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 In a new blind survey of gravitational microlensing, an international team of astronomers has detected likely evidence for four Earth-sized planets wandering freely through interstellar space. Using observations from the aging Kepler Space Telescope, researchers led by Iain McDonald at the University of Manchester picked out key signs of microlensing by the planets in a crowded and noisy field of stars. Their success in the face of challenging circumstances clearly demonstrates the feasibility of blind, space-based microlensing surveys in future missions.

In some star systems, astronomers predict that the strong gravitational tug of large planets could have thrown their smaller planetary neighbours out into interstellar space. Without any host star, these roughly Earth-sized “free-floating planets” (FFPs) would be virtually impossible to detect using conventional exoplanet searching techniques – but should be detectable through the effect of gravitational microlensing.

First predicted by Einstein as part of his theory of general relativity, this effect occurs when a massive object passes in front of a more distant star in our line of sight. As the object’s gravitational field bends more of the star’s light towards us, it causes a brief burst in its observed brightness. In the case of Earth-sized FFPs, these bursts would be extremely faint, and last for little more than an hour.