Prime planet-hunting mission of the NASA's prolific Kepler space telescope has come to an end. Engineers have not been able to find a fix for its hobbled pointing system, leaving no choice than to announce its end. In May, NASA's observatory lost the second of its four reaction wheels, due to which, it could no long hold completely steady as it looks towards the stars.

Despite working through a number of possible solutions, engineers have failed to find one solution that could work. As of now, 135 planets beyond our solar system have been confirmed by Kepler.

However, there are more than 3,500 candidates in its database that are not yet fully investigated. The vast majority of these are likely to be confirmed as planets in due course.

Bill Borucki is the Kepler mission principal investigator. He said, "We really expect the most exciting discoveries are going to come in the next few years as we search through all this data".

The $600m observatory was launched in 2009 to find earth-sized worlds orbiting their host stars.

The spacecraft was launched with four wheels. It experienced its first failure in July 2012 when its hardware set failed. Then, it happened this year in May when it lost its second wheel.