The Kepler telescope has discovered evidence suggesting there are free-floating planets that are alone in deep space, unbound to any host star. The data shows four new discoveries consistent with planets having similar masses to Earth that are free-floating in deep space. The study was led by Iain McDonald from the University of Manchester and used data gathered in 2016 during the K2 phase of the NASA Kepler Space Telescope.
The K2 mission phase lasted for two months. During that time, Kepler monitored a crowded field of millions of stars near the center of the Milky Way galaxy every 30 minutes to discover rare gravitational microlensing events. The team found 27 short-duration candidate microlensing signals that varied on timescales between an hour and ten days.
Many of those events had been previously seen in data obtained on the ground. However, four of the shortest events are discoveries consistent with planets having a mass similar to Earth. The new signals didn’t show an accompanying longer signal expected from a host star suggesting they were free-floating planets. Astronomers believe free-floating planets likely originally formed around a host star before being ejected by the gravitational tug of heavier planets in the system.
Microlensing describes how light from a background star can be temporarily magnified by the presence of other stars in the foreground. The microlensing event produces a short burst in brightness that can last from a few hours to a few days. Scientists have found that approximately one of every million stars in the galaxy is visibly affected by microlensing at any given time. However, only a few percent of the events are believed to be caused by planets.
Interestingly, Kepler wasn’t designed to find planets using microlensing or to study dense star fields inside the inner galaxy. Scientists had to design data reduction techniques to look for signals within Kepler’s data set. McDonald says the signals are extremely difficult to find, but the team could extract characteristic brightenings caused by planets. Discovering additional free-floating planets will be a focus for the NASA Nancy Grace Roman Space Telescope, which is optimized to look for microlensing signals.