Serving Up the Cosmic Leftovers

What’s the oldest stuff in your refrigerator (aside from that rubbery celery you bought but never ate)? Leftovers! The same is true in the solar system. The fragmentary leftovers of the formation of the sun and planets are some of the oldest objects in the solar system. For a long time, few scientists paid much attention to this debris or knew much about it. More recently, however, astronomers have come to realize that many significant clues to the origin and early evolution of the solar system are to be found not in the planets, but in the smaller bodies, the planetary moons and solar system debris. For the most part, the planets are very active places. Atmospheres have produced erosion, and internal geological activity has erased ancient surfaces. On the earth, weather, water, and tectonic motion have long since “recycled” the earth’s original surface.
So studying the planets reveals relatively little about the origins of the solar system. However, on moons and asteroids, atmospheres are sparse or nonexistent, and geological activity is minimal or absent. The result? Many of these bodies have changed little since the solar system was born. They are, in effect, cosmic leftovers.

The Inner and Outer Circles

Astronomers think of the planets as falling into two broad categories—with one planet left over. The four planets (including the earth) closest to the sun are termed the terrestrial planets. The four farthest from the sun (not counting Pluto) are the jovian planets. And Pluto, usually the farthest out of all, is in an unnamed class by itself. Its location is jovian, while its size and composition put it more in a class with the moons of the jovian planets. Some astronomers prefer to think of it as the largest Kuiper Belt object rather than the smallest (and hardest-to-categorize) planet.

Snapshot of the Terrestrial Planets
Mercury, Venus, Mars, and Earth are called the terrestrial planets because they all possess certain Earth-like (terrestrial) properties. These include proximity to the sun (within 1.5 A.U), relatively closely spaced orbits, relatively small masses, relatively small radii, and high density (rocky and solid-surfaced). Compared to the larger, more distant jovian planets, the terrestrials rotate more slowly, possess weak magnetic fields, lack rings, and have few moons or none. In fact, within the terrestrial “club” the earth’s large moon is unique. The moon is only slightly smaller than the planet Mercury and larger than Jupiter’s moon Europa! As we have seen, the moon’s large size is one clue to its origin.

Snapshot of the Jovian Planets
The jovians are far from the sun and travel in widely spaced orbits. They are massive planets with large radii, yet they are of low density with predominantly gaseous makeup and no solid surface. In contrast to the terrestrial planets, they rotate faster, possess strong magnetic fields, have rings, and are orbited by many large moons.
The outermost jovian planet is Neptune.

Planetary Report Card

Let’s make a survey of the planets. Here’s what we’ll be measuring and comparing in the table that follows:

• Semi-major axis of orbit. You’ll recall from Chapter 4 that the planets orbit the sun not in perfectly circular paths, but elliptical ones. The semi-major axis of an ellipse is the distance from the center of the ellipse to its farthest point. This distance does not exactly correspond to the distance from the sun to the farthest point of a planet’s orbit, since the sun is not at the center of the ellipse, but at one of the ellipse’s two foci. We will express this number in A.U.
• Sidereal period. The time it takes a planet to complete one orbit around the sun, usually expressed in Earth years.
• Mass. The quantity of matter a planet contains. The mass of the earth is 5.977 1024 kg. We will assign the earth’s mass the value of 1.0 and compare the masses of the other planets to it.
• Radius. At the equator, the radius of the earth is slightly less than 6,400 km (3,963 miles). We will assign the radius of the earth a value of 1.0 and compare the radii of other planets to it.
• Number of known moons. Self-explanatory—an ever-changing number for the outer planets.
• Average density. This value is expressed in kilograms of mass per cubic meter. The substance of the inner planets is dense and tightly packed; in the outer planets, the densities are typically lower.

Some Points of Interest

The orbits of the planets lie nearly in the same plane, except for Mercury and Pluto, which deviate from this plane by 7 degrees and 17 degrees, respectively. Between the orbit of Mars and Jupiter is a concentration of asteroids known as the asteroid belt. Most of the solar system’s asteroids are here. The orbits of the planets are not equally spaced, tending (very roughly) to double between adjacent orbits as we move away from the sun.
To say that the distances between planets and
the sun are very great is an understatement.
Interplanetary distances are so great that it becomes awkward to speak in terms of miles or kilometers.
For that reason, astronomers have agreed on something called an astronomical unit A.U.), which is the average distance between the earth and the sun—that is, 149,603,500 kilometers or 92,754,170 miles.
Let’s use these units to gauge the size of the solar system. From the sun to the average distance of the outermost planet, Pluto, is 40 A.U.
(3,710,166,800 miles, or almost 6 billion km). At just about a million times the radius of the earth, that’s quite a distance. Think of it this way: If the earth were a golf ball, Pluto would be a chickpea about 8 miles away, Jupiter would be a basketball about 1 mile away, and the sun would go floor-to ceiling in a 10-foot room and be less than a quarter- mile away. However, compared to, say, the distance from the earth to the nearest star (after the sun), even Pluto is a near neighbor. Forty A.U. is less than 1⁄1000 of a light-year, the distance light travels in one year: almost 6 trillion miles. Alpha Centauri, the nearest star system to our sun, is about 4.3 light-years from us (more than 25 trillion miles). On our golf ball scale, Alpha Centauri would be about 55,000 miles away.

