Ira Bell

Introduction

Roughly a decade ago, planet surveys indicated that stars throughout the universe typically host at least one planet on average. For example, a 2011 microlensing study by Cassan et al. (2011) found roughly 1.6 bound planets per star (for planets 0.5–10 AU from the star, with masses between a few Earths and 10 Jupiter masses)​. In other words, planets were inferred to be the rule rather than the exception, with every Milky Way star hosting on average on the order of one planet or more in that orbital range. Early Kepler results similarly suggested “at least one planet per star” on average​. These initial estimates (on the order of ~1 planet per star) set the baseline that more recent studies are now exceeding.

Revised Analyses of Kepler Data

In the last few years, improved statistical analyses of Kepler exoplanet data have pushed the average planet count per star much higher:

  • Hsu et al. (2019) – Using Kepler DR25 data (with Gaia-based stellar radii) and a Bayesian approach, Hsu and colleagues estimated ≈2.5 planets per FGK star detectable in orbits up to 500 days​. Furthermore, when they accounted for planets in parameter space that Kepler was not sensitive to (e.g. small long-period planets), the inferred average rose to ~5.2 planets per star​. This analysis highlights that many planets remain undetected, and the true underlying occurrence is likely several planets per star.
  • Zink, Christiansen & Hansen (2019) – By modeling the Kepler planet yield (periods 0.5–500 days, radii 0.5–16 R⊕), Zink et al. found that ≈72% of Sun-like stars have at least one planet in that range​. Crucially, their population model implies that most of these planet-hosting stars actually harbor multiple planets. They reported an average of 8.4 ± 0.3 planets per planetary system (excluding rare single-planet systems with a giant planet)​. This result “implies that most stars have at least seven planets” within the considered size and orbital range, with only a small minority of planetary systems having fewer than five​. In other words, the typical star with planets may host an entire multiplanet system, far more populated than earlier thought.
  • He, Ford & Ragozzine (2019) – This study introduced a “clustered” forward model to simulate Kepler multis. They found that when planet occurrence is correlated within systems (i.e. planets tend to come in multiples), the average number of planets per FGK star (with 3–300 day orbits, 0.5–10 R⊕) comes out to about 2.3 planets per star​. If one considers only the stars that actually have planets in that range (about ~56% of stars in their model), those planetary systems contain ~4.3 planets on average​. This clustered model better matches the Kepler data than an assumption of independent single-planet occurrences. It reinforces that multi-planet systems are common, and many stars (particularly those that are not completely barren) likely host several planets.

Taken together, these Kepler-based studies suggest that the average number of planets per star is higher than previously estimated, easily on the order of a few planets per star and possibly exceeding three (depending on orbital range and planet size considered).

High Multiplicity in M-Dwarf Systems

Low-mass stars (red dwarfs) have also shown evidence of very rich planetary systems. Mikko Tuomi et al. (2019) combined radial-velocity surveys of 426 nearby M-dwarf stars, detecting 118 planets, and then corrected for detection biases. They concluded that M dwarfs have at least ~2.39 (+4.58/−1.36) planets per star on average, with a best estimate around 3 planets per M dwarf when accounting for the hardest-to-detect planets​. In fact, after accounting for observational incompleteness, the study suggests the true average “is actually closer to 3” planets per red dwarf​. This aligns with other earlier findings that small, low-mass planets are extremely common around red dwarfs, making these diminutive stars major contributors to the galaxy’s planet population. Notably, ~28% of the surveyed M dwarfs had multi-planet systems, and the majority of the detected planets were of low mass (super-Earths/mini-Neptunes), indicating that compact multi-planet systems are the norm for red dwarfs​. Given that red dwarfs are the most numerous stars in the Milky Way, their high planet counts per star significantly boost the galactic average number of planets per star.

