The Nice model also provides interesting insights into the evolution of the outer solar system after the four giant planets were formed. These planets were so much more massive than the surviving planetesimals with which they orbited that they were able to gravitationally fling some of this remaining debris toward the inner solar system. To do so while conserving energy and angular momentum, the planets had to spiral outward in response. Lower-mass Saturn spiraled outward faster than Jupiter. Recall from Kepler’s third law (Section 2-5) that as planets move farther from the Sun, the periods of their orbits lengthen. Eventually, Saturn reached a distance where it orbited the Sun once every time Jupiter orbited twice. This situation, where the periods of two orbits are related by whole numbers, is called a resonance because the resulting gravitational interactions between the two bodies reinforce each other. The process is analogous to pushing someone on a swing—if you push at the right time (that is, in resonance with the person’s motion), the person will go higher and higher.

As Saturn passed through this distance, resonant tugs from Jupiter pulled Saturn forward in its orbit, giving Saturn energy that enabled it to rapidly spiral further outward. Approaching Neptune and Uranus, Saturn’s gravitational attraction in turn gave these bodies extra strong tugs, which in turn forced them to spiral out much farther away from the Sun than they were originally. Recall that Neptune formed closer to the young Sun than Uranus. Because it was closer to Saturn during this transformative period, Neptune received stronger gravitational tugs from Saturn (and Jupiter) than did Uranus. As a result, Neptune was forced out past Uranus.

At that time there were still billions of planetesimals in the realm of the giant planets. The gravitational effect of the outwardly moving planets caused most of these planetesimals to change orbits. Some were sent inward, but most were hurled into orbits much farther away from the Sun than they had been. The outward-flying planetesimals spread way beyond the original disk (which had a radius of 30 AU, as noted above). Billions of them were flung into a bagel-shaped volume beyond the orbit of Neptune that is now called the Kuiper belt (Figure 4-6), named after Gerard Kuiper, who first proposed the existence of such a region in 1951. Many more planetesimals were sent out even further than the Kuiper belt, creating a spherical distribution of debris called the Oort cloud.

Kuiper belt objects, or KBOs, were first observed in 1992. Most of the Kuiper belt objects are ice-rich comets. Some of them have developed highly elongated orbits that occasionally bring them closer to the Sun than Earth’s orbit. When this happens, the Sun’s radiation vaporizes some of their ices, producing the long, flowing tails that we associate with comets.

Kuiper belt objects are distributed from about 30 AU to about 50 AU from the Sun. At least 1500 of them have been cataloged. The smallest Kuiper belt object observed so far, discovered using the Hubble Space Telescope in 2009, is only 1 km across. Astronomers believe that there are many Kuiper belt objects much smaller than this. Even nonscientists, however, are familiar with one of several large bodies that the giant planets flung out—Pluto. Pluto became locked in a 2:3 resonant orbit with Neptune, meaning that it orbits the Sun twice in the same time that Neptune orbits 3 times. The gravitational effects of resonances are so pronounced that thousands of other bodies, called plutinos, have the same type of resonant orbit around the Sun (two orbits of each plutino for every three orbits of Neptune) as does Pluto. Astronomers have observed several other bodies similar in size to Pluto that were also flung far into the Kuiper belt. All bodies orbiting beyond Neptune that have enough mass to pull themselves into spherical shape are called plutoids. Pluto is one such body.

106

The Oort cloud extends out perhaps 50,000 AU, a quarter of the way to the nearest star (see Figure 4-6). Most bodies in that distribution are believed to be a few kilometers in size or smaller. One candidate Oort cloud object, Sedna, is in an extremely elliptical orbit (Figure 4-6a), taking it between 76 and 976 AU. All the objects that orbit the Sun farther than Neptune are called trans-Neptunian objects, or TNOs. Plutinos and plutoids are examples of TNOs.

107

Now we turn to the major reservoir of leftover rock and metal debris remaining in the inner solar system, the asteroid belt, located between Mars and Jupiter. Orbiting in this region are millions of planetesimals and smaller pieces of debris. The total mass of debris in the asteroid belt is about 5% of our Moon’s mass, much less than the mass of a planet. There were more bodies in the asteroid belt in the distant past, but impacts and the gravitational tug of Jupiter pulled some bodies away and kept the remaining debris spread out, thereby preventing it from ever coming together to form a larger body.

The largest asteroid, Ceres, has a diameter of about 900 km (compared to Earth’s diameter of 12,756 km). It was the first asteroid discovered, in 1801. Today Ceres is also classified as a dwarf planet, as we will discuss shortly. The next largest, Vesta and Pallas, are each about 500 km in diameter. Still smaller asteroids are increasingly numerous. Many thousands of kilometer-sized asteroids have been observed. A close-up picture of the asteroid Gaspra is shown in Figure 4-7. Some asteroids have their own moons, such as the asteroid Ida, which is orbited by heavily cratered Dactyl.

Some asteroids in the inner solar system orbit between the asteroid belt and the Sun. Many of these objects have highly elliptical orbits that take them across the paths of some of the planets, including Earth.

Earth, along with every other body that orbits the Sun, has been struck by countless pieces of space debris throughout its life. Indeed, these impacts still occur today. Depending on the speed and angle at which they strike land, impacting bodies larger than about 0.5 m create scars, called craters. Most of the craters on our planet’s surface have been erased, as we will study in Chapter 6, but we can still see numerous ones on our Moon, as well as on nearly all the other bodies with solid surfaces in the solar system (Figure 4-8). Venus is the most notable exception, as it is periodically resurfaced.

During the first 400 million years that the planets existed, the rate of impacts declined steadily. But then, about 4.1 billion years ago, the impact rate skyrocketed as planetesimals in the asteroid belt were knocked out of orbit as debris flung inward by the giant planets (see Section 4-5) struck them. Numerous asteroids were pushed into orbits that crossed the paths of the inner planets. Thus began the Late Heavy Bombardment, which ended as these errant asteroids were destroyed by either striking something, falling into the Sun, or eventually being flung out of the inner solar system.

108

The Moon was struck by so many objects during the Late Heavy Bombardment that large regions of its surface were completely obliterated, creating gigantic craters, which remained on the surface for hundreds of millions of years. These craters were eventually filled in when molten rock from inside the Moon (magma) leaked out through the thin crust below the craters, thereby forming the dark plains called maria that we see today (Figure 4-8). Mercury, which is particularly dense when compared to the other inner planets, may have lost much of its lower-density outer layers in a collision that occurred during this time.

The Moon’s virtual lack of atmosphere has helped preserve important information about the early history of the solar system. Radioactive dating of Moon rocks brought back by the Apollo astronauts indicates that the rate of impacts declined dramatically about 3.8 billion years ago. That time is taken to be the end of the Late Heavy Bombardment. Since that time, impact cratering has continued but at a very low level.