Using data from NASA’s Kepler/K2 mission, astronomers have detected four new microlensing events that are consistent with free-floating planets of similar masses to Earth.
Gravitational microlensing is an observational effect that was predicted in 1936 by Albert Einstein using his general theory of relativity.
When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the gravitational attraction of the foreground star.
This star is then a virtual magnifying glass, amplifying the brightness of the background source star, so astronomers refer to the foreground star as the lens star.
If the lens star harbors a planetary system, then those planets can also act as lenses, each one producing a short deviation in the brightness of the source.
Roughly one out of every million stars in our Milky Way Galaxy is visibly affected by microlensing at any given time, but only a few percent of these are expected to be caused by extrasolar planets.
“These signals are extremely difficult to find,” said Dr. Iain McDonald, an astronomer in the Jodrell Bank Centre for Astrophysics at the University of Manchester and Open University.
“Our observations pointed an elderly, ailing telescope with blurred vision at one the most densely crowded parts of the sky, where there are already thousands of bright stars that vary in brightness, and thousands of asteroids that skim across our field.”
“From that cacophony, we try to extract tiny, characteristic brightenings caused by planets, and we only have one chance to see a signal before it’s gone.”
“It’s about as easy as looking for the single blink of a firefly in the middle of a motorway, using only a handheld phone.”
Dr. McDonald and colleagues analyzed archival data obtained in 2016 during the K2 mission phase of NASA’s Kepler Space Telescope.
They found 27 short-duration candidate microlensing signals that varied over timescales of between an hour and 10 days.
Many of these had been previously seen in data obtained simultaneously from the ground.
However, the four shortest events are new discoveries that are consistent with planets of similar masses to Earth.
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.
“Kepler has achieved what it was never designed to do, in providing further tentative evidence for the existence of a population of Earth-mass, free-floating planets,” said Dr. Eamonn Kerins, an astronomer in the Jodrell Bank Centre for Astrophysics at the University of Manchester.
“Now it passes the baton on to other missions that will be designed to find such signals, signals so elusive that Einstein himself thought that they were unlikely ever to be observed.”
The results appear in the Monthly Notices of the Royal Astronomical Society.
I. McDonald et al. 2021. Kepler K2 Campaign 9 – I. Candidate short-duration events from the first space-based survey for planetary microlensing. MNRAS 505 (4): 5584-5602; doi: 10.1093/mnras/stab1377