Tidal comets
Comets are known to reside in a vast spherical halo called the Oort cloud, extending for about a light year beyond the orbit of Pluto. The forces responsible for their coming towards us are still not known. Now, John Matese and Daniel Whitmire of the University of Southwestern Louisiana, US have come up with a theory that might provide the answer. They analysed comet orbit data and concluded that the tidal forces are responsible for the comets' movement towards us. Though most astronomers agree with the theory, they feel that there is not enough data for any firm conclusion.