Читать книгу Exploring the Solar System - Peter Bond - Страница 29

Our picture of the early Solar System is complicated by the likelihood that the giant planets migrated considerable distances before they ended up in their present positions. Such large‐scale movement is supported by the discovery of numerous large, extrasolar planets that orbit within a fraction of an astronomical unit of a star.

In the case of our Solar System, this migration can be explained by the exchange of orbital momentum between giant planets and innumerable planetesimals. One current model (the Nice model – see Box 1.5 and Figures 1.17 and 1.18) envisages a chaotic early Solar System occupied by the major planets out to a distance of about 15 AU (closer than the present orbit of Uranus). Jupiter may have been born a little farther out in the Solar System than it is today, whereas the other giants were closer to the infant Sun than at present. Beyond the planets was a region swarming with leftover planetesimals.

Whereas Jupiter was massive enough to eject large numbers of planetesimals to the outer reaches of the Solar System or out of the system altogether, the three smaller giants were unable to do this. Instead, they flung similar numbers of planetesimals toward the Sun and away from it. Whenever Uranus or Neptune decelerated a nearby planetesimal, causing the object to move closer to the Sun, the planet gained a tiny amount of momentum. The resultant acceleration caused it to drift away from the Sun.

Over time, after billions of such gravitational interactions, Jupiter spiraled inward a modest distance, while Saturn drifted outward. When Jupiter reached a distance of 5.3 AU and Saturn arrived at 8.3 AU, the two planets were in a 2:1 orbital resonance, so that one orbit of Saturn lasted precisely two Jupiter orbits. The repeated gravitational pull of Jupiter caused Saturn's orbit to become much more elongated.