Last month I wrote an article describing how the Grand Tack model explains the orbital configurations of all eight planets and all five asteroid and comet belts in our solar system. The heart of this model is an inward migration of Jupiter, Saturn, Uranus, and Neptune where, at different times, each planet stops moving inward, proceeds to migrate outwards, and finally settles into its present stable orbit.
The Grand Tack has been consistently successful in explaining the characteristics of the present-day solar system and its history—the mark of a good model. Moreover, the fine-tuning inherent in this model provides further proof that no other planetary system comes close to mimicking the characteristics seen in ours. A recent paper questioned the Grand Tack's explanation for the Late Heavy Bombardment, yet failed to find a suitable alternative scenario.
The Grand Tack, developed by a team in Nice, France, drew the attention of two American astrophysicists, Nathan Kaib and John Chambers. They developed a large ensemble of computer simulations to investigate how the inward-outward migrations of the gas giants would affect the orbits of Mars, Earth, Venus, and Mercury over time.1
Kaib and Chambers' simulations revealed at least an 85 percent probability that the inward-outward migration of Jupiter and Saturn into their present orbits would cause at least one of the terrestrial planets to either be ejected from the solar system or fall into the Sun. For the minority of the simulations that preserved all four of terrestrial planets, less than 5 percent manifested orbital eccentricities (ellipticities) and inclinations as small as those presently exhibited by Mars, Earth, Venus, and Mercury. Furthermore, Kaib and Chambers found a less than 1 percent probability of achieving all eight planets' present-day orbital features.
Kaib and Chambers concluded their research paper by stating that the small probabilities they derived imply "that the giant planet instability [inward-outward migration] is not the source of the Late Heavy Bombardment and that terrestrial planet formation finished with the giant planets in their modern configuration."2
The Late Heavy Bombardment (LHB) refers to a time, about 3.9 billion years ago, when the tens of thousands of large comets and asteroids impacted the terrestrial planets and the Moon. Kaib and Chambers' concluded that Jupiter, Saturn, Uranus, and Neptune likely gained their present orbital configurations before Mars, Earth, Venus, and Mercury completed their formations and several hundred billion years before the LHB.
Two major problems arise from Kaib and Chambers' proposal. The first is that, apart from the Grand Tack model, no other viable scenario (let alone detailed model) exists to explain the LHB. For example, the suggestion made more than a decade ago that a rogue star passed close enough to the Sun to disrupt its primordial comet and asteroid belts fails to explain the LHB and the present orbital configurations of the asteroid and comet belts and the four gas giants. The second major problem is that while Kaib and Chambers' proposal may explain the present-day orbital features of the eight planets, it is unable to explain the orbital features of the five asteroid and comet belts.
Kaib and Chambers may be right, though, about a planet being ejected from the solar system. The latest Grand Tack model produced by the Nice team incorporates a Neptune-sized planet being ejected by the reversal of Jupiter's migration. The latest Grand Tack also tolerates a possible ejection of a fifth terrestrial planet from the solar system.
As I mentioned in the article I wrote last month, the Grand Tack model involves a tremendous amount of solar system fine-tuning—something that unsettles nontheistic astronomers. It is the one model, however, that explains all the characteristics of the Sun's planets and asteroid and comet belts, and in minute detail. It even explains the orbital configurations of both families of the Trojan asteroids that accompany Jupiter.
The fine-tuning inherent in the Grand Tack model is consistent with the observation that no other of the 1,293 planetary systems in the exoplanet catalog (maintained by the exoplanet team) comes close to mimicking the characteristics of ours. Kaib and Chambers have provided yet another confirmation of the rare solar system hypothesis. Their research confirms the Grand Tack model's robustness and provides more evidence for the extraordinary fine-tuning design of our solar system that makes advanced life and civilization possible on Earth.