The question of how many solar systems exist in the universe is one of immense scale. The answer is not a single, fixed number but a vast estimate derived from astronomical observations and mathematical models. Scientists approach this cosmic census by first determining the total number of stars and then calculating the likelihood that each star hosts orbiting planets. Because our own planetary arrangement is unique, the term “solar system” is often used interchangeably with the more general “star system” or “exoplanet system.” These numbers are always a projection, but they paint a compelling picture of a universe teeming with planetary bodies.
What Constitutes a Solar System
The term “Solar System” technically refers only to our home system, defined by the Sun (Sol). Scientifically, any star orbited by planets is referred to more broadly as a planetary system or a star system. These systems share a fundamental composition, beginning with at least one primary star that acts as the gravitational anchor.
The necessary components are the central star and a collection of gravitationally bound orbiting bodies. These orbiting objects can include planets, dwarf planets, moons, asteroids, and comets, all formed from the same protoplanetary disk of gas and dust. While our own system is anchored by a single star, many star systems are known to have two or even three stars, fundamentally changing the dynamics of the planets’ orbits.
Estimating Star Systems in the Milky Way
The initial step involves estimating the number of stars within the Milky Way. Current scientific consensus suggests the Milky Way contains between 100 billion and 400 billion stars. Astronomers arrive at this range by measuring the galaxy’s total mass and modeling the distribution of different star types, since smaller, dimmer stars are harder to detect than larger, brighter ones.
Once the total star count is estimated, the next step is determining the fraction of those stars that host planets. Data from decades of exoplanet surveys, particularly from missions like the Kepler Space Telescope, indicates that having a planetary system is the rule rather than the exception. These surveys showed that planets are extremely common, suggesting that nearly every star in the galaxy is likely to have at least one orbiting planet.
Based on this high prevalence, scientists estimate that the number of star systems in the Milky Way is roughly equal to the number of stars. This means our galaxy alone is home to between 100 billion and 400 billion star systems containing planets. This figure provides the necessary foundation for scaling the estimate to the entire observable universe.
Scaling Up to the Observable Universe
The number of star systems in the Milky Way is the starting point for calculating the universal total. To scale this figure up, astronomers must determine the number of galaxies in the observable universe. Early extrapolations based on deep-field images from the Hubble Space Telescope suggested a count of around 100 to 200 billion galaxies.
More recent analyses and mathematical models, which account for galaxies that are too small, faint, or distant to be directly observed, have dramatically increased this estimate. The current working figure for the number of galaxies in the observable universe is approximately two trillion, or 2 x 10^12 galaxies. This vast number is derived by taking a small, representative patch of sky and multiplying the galaxy density found there by the entire celestial sphere.
Multiplying the number of galaxies by the average number of star systems per galaxy provides a staggering final estimate. If we take the conservative low-end estimate of 100 billion star systems per galaxy and multiply it by the two trillion galaxies, the result is 2 x 10^23, or 200 sextillion, star systems in the observable universe. This number only accounts for the portion of the universe from which light has had time to reach us, meaning the total number of systems in the entire cosmos is likely far greater.
The Ongoing Search and Future Discoveries
Technological advancements have played a fundamental role in refining these cosmic estimates. The Kepler Space Telescope (2009 to 2018) was instrumental in establishing the high occurrence rate of exoplanets. It achieved this by watching for the dimming of starlight as a planet crossed in front of its star. Kepler’s data revealed that small, often rocky planets are incredibly frequent, shifting the scientific view that planetary formation was not a rare event.
The James Webb Space Telescope (JWST) is now building on this foundation by characterizing the systems that Kepler and other missions found. JWST uses its powerful infrared capabilities to perform transit spectroscopy, analyzing the atmospheric composition of distant exoplanets. This work does not increase the raw count of star systems, but it provides detailed information about their physical properties, such as the presence of molecules like water or carbon dioxide. These characterizations refine the understanding of what kinds of planetary systems exist, lending greater certainty to the statistical models that produce the total census.