The solar system is made up of one star, our Sun, and all the bodies that orbit it—planets, moons, and various kinds of debris (called asteroids, comets, and meteoroids). How did the solar system form? How were its varied building blocks of rock, metal, ice, and gas created? How has the solar system changed since its formation, and what does its history tell us about the planets as they are today? Within the past few decades, telescopes and space probes, along with the theories of modern science and computer simulations of these theories, have finally provided some of the answers to these age-old questions.

Our study of the formation of the solar system begins with an inconsistency, namely, that Earth, the Moon, and many other objects that orbit the Sun are composed primarily of heavy elements including, among many others, oxygen, silicon, aluminum, iron, carbon, and calcium. (Note that we capitalize the word moon only when we are referring to Earth’s Moon.) Furthermore, most of these bodies contain extremely little hydrogen and helium. However, observations of the spectra of the Sun, other stars, and interstellar clouds reveal that hydrogen and helium are by far the most abundant elements in the universe. These two elements account for about 99.9% of all observed atoms (or, equivalently, 98% of observed mass). All of the other elements combined account for about 0.1% of the observed atoms (and, hence, 2% of the observed mass of the universe). How is it that Earth, the Moon, Mars, Venus, Mercury, and many smaller bodies in the solar system typically contain less than 0.15% hydrogen and helium? Somehow, these familiar astronomical objects formed from matter that had been enriched with the heavier elements and depleted in hydrogen and helium.

There is a good reason for the overwhelming abundance of hydrogen and helium throughout the universe. Astronomers believe that the universe formed about 13.8 billion years ago in a violent event called the Big Bang, which created not only all the matter and energy that exists but also space and time. (We explore these issues in Chapter 14.) Only the lightest elements—hydrogen, helium, and a tiny amount of lithium—were created as a direct result of the Big Bang. These first elements came into existence as gases. The first stars, composed only of these three elements, condensed out of this primeval matter within a few hundred million years after the Big Bang occurred.

The difference in chemical composition between the gas and stars in the early universe and the solar system today shows that the solar system did not form as a direct result of the Big Bang. Rather, as we shall see shortly, the solar system formed some 9 billion years after the universe came into existence. Let us briefly explore the chemical transformations that led to our being here, which began deep inside those first stars.

Once stars form, gravity compresses the matter in their central cores so much that the hydrogen there is transformed into helium in a process called thermonuclear fusion (fusion, for short). Hydrogen fusion has the interesting property that some of the hydrogen’s mass is converted into electromagnetic energy (photons), a portion of which eventually leaks out through the star’s surface, enabling it to shine. (That is why the Sun shines.) The core’s helium subsequently fuses to create carbon and oxygen, also converting more mass into energy. If the star has enough mass, the pressure in its core will force these elements to transform into even heavier elements, including neon, aluminum, silicon, calcium, and iron, while other elements are fused in shells surrounding the core. We explore details of the creation of these heavier elements in Chapters 11 and 12.

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Stars also shed matter into space (Figure 4-1). Some of the heavy elements that formed inside them, specifically those elements created in shells of fusion occurring outside their cores, are eventually ejected, along with most of the hydrogen and helium remaining in their outer layers. The outer layers of stars are expelled at different rates, ranging from continuous outflows called stellar winds (Figure 4-1a), to the more energetic expulsion of gases called planetary nebulae (Figure 4-1b), to spectacular detonations called supernovae (Figure 4-1c). Planetary nebulae were so named not because they have any direct connection with real planets, but rather because they looked like planets in early, low-resolution telescopes. Supernovae are such powerful events that fusion occurs during these explosions, creating a variety of heavy elements, such as nickel, copper, zinc, silver, and gold, which are also expelled from the star. Planetary nebulae and supernovae leave only tiny, dim, but in many cases very hot stellar cores, remnants of once mighty stars.

Of the matter ejected from stars, some leaves as small particles of dust, similar to the soot formed in a fire or in diesel exhaust. This dust is typically a millionth of a meter (a micron) in size. Over time, enough gas and dust were emitted by enough stars to form interstellar clouds rich in hydrogen and helium that also contained small amounts of metals. (In a curious alternative use of a common word, astronomers define metals to be all of the elements in the universe other than hydrogen and helium.) Some of the atoms in these clouds also combine to form molecules of, for example, water, carbon dioxide, methane, and ammonia. We will turn next to these clouds in order to understand the formation of the solar system and why dense planets like Earth are so rich in metals and poor in light elements.

