TLDR
The Milky Way may contain many more planets wandering without stars than planets orbiting far from stars. Most could be small, but the estimate remains uncertain because it relies on only a handful of brief signals.
Summary
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1 Study Aim
The authors aimed to measure the mass function of free-floating planets (planets without detectable host stars) using nine years of observations. They also included planets on extremely wide orbits, whose stars would not appear in the data. The study tested whether brief gravitational microlensing events (temporary brightening caused by a foreground object bending starlight) require a planetary population beyond known stars and brown dwarfs. The study asks whether short, faint lensing signals reveal a large population of small planets.
2 Study Design
The research analyzed 3,535 carefully selected single-lens events from the MOA-II survey toward the Galactic bulge, collected from 2006 through 2014. The authors modeled each event using its Einstein-radius crossing time (how long the lensing signal lasts) and, when available, its angular Einstein radius (the lensing scale seen in the sky). The sample included 10 events lasting under one day and 13 with finite-source effects (changes caused by the source star’s size). Image-level simulations measured detection efficiency across both measurements. The team then used a Galactic population model and Markov Chain Monte Carlo (MCMC, repeated random sampling) likelihood analysis to fit planetary and known-object populations. The researchers combined thousands of ordinary events with a small, carefully modeled set of very brief events.
3 Findings
The study identifies six likely planetary events lasting under 0.5 days. The shortest lasted 0.057 days and had an angular Einstein radius of 0.90 microarcseconds. The authors find a power-law mass function (a rule describing how object numbers change with mass) with slope 0.96, though uncertainty is large. It implies 21^{+23}_{-13} free-floating or very wide-orbit planets per star from 0.33 to 6,660 Earth masses, totaling 80^{+73}_{-47} Earth masses per star. These objects may outnumber wide-orbit bound planets by 19^{+23}_{-13} times, while holding similar total mass. The authors argue that ejection from planetary systems could explain this pattern. A broken power law gives consistent but less precise results. The study predicts Roman could detect about 988 such objects down to Mars mass. It recommends better measurements of low-magnification events around ordinary stars. The results point to a large population of small wandering planets, but future surveys must test how many truly exist.