactive isotopes) are called "radioisotopes." Some elements, such as uranium, have no stable isotopes. The rate at which unstable elements decay is measured as a "half-life," the time it takes for half of the unstable atoms to have decayed. After one half-life, half the unstable atoms remain; after two half-lives, one-quarter remain, and so forth. Half-lives vary from parts of a second to millions of years, depending on the atom being considered. Whenever an isotope decays, it gives off energy, which can heat and also damage the material around it. Decay of radioisotopes is a major source of the internal heat of the Earth today: The heat generated by accreting the Earth out of smaller bodies and the heat generated by the giant impactor that formed the Moon have long since conducted away into space.

with some silicate and metal components, they could form the cores of the giant planets Jupiter, Saturn, Uranus, and Neptune. Once these giant condensed cores formed, they had enough gravitational attraction to pull in the clouds of hydrogen (H ) and helium (He) gases that form by far the majority of all matter in the universe.

The first solids thought to condense from the protoplanetary disk as it cooled are corundum (Al O ), hibonite (CaAl O ), and per-ovskite (CaTiO3), followed by zirconium oxide (ZrO2, a rare material), titanium oxide (TiO3), and then the minerals pyroxene

CONDENSATION TEMPERATURES FOR SELECTED PLANET-FORMING MATERIALS

Material corundum (Al O )

2,730°F (1,500°C)

olivine ((Fe, Mg) SiO , a common silicate

mineral in the mantles of terrestrial planets)

2,190°F-930°F (1,200°C-500°C)

iron-nickel alloys (common core-forming material in terrestrial planets)

2,010°F-1,830°F (1,100°C-1,000°C)

water (HO)

32°F (0°C)

carbon dioxide (CO )

-112°F (-80°C)

methane (CH , exists as an ice on Pluto)

4

approximately -370°F (-225°C)

ammonia (NH , exists as a gas and an ice on Jupiter)

approximately -410°F (-245°C)

((Ca,Mg,Fe)SiO3), spinel (MgAl2O4), and olivine ((Fe, Mg)2SiO4). Volatile compounds like water and methane remain as gases until temperatures fall below 32°F (0°C), a situation not met in the early pro-toplanetary disk until the material was a great distance from the center (see the table on page 13).This is thought to be the reason that the inner planets—Mercury,Venus, Earth, and Mars—are made primarily of silicates and metals, while the outer planets—Jupiter, Saturn, Uranus, and Neptune—are made primarily of more volatile species and contain a large proportion of hydrogen and helium. Hydrogen and helium are among the most volatile species, meaning that they are least likely to be solids.They both, however, become fluids and then metals in the high pressures of the interiors of Jupiter and Saturn.

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