A popular theory holds that advanced life on Earth depends on Jupiter's presence to shield our planet from cometary bombardment. However, a paper in Astrobiology turns this idea on its head, suggesting Jupiter's value is in peppering us with more comets. The debate may influence how we seek life in other solar systems.

Earth's early development was characterized by regular impacts from comets and asteroids. We don't know if life got started at this time, but if so it couldn't flourish – before too long a huge impact would sterilize the planet again. In the mid-'90s George Wetherill wrote two papers that have been interpreted as arguing that, were it not for the presence of Jupiter, this bombardment would have continued, giving no opportunity for life to evolve past single-celled organisms.

However, NASA's Dr. Kevin Grazier thinks we have this backwards. Having run simulations on the behavior of outer Solar System objects with and without Jupiter he concludes that, rather than collecting comets that might otherwise enter the inner Solar System, Jupiter teams up with Saturn to throw objects into paths that send them towards the Sun, sometimes hitting Earth on the way. Jupiter does play a role in expelling comets from middle distances from the Sun to the outer Solar System, but Grazier concluded that even much smaller objects at its location would do the same thing.

In the quest for inhabited worlds beyond the Solar System, the “Jupiter shield” theory has led many astronomers to conclude that the only places we should look are where at least one large planet lurks suitably far from the parent star. Grazier thinks this might be the right approach, but for the wrong reason.

^{Comets can be a menace, but they are also crucial to the habitability of the Earth. MarcelClemens/Shutterstock}

Too many comets are a problem for evolution, but too few may prevent life from ever getting started. It is believed that Earth lost its water in the initial heat of formation, and it was only through cometary bombardment that it was replenished. Grazier concluded that without Jupiter's influence, Earth might be too dry for life. Indeed his work suggests that not one, but two, large planets may be needed to keep inner rocky world's wet, since Saturn's contribution proved unexpectedly large.

"In this paper, we learn that the overly simplistic 'Jupiter as shield' concept is a thing of the past, and future research in this area will require the continued use of the kinds of robust simulation strategies so effectively employed in Dr. Grazier's work,” said Astrobiology Editor-in-Chief Dr. Sherry Cady in a statement.

Grazier also found that Jupiter changed the orbits of small objects that visited the inner Solar System so that they passed through more slowly. This “potentially improv[es] the odds of accretion of cometary material by terrestrial planets,” Grazier notes. Objects moving more slowly relative to a planet will cause smaller explosions, so it is possible that Jupiter provides a double benefit, ensuring Earth got the water it needed while reducing the destruction of the impact.