Gravitational microlensing is the only method capable of exploring the entire
population of free-floating planets down to Mars-mass objects, because the
microlensing signal does not depend on the brightness of the lensing object. A
characteristic timescale of microlensing events depends on the mass of the
lens: the less massive the lens, the shorter the microlensing event. A previous
analysis of 474 microlensing events found an excess of very short events (1-2
days) - more than known stellar populations would suggest - indicating the
existence of a large population of unbound or wide-orbit Jupiter-mass planets
(reported to be almost twice as common as main-sequence stars). These results,
however, do not match predictions of planet formation theories and are in
conflict with surveys of young clusters. Here we report the analysis of a six
times larger sample of microlensing events discovered during the years
2010-2015. Although our survey has very high sensitivity (detection efficiency)
to short-timescale (1--2 days) microlensing events, we found no excess of
events with timescales in this range, with a 95% upper limit on the frequency
of Jupiter-mass free-floating or wide-orbit planets of 0.25 planet per
main-sequence star. We detected a few possible ultrashort-timescale events
(with timescales of less than 0.5 day), which may indicate the existence of
Earth- and super-Earth-mass free-floating planets, as predicted by
planet-formation theories. [abridged]Comment: published in Nature, authors' version (see nature.com for the
published version