Microlensing & Outer Planet Surveys – Wide Orbits Add More Planets

Beyond the reach of Kepler’s transit survey, microlensing experiments have probed planets on wider orbits and found that these contribute substantially to the overall planet count. Poleski et al. (2021) analyzed ~20 years of OGLE microlensing data for cool outer planets and concluded that wide-orbit planets (5–15 AU) are very common. They infer a planet-to-star ratio of roughly 1.4 planets per star just in the 5–15 AU range. In other words, on average each star hosts >1 giant planet or sub-Neptune in that distant zone – on top of the inner planets that Kepler typically found. This result dovetails with the earlier microlensing work by Cassan et al. (2011), which showed that “stars are orbited by planets as a rule”, with cool Neptunes and super-Earths being especially abundant at a few AU separations​. The implication is that if we combine the inner planetary systems (rich in small planets, as Kepler revealed) with the outer populations (revealed by microlensing), the total number of planets per star climbs even higher. Indeed, astronomers have noted that these findings likely underestimate the true count – there may be additional small, distant, or inclined planets that neither transit nor significantly lens their host stars, meaning the average star could host several planets overall.

Theoretical Models Support Higher Planet Counts

The emerging picture of multiple planets per star is also supported by theoretical population studies. For instance, a 2024 study combined advanced planet formation simulations with galactic stellar population models to estimate planet demographics across the Milky Way. It found that for Sun-like stars, the most common outcome is to form several small planets per system. In simulated Milky Way analogues, Earth-sized and super-Earth planets occur at a rate of about 3.5 to 4 per star on average (for Sun-mass stars) in the galactic disk​. Gas giants are much less frequent, but the dominance of multiple small planets in these models mirrors the empirical findings. Similarly, forward modeling of planetary system architectures (e.g. the clustered model by He et al. 2019 mentioned above) inherently favors scenarios where planets come in packs rather than lone singles. These theoretical frameworks, from N-body formation simulations to statistical occurrence models, consistently produce multi-planet systems as a common outcome, lending credence to the idea that most stars form with, and retain, numerous planets.

Number of Stars in the Universe

When we gaze up at the night sky, it’s easy to forget that the few thousand twinkling dots we see are only a fraction of what’s out there. Astronomers estimate that our own Milky Way galaxy contains hundreds of billions of stars, and that the observable universe holds at least two trillion galaxies. Multiply those two staggering figures, and the result suggests there may be on the order of 10²² to 10²⁴ stars in the cosmos—an unfathomable number often referred to as “stars in the observable universe.” Of course, each new discovery and improved telescope can shift these estimates, but whether it’s 100 sextillion or a quadrillion more, the key takeaway remains the same: the universe holds far more stars than we could ever count—putting into perspective just how immense and mysterious our cosmic neighborhood truly is.

What about Moons?

These data don’t even take into account the average number of moons per planet in the universe. If we simply look at our own solar system, we have the following counts of moons for each planet (though we have a pattern of discovering more for those beyond Earth as time has gone on):

  • Mercury: 0
  • Venus: 0
  • Earth: 1
  • Mars: 2
  • Jupiter: 95
  • Saturn: 146
  • Uranus: 28
  • Neptune: 16

Total count of moons in our solar system: 288

This makes the average moons per planet in our solar system ~36. As we study the moons in our solar system, we are interested several of them with respect to the possibilities of finding life (e.g., Europa, Callisto, Ganymede, Enceladus, Titan, etc.)

Conclusion

Recent research suggests the average star in our galaxy hosts significantly more planets than the older estimates of about 1–1.6 planets per star. Analyses of the complete Kepler dataset (correcting for detection biases) now indicate on the order of 2–5 planets per Sun-like star, with some models implying even 5+ planets on average when including undetected smaller worlds​. When considering only stars that have planets, the numbers are higher still – for example, if a Sun-like star has any planets, it likely has a whole system (5–8 or more planets) rather than a single world​. Add to this the rich systems of red dwarfs (≈3 planets per star on average)​ and the contribution of wide-orbit planets (≥1 on average per star)​, and it becomes clear that planetary systems often contain multiple planets. The consensus emerging from the past few years of exoplanet research is that planetary systems are abundant and typically multi-planet in architecture. Thus, the “more than three planets per star” benchmark is well within reason, and indeed several studies now support such an average. This marks a notable shift from a decade ago, painting a picture of a galaxy teeming with planets – likely several for every star​.