Let’s Take a Stroll

Our solar system is centered on a single star, the sun. We have recently come to appreciate that about 50 percent of all stars form in binary systems (containing two stars), so our sun is a bit lonely as stars go. In orbit around the sun are nine planets (in order of distance from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Around some of these planets orbit moons—more than 70, at latest count. By the 1990s, astronomers had observed more than 6,000 large asteroids, of which approximately 5,000 have been assigned catalog numbers.
(Such an assignment is made as soon as accurate orbital data is recorded.) Most asteroids are rather small; it is estimated that there are 1 million with diameters greater than 1 km (or about 3⁄5 of a mile). Some, perhaps 250, have diameters of at least 62 miles (100 km). About 30 have diameters of more than 124 miles (200 km). All of these planets and asteroids are the debris from the formation of the sun. They coalesced through the mutual attraction of gravity.
In addition, the solar system contains a great many comets and billions of smaller, rock-size meteoroids.

Solar System Family

A snapshot freezes an instant in time. When we think about our solar system, we usually assume that it has always been much as it is now, and always will be. But what we know of the solar system (4,000 years of accumulated knowledge) is only a mere snapshot in comparison to its 4.6 billion–year age. It took humankind millennia to reach the conclusion that our planet is part of a solar system, one of many planets spinning on its axis orbiting the sun. These were centuries of wrestling with the earth-centered planetary system first of Aristotle, then of Ptolemy, trying to make the expected planetary orbits coincide with actual observation. This knowledge arose in some sense as a side product of the real initial goals: to be able to predict the motion of the planets and stars for the purpose of creating calendars and (in some cases) as a means of fortune-telling. However, even the earliest astronomers (of whom we know) wanted to do more than predict the planets’ motions. They wanted to know what was “really” going on. When Copernicus, Galileo, Tycho Brahe, and Kepler finally succeeded in doing this quite well in the sixteenth and seventeenth centuries, it was a momentous time for astronomy and human understanding.
Understanding how the planets move is important, of course, but our understanding of the solar system hardly ends with that. In the last few decades of the twentieth century and now into the twenty-first, astronomers have learned more about the solar system than in all the 400 years since planetary motions were pretty well nailed down. As this chapter will show, the planetary neighborhood is a very interesting place, and our own world, the earth, is unique among the planets as a home to life.

Ashes to Ashes, Dust to Dust

In the chapters of this book’s final section, we will consider questions of time and eternity as they relate to the universe. But as to the solar system, we know that it was born about 4.6 billion years ago, and that it will die when its source of energy (the sun) dies of old age.
Just as the specifics of the formation of the solar system depended on the formation of the sun, so its death will be intimately related to the future of our parent star. The evolution of the sun will presumably follow the same path of other stars of its size and mass, which means that the sun will eventually consume the store of hydrogen fuel at its core. As this core fuel wanes, the sun will start to burn fuel in its outer layers, grow brighter, and its outer shell will expand. It will become a red giant, with its outer layers extending perhaps as far as the orbit of Venus. When the sun puffs up into a red giant, Mercury will slow in its orbit, and probably fall into the sun. Venus and the earth will certainly be transformed, their atmospheres (and, in the case of Earth, also water) being driven away by the intense heat of the swelling sun. Venus and Earth will return to their infant state, dry and lifeless.
Some recent models of solar evolution predict that the sun will slowly grow to this state sooner, giving us only another billion or so years before the earth becomes uninhabitable. But don’t fret. All of this is another one to five billion years away. The sun is in its midlife now, and, we hope, will avoid any crisis. The sun will then eject its outer layers (to become a planetary nebula), leaving behind a burned-out star called a white dwarf. A white dwarf does not have sufficient mass to continue fusing elements. It will slowly cool, radiating its internal heat into space, and eventually become a black dwarf—a strange object composed mostly of oxygen and carbon, the size of a planet with the mass of a star. Let’s hope humanity has pushed on by then!

Do the Pieces Fit?

By combining the nebular and condensation theories, we have arrived at an explanation that appears to address the major constraints that we listed at the beginning of the chapter. Does that mean that this theory is “right”? Perhaps.
But like any model, it is subject to future observations that might cause us to reject or revise it. Let’s revisit some of the constraints that we outlined.

• A rotating cloud of gas, collapsing gravitationally, can account for the “counterclockwise” (as seen from the North Pole) orbit of the planets, rotation of the sun, and rotational orientation of their moons. What we are seeing in all of these is the direction of rotation of that original solar nebula.
• The rocky nature of the inner solar system, and the gaseous nature of the outer solar system follow directly from the temperature of these regions as the dust grains were formed. Only the heaviest materials (metals) survived intact close to the sun, whereas more fragile molecules (like water) survived in the outer reaches.
• The planets are all found close to the ecliptic because, as the solar nebula contracted gravitationally, it naturally flattened. This flattened disk was where the planets most likely formed.
• The existence and location of asteroids, comets, and other debris is a natural byproduct of the accretion and early gravitational interaction process. Yet, as expressed here, the condensation theory does not account for absolutely everything we observe in the solar system.

For the theory allows for an element of randomness, primarily in the form of close encounters and collisions among the planetesimals and protoplanets, which probably influenced certain variations we see in the orbital motions and orientation of some of the planets. As we saw in the last chapter, it is very likely that our own moon is the remnant of a catastrophic collision between the earth and a planetesimal that was the size of Mars. That collision also likely explains the anomalous tip of the earth’s rotational axis, and thus the seasons that grace our planet.