The Originand Fortuitous Stabilityof Jupiter

Why and how did Jupiter form where it did? It is generally believed that Jupiter's formation began with the accretion of a solid core. This core grew by collision and sticking of dust, ice, rocks, and larger bodies—a process similar to the accretion of Earth. Jupiter, however, formed outside the "snow line," a special place in the solar system where water vapor condensed to form ice grains and the presence of "snow" in this region would enhance the density of solid matter and accelerate the accretion process. The mystery is why the proto-Jupiter grew so rapidly. Apparently, Jupiter grew to a mass of 15 Earths before Mars grew to 10% of an Earth mass. David Stevenson at Cal Tech has suggested that outward migration of water vapor and condensation at the "snow line" may have provided larger concentrations of condensed matter at this location, thus speeding up the formation of the embryonic Jupiter.

Jupiter's growth to a giant planet began when the rock-ice core mass reached 15 Earth masses. At this mass, the gravity of the core can pull in and hold hydrogen and helium, the light gases that account for 99% of the mass of the nebula. When this gas accretion process begins, it is very dramatic because the rate of accretion of gas is proportional to the square of the mass already accreted. In other words, the bigger it gets, the faster it grows. If gas could be continually fed to it, it would gobble up the Universe in a relatively short time! What actually happens is that Jovian planet formation depletes its feeding zone of matter, which in turn truncates planet formation. And although the general properties of this process might be modeled, it just seems to have been by chance that our Jupiter formed as it did.

Because it cleans our solar system of dangerous Earth orbit-crossing asteroids and comets, Jupiter has had a beneficial influence on life on Earth. However, it appears that we have been quite lucky that the Jupiter in our solar system has maintained a stable orbit around the sun. A Jupiter and a giant neighbor like Saturn are a potentially deadly couple that can lead to disastrous situations where a planetary system can literally be torn apart. Recently, it has become possible to use powerful computers to determine the stability of the orbits of Jupiter and Saturn over the lifetime of the planetary system. There are minor chaotic changes but no major changes, and the solar system, at least to a first approximation, is stable over its lifetime. However, this would not be the case if either Jupiter or Saturn were more massive or if the two were closer together. It would also be dangerous to have a third Jupiter-sized planet in a planetary system. In an unstable system the results can be catastrophic. Gravitational perturbations among the planets can radically change orbits, make them noncircular, and actually lead to the loss of planets ejected into interstellar space. It is possible for chaotic disruption to occur even after a system has been stable for billions of years, and in the worst cases, planets can be spun out of the planetary system, escaping the gravitational hold of the star. A life-bearing planet ejected into galactic space would have no external heat source to warm its surface and no sunlight to provide energy for photosynthesis. Although instability might start with just two planets, the effects would spread to them all. In less severe cases, the orbits of the planets would become highly elliptical, and the changing distance between planets and the central star would prevent the persistence of conditions required for stable atmospheres, oceans, and complex life.

Numerical calculations first indicated that some planetary systems might become unstable, and recent observations provide evidence that this actually does occur. At the present time, planets are being discovered around other stars by detecting small velocity changes of the central star. Of the planets that have been detected, many are Jupiter-mass planets far from the star, with highly noncircular orbits. This is quite different from the solar system, where all giant planets have quite circular orbits. It is generally agreed that the best explanation for the elliptical orbits is that these are planets whose orbits have been altered by other planets, possibly by ejection of another planet into interstellar space.

It is during the formation of planetary systems that Jupiter-like planets pose the most draconian threat to terrestrial planets. With radial order similar to that in the solar system, the terrestrial planets in the habitable zone of typical planetary systems would form close to the star, the Jovian planets farther out. There is reason to believe that this is the "natural way," because the formation of Jovian planets probably occurs only in the colder, more distant regions outside the "snow line," mentioned previously. It is also to be expected that giant gaseous planets like Jupiter could not form close to a star because tidal forces (the differential force of gravity) would disrupt a planet in its diffuse, early stages. When a proto-Jupiter was very diffuse and too close to the star, the differential force of gravity between the sides near the star and far from it would pull the forming planet apart. It was quite surprising, then, that several of the first extrasolar planets discovered were found to be Jupiter-mass planets very close to their central stars—closer than Mercury to the sun. All of these "hot Jupiters" have highly circular orbits, and it is difficult to imagine that they actually formed in these locations.

A popular explanation of this phenomenon is that the giant planets in these systems actually did form at Jupiter-like distances but that their orbits decayed and the planets spiraled inward. This cannot happen in an evolved planetary system, but it can in the early, solar nebula phases when extensive gas and dust still exist in the regions between the planets. Doug Lin at the University of California at Santa Cruz has calculated that spiral waves generated in the solar nebula phase can extract energy from a young Jupiter and cause its orbit to spiral inward. In many cases the planet actually hits the star,- in others the inward drift stops before collision occurs. The observed giant planets that are very close to stars may be examples of this inward drift. Events such as this can be calamitous for terrestrial planets. When a Jupiter spirals inward, the inner planets precede it and are pushed into the star. If our Jupiter had done this, Earth would have been vaporized long before life-tolerant conditions were ever established on its surface. Lin has suggested that our solar system may have had several Jupiters that actually did spiral into the sun, only to be replaced with a newly formed planet. Perhaps Jupiter is at its "right" distance from the sun only because it was the last one to form and it formed at a time when the solar nebula had weakened to the point where orbital decay ceased to be important.

The programs to detect extrasolar planets have revealed that nearly all of the planets found either are "hot Jupiters" in circular orbits close to the star or describe elliptical orbits farther from the star. All of these are "bad" Jupiters whose actions and effects should preclude the possibility of these systems having animal life on Earth-like planets in the habitable zones of the parent stars. These life-unfriendly planetary systems have been found around 5% of the nearby stars. The search techniques are most effective for detecting Jupiters close to stars, however, and at present they cannot detect Jupitermass planets at our Jupiter's distance from the sun, nor can they detect planets less massive than Jupiter. They could not presently detect our solar system from the distance of nearby stars, and so it is possible that up to 95% of the nearby solar-like stars have "regular" planetary systems similar to our own, with terrestrial planets close to the star and Jovian planets in circular orbits farther out. On the other hand, it appears that most other "Jupiters" so far detected would have prevented the development of animal life anywhere in their respective solar systems.

The Moon and Jupiter are two factors causing us to believe that complex life requires disparate influences. We shall see, in the next chapter, how we might put this hypothesis to the test.

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