Constraining the Frequency of Free-Floating Planets From a Synthesis of Microlensing, Radial Velocity, and Direct Imaging Survey Results

Abstract

A microlensing survey by Sumi et al. (2011) exhibits an overabundance of short-timescale events (t E 2 days) relative to that expected from known stellar populations and a smooth power-law extrapolation down to the brown dwarf regime. This excess has been interpreted as a population of approximately Jupiter-mass objects that outnumber main-sequence stars by nearly twofold; however the microlensing data alone cannot distinguish between events due to wide-separation (a 10 AU) and free-floating planets. Assuming these short-timescale events are indeed due to planetary-mass objects, we aim to constrain the fraction of these events that can be explained by bound but wideseparation planets. We fit the observed timescale distribution with a lens mass function comprised of brown dwarfs, main-sequence stars, white dwarfs, neutron stars, and black holes, finding and thus corroborating the initial identification of an excess of short-timescale events. Including a population of bound planets with distributions of masses and separations that are consistent with the results from representative microlensing, radial velocity, and direct imaging surveys, we then determine what fraction of these bound planets are expected not to show signatures of the primary lens (host) star in their microlensing light curves, and thus what fraction of the short-timescale event excess can be explained by bound planets alone. We find that, given our model for the distribution of planet parameters, bound planets alone cannot explain the entire excess without violating the constraints from the surveys we consider, and thus some fraction of these events must be due to free-floating planets, if our model for bound planets holds. We estimate a median fraction of short-timescale events due to free-floating planets to be f = 0.67 (0.23-0.85 at 95% confidence) when assuming "hot-start" planet evolutionary models and f = 0.58 (0.14-0.83 at 95% confidence) for "cold-start" models. Assuming a delta-function distribution of free-floating planets of mass m p = 2 M Jup yields a number of free-floating planets per main sequence star of N = 1.4 (0.48-1.8 at 95% confidence) in the "hot-start" case and N = 1.2 (0.29-1.8 at 95% confidence) in the "cold-start" case.

A Neptune-mass Free-floating Planet Candidate Discovered by Microlensing Surveys

“…However, the statistical significance of Sumi et al's results is largely based on three shortest-timescale events (t E < 1 days). As we mentioned above, one measurement is incorrect and the model by Clanton & Gaudi (2017) shows that the remaining two are statistically consistent with being wide-orbit planets. That model still cannot account for a small overabundance of events with timescales between 1-2 days (see Figures 4 and 5 from Clanton & Gaudi 2017), but the statistical significance of the remaining excess relative to the short-timescale events expected from stars and brown dwarfs is small.…”

“…They found an excess of nine short 2 events, relative to what was expected from brown dwarfs and stars, and they attributed this excess to a large population of Jupiter-mass planets, which should be nearly twice as common as mainsequence stars. Clanton & Gaudi (2017) modeled the microlensing signal expected from exoplanets on wide orbits using constraints from microlensing, radial velocity, and direct imaging surveys and concluded that at most ∼ 40% of short-timescale events detected by Sumi et al (2011) can be interpreted as due to wide-orbit planets. However, the statistical significance of Sumi et al's results is largely based on three shortest-timescale events (t E < 1 days).…”

Exobodies in Our Back Yard: Science from Missions to Nearby Interstellar Objects

“…In addition, stars can eject dust and gas and planetary debris after their time on the main sequence, and it is even possible that small bodies form directly in interstellar space. Although spacecraft, radar and visual detection arrays definitely observe streams of dust-sized interstellar micrometeorites Strub et al [2015], Baggaley [2000], and although there appears to be a population of nomadic interstellar planets Sumi et al [2011], Strigari et al [2012], Clanton and Gaudi [2017], until recently there was no firm evidence for even a single galactic asteroid or comet coming into the solar system from interstellar space Engelhardt et al [2017].…”

Spitzer Microlensing Parallax Reveals Two Isolated Stars in the Galactic Bulge

“…At the low-mass end, microlensing has detected several free-floating planet candidates based on their short microlens timescale, t E 2 days (Sumi et al 2011;Mróz et al 2017Mróz et al , 2018Mróz et al , 2019, including a few possible Earth-mass objects. Such discoveries are crucial for testing theories about the origin and evolution of free-floating planets (Veras & Raymond 2012;Pfyffer et al 2015;Ma et al 2016; Barclay et al 2017;Clanton & Gaudi 2017). For more massive objects (i.e., isolated brown dwarfs), five have been discovered by microlensing: OGLE-2007-BLG-224L (Gould et al 2009, OGLE-2015-BLG-1268L (Zhu et al 2016), OGLE-2015-BLG-1482 58 (Chung et al 2017), OGLE-2017-BLG-0896 (Shvartzvald et al 2019), and OGLE-2017-BLG-1186 59 (Li et al 2019).…”