Artist's conception of the distribution of cold planets throughout the Milky Way. The Kepler transit field field is represented by the cyan cone. The inset depicts an artistic representation of a planetary system within the galactic bulge. Credit: Osaka University
Many planets have been found in the Milky Way. However, they are located less than a few thousand miles from Earth. Our galaxy spans more than 100,000 light-years, which makes it difficult to study the distribution of planets in our galaxy. A research team has now found a way around this problem.
Researchers from Osaka University, NASA and other institutions used modeling and observations to calculate how planet-hosting probabilities change with distance from the galactic center. This study was published in the Astrophysical journal Letters.
These observations were made using gravitational microlensing. This is a phenomenon where objects like planets act as lenses and bend or magnify the light of distant stars. This effect can be used in order to detect cold planets such as Jupiter and Neptune across the Milky Way from the galactic disc to the central region of our galaxy.
Daisuke Suzuki, coauthor of the study, said that GRAvitational microlensing is currently the only way to examine the distribution of planets within the Milky Way. "But, until now, little has been known due to the difficulty in measuring distances to planets more than 10,000 light-years from the Sun."
Instead of trying to solve the problem, researchers looked at the distribution of a quantity which describes the relative motion between the lens and the distant light source during planetary microlensing. The research team was able to infer the galactic distribution by comparing the microlensing distribution with the predicted galactic model.
These results indicate that the distance from the galactic centre does not affect the distribution of planets. Cold planets that orbit far from their stars appear to be universally found in the Milky Way. This includes the galactic bulge which is in a different environment than the solar neighborhood and where planets are long unknown.
Lead author Naoki Koshimoto explains that stars in the bulge area are older and more closely to one another than stars in their solar neighborhood. Our discovery that planets can reside in both stellar environments could help us understand how they form and the history behind planet formation in the Milky Way.
Researchers believe that the next step is to combine these findings with measurements of microlens brightness or microlensparallax, two other important quantities related to planetary microlensing.
Further information: Naoki Kashimoto et., No Large Dependence on Planet Frequency at Galactocentric Distance. The Astrophysical Journal Letters (2021). Information for the Journal: Astrophysical Journal Letters Naoki Kashimoto et. al, No Large Dependence on Planet Frequency at Galactocentric Distance. (2021). DOI: 10.3847/2041-8213/ac17ec
