Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. II. Free-floating Planet Detection Rates*

Abstract

The Nancy Grace Roman Space Telescope (Roman) will perform a Galactic Exoplanet Survey (RGES) to discover bound exoplanets with semimajor axes greater than 1 au using gravitational microlensing. Roman will even be sensitive to planetary-mass objects that are not gravitationally bound to any host star. Such free-floating planetarymass objects (FFPs) will be detected as isolated microlensing events with timescales shorter than a few days. A measurement of the abundance and mass function of FFPs is a powerful diagnostic of the formation and evolution of planetary systems, as well as the physics of the formation of isolated objects via direct collapse. We show that Roman will be sensitive to FFP lenses that have masses from that of Mars (0.1 M ⊕) to gas giants (M100 M ⊕) as isolated lensing events with timescales from a few hours to several tens of days, respectively. We investigate the impact of the detection criteria on the survey, especially in the presence of finite-source effects for low-mass lenses. The number of detections will depend on the abundance of such FFPs as a function of mass, which is at present poorly constrained. 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 ⊕). We also predict that Roman will improve the upper limits on FFP populations by at least an order of magnitude compared to currently existing constraints. Unified Astronomy Thesaurus concepts: Gravitational microlensing (672); Free floating planets (549); Space telescopes (1547); Exoplanet detection methods (489)

KMT-2017-BLG-2820 and the Nature of the Free-floating Planet Population

“…The launch of the Nancy Grace Roman (formerly known as WFIRST) satellite will provide another path to FFPs. Johnson et al (2020) estimated that FFP population models consistent with the Mróz et al (2017) short-t E events would lead to several hundred Roman detections (see their Figure 7). For the events among these that are generated by wide-separation planets (as opposed to genuine FFPs), a substantial fraction of the hosts will be directly detected as blended flux.…”

OGLE-2018-BLG-1428Lb: a Jupiter-mass planet beyond the snow line of a dwarf star

“…Also, the Nancy Grace Roman (Roman, formerly WFIRST) satellite will be launched in near future (Spergel et al 2015). With this satellite, it is expected to detect ∼1400 bound exoplanets (Penny et al 2019) and ∼250 free-floating planets (Johnson et al 2020). Hence, the masses of over 1000 planetary systems can be measured from the Roman together with ground-based observations, such as the Korea Microlensing Telescope Network (KMTNet; Kim et al 2016).…”

A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event

“…This issue may become more important for the Roman telescope, which is being designed to carry out observations in W146 filter with a 15 minute cadence and in Z087 filter with a 12 hr cadence. Johnson et al (2020) estimate that only approximately 10% of short-timescale events due to Å M 1 lenses would have a color measurement. We thus advocate that the frequency of Z087 filter observations should be increased.…”

“…Thus, we do not find any significant evidence for the host star up to the projected distance of 8.0 au from the planet (assuming p = 0.1 rel mas). The properties of OGLE-2016-BLG-1928 place it at the edge of current limits of detecting short-timescale microlensing events and highlight the challenges that will be faced by future surveys for extremely short-timescale events (for example, by the Nancy Grace Roman Space Telescope, formerly known as WFIRST, Johnson et al 2020). Despite the fact that the event was located in high-cadence survey fields, only 15 data points were magnified (11 from OGLE and 4 from KMTNet), rendering the event difficult to detect.…”