Free-floating planets from core accretion theory: microlensing predictions

TL;DR

This paper predicts microlensing event rates and time-scales for free-floating planets based on core accretion theory, finding significantly lower values than observed, suggesting a need for revised theories or alternative explanations.

Contribution

It provides the first detailed microlensing predictions for free-floating planets from core accretion theory, highlighting discrepancies with observations.

Findings

Event rate is about 0.0018 times that of stars.

Median microlensing event time-scale is approximately 0.1 days.

Predicted values are much lower than observed short-time events.

Abstract

We calculate the microlensing event rate and typical time-scales for the free-floating planet (FFP) population that is predicted by the core accretion theory of planet formation. The event rate is found to be ~$1.8\times 10^{-3}$ of that for the stellar population. While the stellar microlensing event time-scale peaks at around 20 days, the median time-scale for FFP events (~0.1 day) is much shorter. Our values for the event rate and the median time-scale are significantly smaller than those required to explain the \cite{Sum+11} result, by factors of ~13 and ~16, respectively. The inclusion of planets at wide separations does not change the results significantly. This discrepancy may be too significant for standard versions of both the core accretion theory and the gravitational instability model to explain satisfactorily. Therefore, either a modification to the planet formation theory is required, or other explanations to the excess of short-time-scale microlensing events are needed. Our predictions can be tested by ongoing microlensing experiment such as KMTNet, and by future satellite missions such as WFIRST and Euclid.