For a long time humanity has wondered if we are alone in the Universe, and finding the answer is difficult, considering we cannot just fly to another planet to check. For now one of the best things we can do is try to understand why life exists here, and then look for places with similar conditions. Among this special conditions may be the presence of an asteroid belt, provided it is in the correct position.
Asteroids likely influenced the evolution of life on Earth in two ways. Asteroids, when they impact the Earth, carry materials with them, possibly including water and some organic materials. Also the impact itself may so disrupt the environment that species have to adapt and evolve, or go extinct. Now researchers, armed with the Spitzer Space Telescope, have looked into how common this could be, and it likely is not. The asteroid belt only exists because of Jupiter, which has so much gravity the mass in the belt could not coalesce into a large object. The asteroid belt also has to be at or past the snow line, because if it were not then water and other materials could be boiled off, so the Jupiter-equivalent in other systems will also have to be past the snow line.
The researchers crunched the numbers for how many systems have a large planet outside of the snow line and found that less than 4% of observed systems do. Assuming their theory is correct, that drastically narrows the field of what planets may support life.