Iain notes: "These signals are extremely difficult to find.
This meant that new data reduction techniques had to be developed to look for signals within the Kepler dataset. Kepler was not designed to find planets using microlensing, nor to study the extremely dense star fields of the inner Galaxy. Roughly one out of every million stars in our Galaxy is visibly affected by microlensing at any given time, but only a few percent of these are expected to be caused by planets. This produces a short burst in brightness that can last from hours to a few days. Predicted by Albert Einstein 85 years ago as a consequence of his General Theory of Relativity, microlensing describes how the light from a background star can be temporarily magnified by the presence of other stars in the foreground. Such planets may perhaps have originally formed around a host star before being ejected by the gravitational tug of other, heavier planets in the system. These new events do not show an accompanying longer signal that might be expected from a host star, suggesting that these new events may be free-floating planets. However, the four shortest events are new discoveries that are consistent with planets of similar masses to Earth. Many of these had been previously seen in data obtained simultaneously from the ground. The study team found 27 short-duration candidate microlensing signals that varied over timescales of between an hour and 10 days. During this two-month campaign, Kepler monitored a crowded field of millions of stars near the centre of our Galaxy every 30 minutes in order to find rare gravitational microlensing events. The study, led by Iain McDonald of the University of Manchester, UK, (now based at the Open University, UK) used data obtained in 2016 during the K2 mission phase of NASA's Kepler Space Telescope.
The results include four new discoveries that are consistent with planets of similar masses to Earth, published today in Monthly Notices of the Royal Astronomical Society. () Tantalising evidence has been uncovered for a mysterious population of "free-floating" planets, planets that may be alone in deep space, unbound to any host star.
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