Questioning Jupiter's role as protector of life on Earth

[Junghalli]

An interesting collection of papers I found questioning the idea that Jupiter is a vital shield for life on Earth against comet impacts. I wouldn't be surprised if it's made the rounds here already but I'd be interested in hearing what people here have to say about it.

The estimate of Earth experiencing a 1000 times greater rate of bombardment without Jupiter was based on a simulation of a population of bodies initially occupying eccentric, low-inclination orbits with semi-major axes in the 5-75 AU range. These papers describe simulations that attempt to more realistically simulate the major impactor sources in the solar system. I will summarize their results.

Jupiter – friend or foe? I: the asteroids:
The Earth experienced ~3.5 times more impacts in the simulation with Jupiter at its actual mass as in the one with Jupiter at .01 times its actual mass (~3.17 Earth masses). The worst bombardment occurred in the simulation with Jupiter at .15 times its actual mass, in which Earth recieved around twice as many impacts as in the simulation with Jupiter at its actual mass. This is not taking into account that at low masses for Jupiter the asteroid belt would probably have conglomerated into a planet, leaving comets as the only major potential impactors for Earth.

Jupiter – friend or foe? II: the Centaurs:
The impact rate from Centaur comets pulled into Earth-crossing orbits was only slightly higher in the simulation with no Jupiter at all than in the one with Jupiter at its actual mass. The worst bombardment occured in the simulation with Jupiter at .2 times its actual mass, in which Earth recieved ~4.5 times as many impacts as it did in the simulation with Jupiter at its actual mass.

Jupiter – friend or foe? III : the Oort cloud comets:
For this simulation it was impractical to track impacts due to the enormous timescales of movement for the simulated comets, so the number of surviving simulated comets (not ejected, collided with one of the planets etc.) was measured instead. The greatest number of surviving simulated comets at the end of the simulation was found in the simulation with no Jupiter. ~2.5 times more comets survived at the end of that simulation than at the end of the simulation in which Jupiter was simulated at its actual mass.

If accurate these are some very exciting findings because they mean solar systems without a planet comparable to Jupiter are still good potential habitats for complex life.

Thoughts?

[madd0ct0r]

that's interesting, but it seems a shame they didn't try the small Jupiter and extra planet from asteroids.

Could two small planets be more effective then one huge one?

[SilverHawk]

It would seem the actual line of thought to take would be that variable planet formations has a larger effect on the impact rate of rockey planets then a gas giant that orbits around the outside rim of the habitable zone of the star in question.