"Planets are too dim to be detected with existing equipment, far away, except in these very special circumstances where they're seen by their gravitational effect"

This quote by Murray Gell-Mann is referring to the difficulty of finding planets that are far away from us. He is saying that due to the dimness of worlds, it is tough to spot them with the existing technology available. He goes on to state that there are some special scenarios where planets can be seen due to their gravitational impact. This implies that the gravitational pull of the world can be found, even if the world itself can not be seen. This is since the gravitational pull of the world can impact the movement of other items in area, such as stars and other planets. This can be used to presume the existence of a planet, even if it can not be seen directly. This quote highlights the problem of finding worlds that are far away from us, and the value of understanding the gravitational impacts of planets in order to spot them.