Jupiter also played a crucial role in purging the inner solar system of bodies left over from planet formation. Jupiter is 318 times more massive than Earth, and it exerts enormous gravitational influence. Its gravitational interactions very efficiently scatter bodies that approach it, and it has largely cleaned out stray bodies from a large volume of the solar system. In the early solar system, there were tremendous numbers of small bodies that had escaped incorporation into planets, but over half a billion years, most of the larger ones inside the orbit of Saturn disappeared. They were accreted by planets, ejected out of the solar system, or incorporated into the Oort cloud of comets. Jupiter was the major cause of this purging of the middle region of the solar system.
The objects that still impact Earth today are planetesimals that managed to survive in three special ecological niches: the Oort comet cloud beyond Pluto, the Kuiper belt of comets just beyond the outer planets, and the asteroid belt, that special refuge located between Mars and Jupiter. The current impact rate averages one 10-kilometer body every 100 million years. The impact of just such a body occurred 65 million years ago, the time of the K/T extinction that ended the age of the dinosaurs. George Wetherill of the Carnegie Institute of Washington has estimated that the flux of these 10-kilometer bodies hitting Earth might be 10,000 times higher if Jupiter had not come into being and purged many of the leftover bodies of the middle region of the solar system. If Earth had been subject to collisions with extinction-causing projectiles every 10,000 years instead of every 100 million years, and fairly frequently with even larger bodies, it seems unlikely that animal life would have survived.
Do most planetary systems harbor planets like Jupiter? Ours has two (Jupiter and Saturn), and the detection of Jupiter-mass planets around other stars suggests that Jupiters exist in other planetary systems, but their frequency is still unknown. It is likely that many planetary systems do not have Jovian planets. The standard formation model of Jupiter has it accreting a large, solid core before it can begin accreting gas, its main constituent. The necessary conditions (available mass and rapid accretion before gas is lost from the planetary system) may not be common, and so Jupiter-mass planets may be rare in other planetary systems. When planetary systems lack a Jovian planet to guard the outer boundary of the terrestrial planet region, the inner planets may not be capable of supporting more than microbial life.
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