# MOA-bin-29b : A Microlensing Gas Giant Planet Orbiting a Low-mass Host Star

## TL;DR

This paper reports the discovery of a gas giant planet orbiting a low-mass host star in the Galactic bulge through microlensing, with detailed analysis of the system's physical properties and degeneracies.

## Contribution

First detection of a gas giant orbiting a low-mass host star via microlensing, with Bayesian analysis estimating the system's physical parameters.

## Key findings

- Gas giant mass: approximately 0.63 Jupiter masses
- Host star likely a brown dwarf or late M-dwarf
- System located about 6.89 kpc in the Galactic bulge

## Abstract

We report the discovery of a gas giant planet orbiting a low-mass host star in the microlensing event MOA-bin-29 that occurred in 2006. We find five degenerate solutions with the planet/host-star mass ratio of $q \sim 10^{-2}$. The Einstein radius crossing time of all models are relatively short ($\sim 4-7$ days), which indicates that the mass of host star is likely low. The measured lens-source proper motion is $5-9$ ${\rm mas}\ {\rm yr}^{-1}$ depending on the models. Since only finite source effects are detected, we conduct a Bayesian analysis in order to obtain the posterior probability distribution of the lens physical properties. As a result, we find the lens system is likely to be a gas giant orbiting a brown dwarf or a very late M-dwarf in the Galactic bulge. The probability distributions of the physical parameters for the five degenerate models are consistent within the range of error. By combining these probability distributions, we conclude that the lens system is a gas giant with a mass of $M_{\rm p} = 0.63^{+1.13}_{-0.39}\ M_{\rm Jup}$ orbiting a brown dwarf with a mass of $M_{\rm h} = 0.06^{+0.11}_{-0.04}\ M_\odot$ at a projected star-planet separation of $r_\perp = 0.53^{+0.89}_{-0.18}\ {\rm au}$. The lens distance is $D_{\rm L} = 6.89^{+1.19}_{-1.19}\ {\rm kpc}$, i.e., likely within the Galactic bulge.