Observations of clouds, such as the Cone Nebula and the nearby Orion Molecular Cloud (Figure 4-2), reveal that most stars form from gas and dust inside such giant interstellar clouds. Between a few and a few thousand stars typically form at the same time in these clouds, creating groups called open clusters of stars. It is likely that the solar system formed in such a cluster. Stars in open clusters eventually drift apart, which is why even the closest stars to the solar system are light-years away from us today.

Such clouds do not spontaneously collapse to form stars because the particles in these clouds are moving fast enough (meaning they are warm enough, since motion is a measure of how hot gases are) to avoid being pulled together by their mutual gravitational attraction. Another way to say this is that the gases comprising these clouds have high enough internal pressures to prevent them from collapsing together and forming stars. Pressure is the force that the gas exerts on any area of either itself or anything else (think of the pressure that the gas in a balloon exerts on the surface of the balloon to keep it inflated).

With sufficiently low temperature and pressure and sufficiently high gas density, however, fragments of clouds collapse under the influence of their own gravitational attraction. Density is the amount of mass of any substance within a given volume. We say that each of these regions has a Jeans instability, after the British physicist Sir James Jeans, who calculated the necessary conditions for such collapse in 1902. Pieces of interstellar clouds become “Jeans unstable,” allowing stars and their hosts of orbiting bodies to form, for at least three reasons. First, winds from nearby stars compress gas and dust in the cloud. Second, the explosive force of a nearby supernova compresses regions of gas and dust. Third, pairs of clouds collide and compress each other. Each collapsing fragment destined to become a star (or a pair of stars orbiting each other called a binary star system) and possibly planets is called a dense core.

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Some chemical evidence in space debris that we have analyzed on Earth suggests that the dense core in which our solar system formed may have become Jeans unstable when its gas and dust were hit by the shock wave from a supernova. We call that dense core the solar nebula (also called the protosolar or presolar nebula).

Many of the concepts in what follows comprise the Nice model (pronounced like “niece” and named after the city in France where it was first developed) of the formation of the solar system. This model is based on computer simulations of the evolution of the outer regions of the solar nebula. We are in an epoch of astronomical research in which insights into the process by which the solar nebula transformed into the Sun and its host of orbiting bodies are coming at an unprecedented rate. The details of this theory are tentative and are likely to evolve as simulations become more realistic and more solar system debris left over from the formation process is analyzed. Therefore, the following scenario is a prime example of how science is a work in progress rather than a presentation of complete understanding of nature.

The solar nebula initially had a diameter of at least 1000 AU (1000 times the average distance from Earth to the Sun) and a total mass about 2 to 3 times the solar system’s present mass (Figure 4-3). At first that nebula was a very cold collection of gas and dust—well below the freezing point of water. Although most of the solar nebula was hydrogen and helium, ice and ice-coated dust grains composed of heavy elements were scattered across this vast volume. Deep inside the nebula, gravitational attraction caused gas and dust to fall rapidly toward its center. (In terms of the physics presented in Appendix P: Energy and Momentum, the cloud’s gravitational potential energy was being converted into kinetic energy—energy of motion.) As a result, the density, pressure, and temperature at the center of the nebula began to increase, producing a concentration of matter called the protosun.

As the protosun continued to increase in mass and to contract, atoms within it collided with increasing speed and frequency. This activity created growing pressure inside the protosun, which caused its contraction to slow dramatically. Such collisions also created heat, causing the protosun’s temperature to soar. Therefore, the first heat and light emitted by our Sun came from colliding gas, not from thermonuclear fusion, as it does today. At the same time, there was also considerable activity in the outer regions of the disk.

