In my upcoming novel “Andromeda: The Encounter,” an Earth-sized planet is wandering lonely through space. Planets far from any star – how common are they? Apparently, this is not an uncommon phenomenon. Star systems can become dynamically unstable and eject single planets. This could have happened to our solar system in early times. That it is a normal sight is also shown by a research work of British scientists. They have discovered evidence of a mysterious population of such free-floating planets. The findings were published in the Monthly Notices of the Royal Astronomical Society.

The study, led by Iain McDonald of the University of Manchester, used data obtained in 2016 during the K2 mission phase of NASA’s Kepler Space Telescope. During that two-month campaign, Kepler monitored a dense field of millions of stars near the center of our galaxy every 30 minutes to find rare gravitational microlensing events.

The study team found 27 short-duration candidate microlensing signals that varied on time scales from one hour to 10 days. Many of them had been seen previously in data obtained simultaneously from the ground. However, the four shortest events are new detections that match planets with masses similar to Earth. These new events show no accompanying longer signal that would be expected from a host star, suggesting that these new measurements may be of free-floating planets. Such planets may have originally formed around a host star before being ejected by the gravitational pull of other, heavier planets in the system.

Predicted by Albert Einstein 85 years ago as a consequence of his theory of general relativity, microlensing describes how the light from a background star can be temporarily magnified by the presence of other stars in the foreground. This produces a brief burst of brightness that can last from hours to a few days. About one in a million stars in our galaxy is visibly affected by microlensing at any given time, but only a few percent of these are thought to be caused by planets.

“These signals are extremely difficult to find. Our observations pointed an older telescope with blurry vision at one of the most densely populated parts of the sky, where there are already thousands of bright stars varying in brightness and thousands of asteroids gliding across our field. From this cacophony, we try to extract tiny, characteristic brightenings caused by planets, and we have only one chance to see a signal before it is gone. It’s about as easy as using only a cell phone to look for the single blink of a firefly in the middle of the highway,” says study leader McDonald, describing the particular challenges.