Planets, Planets Everywhere (Not Just Where You’d Think)

Editor’s note: AAS Nova is on vacation until 22 September. Normal posting will resume at that time; in the meantime, we’ll be taking this opportunity to look at a few interesting AAS journal articles that have recently been in the news or drawn attention.

Most of the exoplanets we’ve discovered are located within about 3,300 light-years of Earth, leaving the distribution of planets across the rest of the Milky Way a mystery. To tackle this question, a team of astronomers led by Naoki Koshimoto (NASA Goddard Space Flight Center) analyzed observations of 28 planets discovered with gravitational microlensing — a technique that can detect planets at far greater distances than the transit or radial-velocity techniques. They compared the characteristics of the observed microlensing events against what would be expected if planets tend to be clustered near the galactic center, sequestered on the edges of the galactic disk, or distributed more evenly throughout the Milky Way.

Based on the observations, the team found that planet frequency is only weakly dependent upon distance from the galactic center. This result suggests that planets are likely to be found throughout the galaxy, though the results don’t fully rule out the possibility that planets could be rare near the galactic center — especially if the masses of the lensing objects tend to be small. As the number of planets discovered with gravitational microlensing grows, astronomers should gain a better understanding of how planets are distributed throughout the Milky Way.

Original article: “No Large Dependence of Planet Frequency on Galactocentric Distance,” N. Koshimoto et al 2021 ApJL 918 L8. doi:10.3847/2041-8213/ac17ec

Osaka University press release: Cold Planets Exist Throughout Our Galaxy, Even in the Galactic Bulge