The Milky Way’s bulge is the ancient and crowded central hub of our Galaxy. It contains about one quarter of the total stellar mass of the Milky Way and has a very different stellar environment from the solar neighborhood, including stellar densities over 10 times higher and an older stellar population.
An artist’s conception of exoplanets in a dense stellar environment. Image credit: Michael Bachofner.
“Although more than 3,000 planetary systems have been discovered to date, most reside at distances over 1,000 parsecs (3,262 light-years) from the Sun,” said lead author Dr. Naoki Koshimoto and colleagues from NASA’s Goddard Space Flight Center, Osaka University, the University of Maryland, and Massey University.
“In this regard, gravitational microlensing is a unique technique because it is sensitive to planetary systems over a wide range of distances in our Galaxy, from the Galactic disk to the Galactic bulge.”
“Microlensing is currently the only technique capable of investigating the Galactic distribution of planets.”
“However, no study measuring the distribution has yet been reported, mainly owing to the difficulty in distance measurement.”
Using a combination of gravitational microlensing observations and modeling, the astronomers determined how the planet-hosting probability varies with the distance from the Galactic center.
“We considered the distribution of a quantity that describes the relative motion of the gravitational lens and distant light source in planetary microlensing,” they said.
“By comparing the distribution observed in microlensing events with that predicted by a Galactic model, we could infer the Galactic distribution of planets.”
Their results show that the dependence of the planet frequency on the Galactocentric distance is not large, and suggest that the Milky Way’s bulge does have planets.
“Stars in the bulge region are older and are located much closer to each other than stars in the solar neighborhood,” Dr. Koshimoto said.
“Our finding that planets reside in both these stellar environments could lead to an improved understanding of how planets form and the history of planet formation in the Milky Way.”
The study was published in the Astrophysical Journal Letters.
_____
Naoki Koshimoto et al. 2021. No Large Dependence of Planet Frequency on Galactocentric Distance. ApJL 918, L8; doi: 10.3847/2041-8213/ac17ec
