Estimating the number of solar systems in the Milky Way requires astronomers to rely on complex mathematical models and observational data, as a direct, star-by-star count is impossible from our position. The answer is a vast range, derived by calculating the total number of stars and then applying the probability that a star hosts a planetary system.
The Foundational Count of Stars
Astronomers estimate the total number of stars in the Milky Way to be between 100 billion and 400 billion. This figure is calculated by measuring the galaxy’s total mass, based on the gravitational influence it exerts on orbiting matter. Observing the rotational speed of visible matter allows scientists to determine the overall gravitational pull and total mass of the galaxy.
This total mass includes stars, gas, dust, and dark matter. The challenge is isolating the mass contribution of the stars from the total galactic mass.
Once the total stellar mass is estimated, it is divided by the average mass of a single star, which introduces uncertainty. Low-mass red dwarf stars are far more numerous than stars like our Sun, making up the vast majority of the stellar population. Since these smaller stars contribute less mass individually, the assumed average stellar mass drastically alters the final count, explaining the wide range.
Converting Stars to Planetary Systems
A solar system is a star orbited by at least one planet. Data from exoplanet-hunting missions, such as the Kepler Space Telescope, show that planet formation is a common byproduct of star formation.
Statistical analysis of Kepler’s findings, which used the transit method to detect planets, suggests that the vast majority of stars possess at least one planet. The scientific consensus points toward an occurrence rate where 80% or higher of stars host a planetary system.
Applying this high occurrence rate to the stellar count provides the final estimate for solar systems. If the Milky Way holds 100 billion to 400 billion stars, and 80% host planets, the resulting number of planetary systems falls between 80 billion and 320 billion. The ubiquity of planets means the number of solar systems is nearly the same as the number of stars.
Why the Number Remains an Estimate
The wide range in the final count persists because astronomers face several fundamental observational and methodological challenges. One major difficulty is the extensive amount of interstellar dust and gas that obscures our view of stars, particularly those toward the galactic center. This means that large populations of distant stars are hidden from direct observation, making any complete census impossible.
Another significant source of imprecision stems from the fact that the total mass of the galaxy is overwhelmingly dominated by dark matter. Dark matter does not emit or absorb light and is only detected through its gravitational effect. Although dark matter accounts for an estimated 90% of the total galactic mass, its mass must be subtracted from the total to isolate the stellar mass, introducing a substantial margin of error.
Furthermore, the most common stars, the dim red dwarfs, are a challenge to count precisely because they are inherently faint. Accurately determining the number of these low-mass stars is particularly difficult at great distances. The uncertainty in the number of these prevalent stars is the largest factor driving the wide margin of error in the total stellar count, and consequently, the estimated number of solar systems.