The question of how many planets exist in the universe requires astronomers to perform a cosmic census using the limits of our current observational technology. This ultimate number is calculated through a series of immense extrapolations, starting with the volume of the observable universe and progressively multiplying by the density of galaxies, the stars within them, and finally, the planets orbiting those stars. Since we cannot count every individual world, the answer is an estimate derived from statistical modeling, focusing only on the section of space from which light has had time to reach us since the Big Bang. This enormous calculation reveals the sheer scale of planetary formation beyond our solar system.
The Observable Universe and Its Galaxies
The first multiplier in this grand estimation is the number of galaxies contained within the observable universe, the spherical region where light has traveled to Earth over the age of the cosmos. Early observations, such as those from the Hubble Deep Field, suggested a lower limit of approximately 100 to 200 billion galaxies. This figure was based on extrapolating the number of galaxies seen in a tiny, representative patch of the sky.
More advanced theoretical modeling and subsequent analysis of deep-field images, however, indicated that many galaxies are too small, faint, or distant to be directly seen with current technology. This led to a significant upward revision of the estimate, with the current consensus placing the number closer to two trillion galaxies in the observable volume. This two trillion figure is ten times the previous estimate.
The “observable” nature of this volume is a limitation, as the universe continues indefinitely beyond this boundary. Even with this boundary, the two trillion galaxies represent the first factor in the planetary count, setting the stage for the following calculation of the stars contained within them.
Estimating Stars Per Galaxy
To move from a count of galaxies to a count of stars, astronomers must determine the average stellar population of a typical galaxy. The number of stars varies wildly depending on the galaxy type, ranging from small dwarf galaxies with only a few thousand stars to giant elliptical or spiral galaxies containing hundreds of billions. For instance, our own Milky Way galaxy is estimated to contain between 100 billion and 400 billion stars.
The figure used for extrapolation must account for this diversity, balancing the large, bright spiral galaxies with the far more numerous, but dim, dwarf galaxies. A common average used in these large-scale calculations is approximately 100 billion stars per galaxy, often using the Milky Way as a practical reference point for a large, mature system.
Multiplying the two trillion galaxy estimate by the 100 billion star average yields a total number of stars in the observable universe that is in the septillions. This average is merely a tool for large-scale estimation, as some studies suggest the true average is much lower, closer to two billion stars per galaxy, due to the sheer number of small dwarf galaxies. However, the use of the Milky Way-like average provides a robust figure for the density of suns available to host planets.
The Exoplanet Factor: Planets Per Star
The most crucial and dynamic factor in the calculation is the planet-to-star ratio, which has been revolutionized by dedicated exoplanet survey missions. Before these surveys, scientists could only speculate on the frequency of planets orbiting other stars. The Kepler Space Telescope, launched by NASA, fundamentally changed this by observing a single patch of sky for years, using the transit method to detect the subtle dimming of starlight caused by orbiting planets.
Kepler’s data demonstrated that planet formation is not rare, but rather nearly universal, suggesting that most stars in the galaxy host at least one planet. Statistical analyses of this data indicate that the occurrence rate of planets for Sun-like stars is extraordinarily high, with some estimates suggesting that 50% to 100% of sun-like stars have planets. This means that the number of planets is at least equal to the number of stars.
A more detailed approach involves calculating the statistical probability of finding an Earth-sized planet in the habitable zone of its star, a metric known as Eta-Earth (\(\eta_{\oplus}\)). While Eta-Earth estimates vary widely based on the definition of ‘Earth-sized’ and ‘habitable zone,’ a key statistical takeaway is that the average number of planets per star is generally considered to be greater than one, with some models suggesting an average of two planets per star in a given system. The discovery of systems with multiple worlds supports the use of a planet-to-star ratio greater than one in the final calculation.
The Grand Total and Current Consensus
Combining the three immense multipliers—galaxies, stars per galaxy, and planets per star—leads to the final estimate for the number of planets in the observable universe. Using the consensus figures of two trillion galaxies, 100 billion stars per galaxy, and at least one planet per star, the calculation produces a lower bound of roughly \(2 \times 10^{23}\) planets, or 200 sextillion worlds.
Astrophysicists often refine this total based on the two-planets-per-star model, which results in an estimate of approximately \(4 \times 10^{23}\) planets, or 400 sextillion. The most common figure cited by astronomers today for the total number of planets orbiting stars in the observable universe is around \(10^{25}\), or 10 septillion. This number represents only the worlds orbiting stars, excluding the potentially trillions of rogue planets that drift through interstellar space without a host sun.