Jupiter as a Menace
Scientists believe that Jupiter's gravitational field is pushing comets and asteroids which are approaching the Earth away from the inner planets, diverting them to the outer planets and thus averting possible catastrophe. Basically, three types of objects, namely, Main belt asteroids, Kuiper belt comets and Oort cloud comets pose a threat to both Jupiter and the Earth. Of these, the Main belt asteroids follow an orbit which is between that of Mars and Jupiter. Kuiper belt comets are found to follow a path which is close to the orbit of the Neptune. Long term comets are known as Oort cloud comets.
The majority of asteroids which hit the Earth are Main belt asteroids. Scientists believe that during the evolutionary stages of the solar system there was a possibility for another planet near Jupiter. But its massive gravitational field prevented the primordial matter near to it from forming another planet . The gravitational field perturbed it in such a way that it started to follow an orbit of eccentricity .33, that is, it moved in to a highly elliptical orbit. As a result of this, many of the primordial material in that orbit collided with the Sun and other solar bodies and thus formed more and more fragments in the early solar system. These fragments are collectively known as the Main belt asteroids. Thus in the absence of Jupiter, most probably, the Main belt asteroids might have joined to form a fifth planet.
A similar situation exists in the case of Kuiper belt objects. Even though the possibility of the Earth being hit by a Kuiper belt objects is minimal, a single one of them wouldn’t have even reached the Earth in the absence of Jupiter. This is because when a Kuiper object falls out of the belt, Jupiter's gravity redirects its trajectory towards the inner solar system.