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 counter-balanced by the aggregate of forces external to the star.

In 2370, to detect Maquis ships coming into the Bryma 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.