Oort cloud

An Oort cloud is a spherical cloud around a star or other celestial body, composed of tiny ice particles, dust, and other debris, situated where the star's influences are counterbalanced by the aggregate of forces external to the star.

In 2370, to detect Maquis ships coming into the system to launch a possible attack on the Cardassian colony on Bryma, Chief O'Brien hid a dozen sensor probes in the Oort cloud of the star system, where the Maquis wouldn't be able to detect them.