A couple of months ago Dave Rogstad reported on a study assessing the current importance of Jupiter in shielding Earth from cometary impacts. An article appearing in Science Daily prompted the discussion. It seems that for one class of comets (the “Jupiter Family Comets”), Jupiter plays an ambiguous role. First, the study revealed that the number of impacts Earth experiences is unchanged whether Jupiter exists or not. Even more interesting, a smaller Jupiter would lead to even more Earth impacts.
Prior to this research scientists had traditionally considered Jupiter’s role (including its size and position) as an impact-absorbing planet critical for life on Earth. When understanding this new study one must also remember the likely role of Jupiter and Saturn in clearing the solar system of comet and asteroid material during the late heavy bombardment. In effect, the early migration of Jupiter and Saturn’s orbits caused most of the comets and asteroids to impact the inner solar system bodies around four billion years ago—before the vast majority of life’s history on Earth.
This migration plays a significant role when considering an article published in the Astrophysical Journal. A team of Penn State and NASA scientists demonstrated via simulations that Earth-sized planets with water could form in planetary systems where the giant planet (a gas giant like Jupiter) migrated in close to the parent star. The key criteria for the terrestrial planet formation are that the gas giant forms and migrates quickly and that the disk surrounding the star contains sufficient material for planets to form after migration. The authors highlighted that the stellar nebula (gas and dust cloud) must contain sufficient gas and friction sources to settle the remaining material back into a disk so the terrestrial planet(s) can form.
Note the contrast with the role of Jupiter’s migration in our solar system. Whereas Jupiter clears the solar system of material which would impact Earth, the extrasolar system (from the simulation) needs to refresh this material in order for terrestrial planets to form. While the authors do find that watery, earth-sized planets often form in systems with significant gas-giant migration, these planets are likely subject to immense bombardment over substantial time periods. Advanced life, such as human beings, requires a large number of significant changes after planet formation. But terrestrial planets in extrasolar systems with close-in Jupiters would experience intense comet and asteroid bombardment making it unlikely that these required changes will occur.
Thus, the features of Earth’s solar system, including Jupiter’s migration history, appear conspicuously beneficial for advanced life.
For those interested in a more detailed discussion of this issue, hear the September 18, 2007 edition of Creation Update.