Since the outer planets could form larger cores faster, they swept up most of the hydrogen and helium as well, growing to huge sizes compared to the inner planets. The solar system also contains billions or trillions of comets. Most of them reside in the Oort cloud, far beyond the orbit of Pluto, near the perimeter of the original gas cloud that formed the sun. It is not clear yet how they got there, but the current speculation is that they were scattered out to such large distances by interacting with the planets. This is common whenever there are many objects orbiting a central star many of the objects get flung out into distant, highly elongated orbits.
Comets consist mostly of dirty ice.
Meteorites Meteorites are usually rock, but once in a while, meteorites are iron, nickel and other metals, and occasionally they are carbonaceous chondrites. We can tell that Mercury and the moon do not have plate tectonics because their surfaces are heavily cratered. Earlier in their histories they had fluid cores, which spewed magma when the surface was punctured by meteors. Human Exploration in space Recall our class discussion of human travels into space and to the moon, including the details of the rockets, milestones of the early space exploration and sequence of events required to carry out the most ambitious missions.