From the June 2012 issue

Is it possible that the gravitational effects attributed to dark matter could be caused by Jupiter-sized planets discovered orbiting other stars and also floating freely outside planetary systems?

Martin J. Grumet, Boise, Idaho
By | Published: June 25, 2012
Scientists think billions of free-floating planets drift through space. These worlds, however, aren’t responsible for the universe’s mysterious missing mass. Credit: NASA/JPL-Caltech
The current number of detected Jupiter-sized planets either gravitationally bound to a central star or free-floating is large, and the count is increasing. The gravitational effects of these worlds on other planets could be substantial, but for the most part such dynamic interactions are confined to their nearby environs.
Ordinary, or baryonic, matter — like the material from which these planets form — interacts with photons to produce scattering, emission, or absorption against luminous sources. Dark matter, which is composed of nonbaryonic subatomic particles, does not interact with light in the same fashion.
A large number of detected and undetected Jupiter-sized planets could not mimic the gravitational effects produced by dark matter for two reasons. First, planets, although faint, are made of luminous matter and therefore are ultimately detectable. Second, their estimated numbers and locations, primarily around the galactic plane, do not support the large-scale gravitational effects similar to those of dark matter (like the ones explaining galactic rotation or the large-scale bending of light, called gravitational lensing).
Given the proper geometric alignment, however, Jupiter-sized planets could produce localized and temporary signal increases from background sources, a phenomenon known as the microlensing effect. This is precisely the mechanism by which we first detected free-floating planets. — Mario Perez, NASA Headquarters, Washington, D.C.