JupiterMany planetesimals that formed out beyond the snow line remained out there in substantially noncircular orbits. As a result, they crossed paths and a few of them collided and merged with each other, creating larger bodies. One planetesimal located 1 or 2 AU beyond the snow line accreted enough of its companions to become an Earth-like body containing perhaps 10 Earth masses of rock and metal. This body had enough gravitational attraction to pull onto itself vast amounts of the hydrogen, helium, and ices that existed near its orbit. Within only a few thousand years of forming, it grew into the planet Jupiter. Like the heating of the protosun, impacts from infalling gases heated Jupiter up; in fact, the planet was heated so much that for a short time it outshone the protosun, if those two bodies had been observed then from equal distances. Jupiter, however, never became a star because its total mass is too low for it to compress and heat its core enough to enable hydrogen fusion to commence. Indeed, Jupiter would need to be 75 times more massive for it to sustain core fusion and therefore be classified as a star, as discussed in Section 4-1.

Plowing through the surrounding gas and dust, the growing Jupiter lost energy and was thereby forced to spiral inward. Calculations suggest that it may have reached as close as Mars’s present orbit before it stopped its migrating toward the protosun. On the way in, Jupiter’s gravitational attraction and the heat it emitted stirred up the surrounding gas enough so that some of the gas and dust it passed was sent inward toward the protosun and some was sent outward. In this way, Jupiter eventually “cleared its neighborhood,” and its growth halted. By this point, Jupiter had accumulated about 300 Earth masses of hydrogen and helium, plus several Earth masses of water and other simple molecules. From the inside out, Jupiter was a rocky world, surrounded by a layer of water, which was surrounded in turn by a much thicker layer of predominantly hydrogen and helium. Jupiter rotates today because it formed from swirling debris in the solar nebula. Likewise, all the other planets in our solar system rotate.

Jupiter’s inward migration had two long-term effects on our solar system. First, as it passed through what is now the asteroid belt, it dispersed most of the debris there, preventing a planet from ever forming in that region. Second, it had a similar, but not quite as effective, influence on stirring up the debris in the vicinity of what is now Mars’s orbit. Astronomers believe that is why Mars is such a low-mass planet compared to Earth and Venus. As we will discuss in Chapter 5, many Jupiterlike planets orbiting other stars are even closer to their stars than Mercury is to the Sun. Jupiter is likely to have continued inward as it plowed through the surrounding gas and dust if it had not been stopped by the gravitational attraction of the next planet to form, Saturn.

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SaturnA few million years after Jupiter formed, collisions of planetesimals led to the formation of a second rocky world of about 10 Earth masses located a few astronomical units out beyond Jupiter. This second world underwent the same process of accreting hydrogen, helium, and ice, thereby creating Saturn. By the time Saturn formed, however, a lot of gas in its vicinity had already been stirred up and dissipated by Jupiter, leaving Saturn with less matter to accrete. Saturn has about 80 Earth masses of hydrogen and helium, along with a few Earth masses of water. Note that when it formed, Saturn was closer to the protosun than it is to the Sun today. Furthermore, the gravitational attraction of the young Saturn, combined with that of Jupiter, cleared their neighborhoods of the gas and dust that were causing them to spiral inward. As a result of their gravitational interaction with each other, they forced each other to spiral back out until Jupiter arrived at its present location, with Saturn farther out.

Neptune and UranusThe gravitational effects of Jupiter and Saturn forced many planetesimals to become concentrated out beyond Saturn’s orbit, leading to the formation of two more rocky planets destined to become the cores of Neptune and Uranus. The debris that far from the protosun had been spiraling inward for several million years; thus, these two planets are believed to have formed substantially closer to Saturn than they are today. Furthermore, computer simulations suggest that at the time of their formation, Neptune was closer to the protosun than Uranus (the opposite of their positions now!).

While substantial ice remained for Neptune and Uranus to collect, they formed so late that Jupiter and Saturn had dissipated most of the hydrogen and helium in their neighborhoods. Therefore, Neptune and Uranus became rich in water, along with carbon dioxide, methane, and ammonia—generically called “ices”—from their surroundings, but each accreted only a couple of Earth masses of hydrogen and helium. While Jupiter and Saturn are called gas giants, Neptune and Uranus are called ice giants.

The details of how the inner four rocky planets—Mercury, Venus, Earth, and Mars—formed are less well known than for the giant planets just discussed. It appears likely that the inner planets formed as a result of collisions between the Moon-sized and larger planetesimals discussed at the end of Section 4-2. Within 100,000 years of the dust beginning to coalesce and to spiral inward, as described above, bodies with 0.1 Earth masses were able to form in it and collide with each other inside the snow line. It is unknown whether most of these planetesimals formed inside the snow line in relatively circular orbits and then collided to build planets in relatively circular orbits, or whether the four inner worlds formed from collisions of planetesimals that came from beyond the snow line in highly elliptical orbits that then somehow became circular. Whether the inner planets formed in orbits near their present average distances from the Sun or whether they migrated inward, like the planetesimals outside the snow line described above, is also unknown.

In the end, however, the collisions of hundreds of Moon-sized planetesimals led to the existence of four rocky inner planets: Mercury, Venus, Earth, and Mars (Figure 4-5). We are literally made of stardust! Because these planets were located inside the snow line, they did not acquire coatings of hydrogen and helium like those still found on the giant planets today. However, enough hydrogen and helium were part of the debris that formed the inner four planets that these gases became their first atmospheres. Being so light, these gases quickly drifted into space, as we explore further in Chapter 6.

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Because Mercury, Venus, and Mars have similar densities as Earth (in Latin, terra), they are composed of similar chemicals. Consequently, all four bodies are called terrestrial planets.

The large quantities of water that exist on Earth today, that existed on Mars, and that are likely to have existed briefly on Venus came in part from ices in the planetesimals that created these worlds and in part from impacts of water-rich debris called comets. Thus, the Nice model shows how our world came to have so much metal and so little hydrogen and helium, as proposed earlier in the chapter.

One planetesimal impact on the young Earth was particularly notable. When the young Earth had between 80% and 90% of its present mass, it was struck by a planetesimal with a mass that may have been a few times the mass of Mars (Mars has roughly 10% of Earth’s present mass). Computer simulations of such an impact indicate that it would have splashed a lot of debris off Earth’s surface and into orbit and tilted Earth’s rotation axis. This event occurred about 4.5 billion years ago, within 100 million years after Earth started forming. The orbiting debris created a short-lived ring, which clumped together (in a process analogous to how planetesimals grew from collisions) and formed our Moon.