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Rotation played a key role in the formation of the solar system. When it first developed, the solar nebula was a very slowly rotating ensemble of particles. This rotation occurred because the formation of interstellar clouds is turbulent, so these clouds are created with many slowly swirling regions, like smoke rising from a fire. As a result of this rotation, the solar nebula, created from one of these slowly swirling regions, had angular momentum (see Appendix P: Energy and Momentum). Because angular momentum is conserved (see Section 2-7), rotating matter spiraling inward in the young solar nebula revolved faster and faster, like the skater in Figure 2-12 when she pulls her arms in. This motion had the effect of flattening the nebula into a disk. You can see an analogous effect of rotation in the way the ice skater’s dress in Figure 2-12 is forced into a plane as she spins. (Had the solar nebula been nonrotating, it would have collapsed en masse to create a star without any planets or other orbiting matter.) Even the debris in the inner solar nebula that formed the protosun was orbiting as it fell inward, like water spiraling before it goes down a drain. Therefore, the protosun was spinning and the Sun that formed from it rotates. We will discuss the Sun’s rotation further in Chapter 9.

The outer region of the young solar nebula was probably very ragged, but mathematical studies show that the combined effects of gravity, collisions, and rotation would transform even an irregular dense core into a rotating disk with a warm center and outer cold edge about 30 AU from the protosun (see Figure 4-3). Eventually, the gas and dust in the disk of the solar nebula were orbiting fast enough to stop spiraling inward.

Although we cannot see our solar system as it was before planets formed, astronomers have found what we think are similar disks of gas and dust surrounding other young stars, including those shown in Figure 4-4. Called protoplanetary disks, these systems are undergoing the same initial stage of evolution that we have described here for our solar system.

As occurs with all gases, the protosun’s temperature increased as it became denser. As the protosun radiated more and more heat, the temperature around it began to increase. The protosun’s increasing temperature vaporized all common icy substances (including carbon dioxide, water, methane, and ammonia) in the inner region of the disk. These gases then blew away, along with the abundant hydrogen and helium gas that also initially existed there. Within a hundred thousand years or so of the disk forming, no gases or ices orbited closer than roughly 3 AU, between what are now the orbits of Mars and Jupiter. Temperatures were low enough beyond this distance that gases, primarily hydrogen and helium, and ices remained in orbit around the protosun. The boundary beyond which these gases and ices of water, carbon dioxide, methane, and ammonia persisted is called the snow line (or frost line or ice line). Within a million years of when the Jeans instability occurred, the solar system had differentiated into an inner region containing just metal-rich dust grains (that are not easily vaporized) and an outer region rich in hydrogen gas, helium gas, and ice-covered dust particles.

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Neighboring dust grains beyond the snow line began colliding with each other. The early motion of this debris was very chaotic; many of these collisions caused dust grains to grow, while many other collisions broke apart larger pieces of this debris. When dust particles finally reached sizes of a few millimeters, they began plowing through the surrounding gas. As the dust particles pushed aside this gas, the gas took energy away from (that is, slowed down) the dust particles, much as you slow down when walking into a headwind. As a result of losing energy (slowing down), this dust spiraled inward toward the protosun. The collision and accretion—the coming together of smaller pieces of matter to form larger ones—of dust particles continued as they spiraled inward. Like static cling between clothes, accretion of dust-sized particles is due to the attraction of their charged particles—protons and electrons—while accretion of much larger pieces of debris is primarily due to their mutual gravitational attraction. Through accretion, the dust grew into pieces of debris meters across. Calculations show that a piece of debris a meter across could have spiraled inward to half its original distance from the protosun in just 1000 years! Collisions destroyed some of these bigger bodies, while some of them merged and grew even larger. Eventually, a few billion of these boulders reached dimensions of a kilometer or so, large enough for their mutual gravitational attractions to enhance the rate at which they collided. Such pieces of debris are called planetesimals.

The formation of planetesimals that had migrated inside the snow line was a turning point in the young solar system’s evolution. They collided with countless dust particles as well as with each other. Sometimes colliding planetesimals destroyed each other, and returned to dust. Eventually, however, these collisions led to the growth of several hundred substantial bodies that persisted and eventually reached the size and mass of our Moon. At this size, the gravitational attraction of the planetesimals was strong enough for them to either sweep up the surrounding rubble or to fling it far away, thereby clearing out nearby space and limiting their own growth. These bodies led to the formation of the terrestrial planets, to which we will return shortly.