With the Kepler launch scheduled for no earlier than Friday, I’m keeping one eye on the mission site while I develop today’s material. Kepler launches aboard a Delta II, but engineers are now having to check common hardware between that rocket and the Taurus XL launch vehicle that failed to get NASA’s Orbiting Carbon Observatory into orbit last week. Thus the March 5 launch date slips to March 6, which itself is still tentative.

Meanwhile, an unusually interesting story in Nature also has my attention, dealing not with exoplanets but with the early Solar System and what may have been a period of planet migration that caused heavy asteroidal bombardment of the inner planets. This one comes out of the University of Arizona, where scientists have been looking at the distribution of asteroids with diameters greater than fifty kilometers. UA’s David Minton and Renu Malhotra ran simulations beginning with a uniform asteroid belt to see how the present-day gaps in the belt may have arisen over time.

At issue are the so-called Kirkwood gaps, which occur in areas where gravitational effects from both Jupiter and Saturn perturb the belt and eject asteroids. The result of the simulations: Areas exist where asteroid orbits are stable but no asteroids exist. But if the effects of giant planet migration in the early system are added to the mix, the simulated belt matches up well to what we see today. Those extra areas of asteroid depletion, then, may be the signature of planetary migration. Says Malhotra:

“Our interpretation is that as Jupiter and Saturn migrated, their orbital resonances swept through the asteroid belt, ejecting many more asteroids than is possible with the planets in their current orbits. And the particular pattern of missing asteroids is characteristic of the pattern of Jupiter’s and Saturn’s migration.”

Thus we have evidence that the giant planets formed with closer spacing than in today’s Solar System, with Jupiter then moving slightly closer to the Sun, while Saturn, Uranus and Neptune moved farther from the Sun and from each other. These events would have been destabilizing enough for the asteroid belt that the possibility of linking them to early, heavy bombardment of the inner planets is worthy of further study.

Graduate student Minton and planetary sciences professor Malhotra seem to make a good team — you may also want to have a look at their recent study of the extrasolar system OGLE-2006-BLG-109L, which examines a possible system architecture that includes the two detected massive planets and has the potential for two terrestrial-class worlds which, if they exist, could create an environment that mimics our Solar System. That paper is “Prospects for the Habitability of OGLE?2006?BLG?109L,” Astrophysical Journal Letters 683 (August 10, 2008), pp. L-67-70 (abstract). The asteroid paper is “A record of planet migration in the main asteroid belt,” Nature 457 (February 26, 2009), pp. 1109-1111 (abstract).