Caption: Artist illustration of a Jupiter-like planet alone in the dark of space, floating freely without a parent star.
Credit: NASA/JPL-Caltech/R Hurt.

A unique aspect of the microlensing phenomenon is that it does not require the lens object to emit any light of its own. This means microlensing is sensitive to dark objects like black holes, neutron stars, and free-floating planets. Roman is expected to discover between 200 and 1,000 free-floating planets (FFPs). This estimate depends on the true mass function of FFPs which is currently unknown.

Several recent independent studies by RGES PIT members have attempted to estimate the expected yield of FFPs from Roman;

 
Johnson et al. 2020 conducted simulations of FFP microlensing events at Roman-like photometric sensitivity. Assuming that FFPs follow the fiducial mass function of cold, bound planets adapted from Cassan et al., we estimate that Roman will detect ∼250 FFPs with masses down to that of Mars (including ∼60 with masses < M⊕).

Caption: Two examples of simulated FFP events as observed by Roman. Black points are observations in the F146 filter, and the orange line is the input lensing model. Top: light curve due to a roughly Mars-mass FFP, with relatively mild finite-source effects. Bottom: light curve due to a ~0.6 M_earth FFP, in this case lensing a giant source, thereby exhibiting strong finite-source effects.
Credit: Johnson et al. 2020

 
Sumi et al. 2023 presented a new estimate of the mass function for FFPs or very wide orbit planets down to Earth-mass. The study analyzed data from the MOA-II ground-based microlensing survey between 2006 and 2014. Utlimately, the authors report the FFP occurrence rate may be between 17-21 FFPs per star in the galaxy. The study estimates that the Roman microlensing survey should detect of order 1,000 FFPs with masses down to that of Mars.

Caption: Initial Mass Function (IMF) of the best-fit power-law results from Sumi et al. 2024. The red line indicates the best fit for all populations. The blue dotted line and green dashed line show the IMFs for the stellar and brown dwarf (BD) population and for the planetary-mass population, respectively. The shaded areas indicate 1σ error. The gray dashed line and the shaded area indicate the best fit and 1σ range of the bound planet MF by Suzuki et al. (2016) via microlensing. The pink shaded area indicates 1σ uncertainty for the broken power-law FFP model.
Credit: Sumi et al. 2023