No large population of unbound or wide-orbit Jupiter-mass planets

TL;DR

This study analyzed a large sample of microlensing events and found no evidence for a large population of free-floating or wide-orbit Jupiter-mass planets, challenging previous claims and supporting planet formation theories.

Contribution

It provides the first stringent upper limit on the frequency of Jupiter-mass free-floating planets using microlensing data from 2010-2015.

Findings

No excess of 1-2 day microlensing events was observed.

Upper limit of 0.25 Jupiter-mass free-floating planets per star.

Possible detection of Earth-mass free-floating planets.

Abstract

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]