Planet-hosting stars constitute the dominant mass, energy source, and dynamical centre of planetary systems; their fundamental properties set the initial conditions for planet formation and govern subsequent orbital and atmospheric evolution. Stellar mass and spectral type largely determine disk mass and temperature structure, producing systematic differences in planetary architectures: early-type (A–F) stars, with higher masses and hotter disks, preferentially yield more massive planets at wider separations, whereas late-type M dwarfs more often harbour compact systems of lower-mass, short-period rocky planets. Metallicity—the abundance of elements heavier than helium—modulates solid available for core assembly and thus correlates with planet class: metal-rich stars show an elevated incidence of gas giants, while lower-metallicity hosts more frequently possess small, terrestrial-sized planets.

Luminosity and effective temperature set the scale and location of the circumstellar habitable zone and, through spectral energy distribution and lifetime on the main sequence, influence planetary climate, atmospheric chemistry, and prospects for long-term habitability; hotter, more luminous stars produce broader, more distant habitable zones but typically evolve more rapidly. Stellar age and evolutionary state trace different stages of system development: young, disk-bearing stars are sites of active accretion, migration and dynamical rearrangement, whereas older systems reveal the integrated outcomes of processes such as migration, scattering and tidal or radiative erosion of atmospheres. Magnetic activity, high-energy (X-ray/UV) emission and rotation further modify planetary atmospheres and surfaces by driving escape, photochemistry and space-weather interactions; such effects are pronounced for young stars and many low-mass stars, with important implications for the retention and composition of close-in atmospheres.

Multiplicity and local stellar environment also shape planet formation and stability: companions can truncate disks, alter planetesimal dynamics and restrict stable orbital zones, and encounters in dense clusters can perturb or strip planetary systems. Interpreting empirical correlations between stellar properties and planet populations requires careful correction for observational selection effects—radial-velocity surveys preferentially detect massive, short-period planets around bright FGK stars, while transit surveys are more sensitive to small-radius planets orbiting small stars—so population-level inferences depend on survey completeness and bias mitigation. Together, these measurable stellar parameters (mass, spectral type, metallicity, luminosity, age, activity, multiplicity) provide predictive constraints for target selection in exoplanet searches and furnish essential boundary conditions for theoretical models of planet formation, orbital architecture and habitability.

Surveys using multiple detection methods now converge on the conclusion that planets are commonplace in the Galaxy, with an average of order unity planets per star. For the purposes of demographic figures discussed here, “Sun‑like” stars are taken to be main‑sequence late‑F, G, or early‑K stars without a close stellar companion; reported percentages below refer to this subset.

The radial‑velocity and transit techniques account for most confirmed exoplanets and are intrinsically most sensitive to high‑mass planets on short‑period orbits, producing a strong detection bias toward close‑in giants. Consequently many early, numerous detections were Hot Jupiters—Jovian‑mass bodies with orbital periods of only a few days. A 2005 radial‑velocity survey measured a Hot‑Jupiter occurrence of ≈1.2% among Sun‑like stars; Kepler field results imply a substantially lower Hot‑Jupiter rate, a discrepancy that may reflect differing Galactic environments (e.g., stellar metallicity) sampled by the surveys. More broadly, the frequency of giant planets (≥~30 Earth masses) with orbital periods ≤100 days is estimated at roughly 3–4.5%, a range shaped by the superior detectability of short‑period giants.

When planetary mass and orbital distributions are taken into account, lower‑mass planets (roughly Earth mass to a few Earth masses) outnumber giants, and planets become more common at larger separations than at very small orbital radii. From these trends it is estimated that about one in five Sun‑like stars hosts at least one giant planet, while at least two in five may host lower‑mass planets. Independent gravitational microlensing analyses (2002–2007 data, synthesized in a 2012 study) inferred an average of ≈1.6 planets per star in the 0.5–10 AU range, supporting the view that planetary systems are typical rather than exceptional.

Scaling these demographic rates to the Milky Way’s stellar population (≥10^11 stars) yields a Galactic planetary inventory on the order of tens to hundreds of billions of planets.

Type of star and spectral classification

The Morgan–Keenan system organizes stars by spectral type (O, B, A, F, G, K, M, extending to L, T, Y for sub-stellar objects), and both intrinsic stellar properties and observational selection effects strongly shape the demographics of detected exoplanets. Most known planets orbit main-sequence F, G and K stars because planet searches have preferentially targeted Sun-like hosts and because detection sensitivity and planet occurrence change with stellar mass and temperature. Red (M) dwarfs, though numerous, present mixed prospects: radial-velocity surveys have been relatively insensitive to many M-dwarf planets—partly because those systems tend to host lower-mass planets and because observational limitations reduce Doppler precision—whereas transit surveys such as Kepler have been highly successful at finding small planets around M stars because the smaller stellar radii yield deeper, more easily detectable transits. Survey results vary by method and host mass: Doppler surveys report that roughly 1 in 6 stars with ≈2 M☉ host at least one Jupiter-class planet, compared with about 1 in 16 for solar-type stars and about 1 in 50 for red dwarfs; microlensing, by contrast, indicates that long-period Neptune-mass planets are common around red dwarfs (on the order of one in three). Kepler data further show that the occurrence of 1–4 R⊕ planets increases systematically toward cooler, lower-mass stars (sequence M > K > G > F). At the hot and massive extreme, A-type main-sequence stars are challenging targets for Doppler techniques because rapid rotation broadens spectral lines; many A stars that later evolve into more slowly rotating red giants have nevertheless revealed a population of tens of planets via radial-velocity monitoring. Extremely massive O-type stars produce intense ultraviolet irradiation that drives photo-evaporation of nearby protoplanetary material and so suppresses planet formation in their vicinity; when such stars explode as supernovae the abrupt mass loss can unbind surviving planets, producing free-floating objects unless a fortuitous natal kick keeps them bound. Supernovae can also leave bound fallback disks of retained ejecta around compact remnants, providing a secondary channel for planet formation around neutron stars and black holes. At the low-mass end, brown dwarfs and sub-brown dwarfs (spectral classes L, T, Y) are observed to host protoplanetary disks and, in some cases, planetary-mass companions (e.g., disks detected around objects like OTS 44). Finally, dynamical evolution within systems can eject planets to produce rogue planets; such ejected bodies may nonetheless retain bound satellite systems, implying that free-floating planets can carry attendant moons.

Metallicity

Stellar metallicity denotes the fraction of a star’s mass in elements heavier than hydrogen and helium and is customarily expressed on a logarithmic scale [m/H], with [m/H] = 0 set at the solar value. Observational surveys have established a clear link between a star’s metal content and its propensity to host planets: higher [m/H] systematically increases planet occurrence, an effect that is strongest for large, gas-dominated planets and thus shapes the overall architecture of planetary systems.

Analyses of Kepler detections (2012) showed that planets smaller than Neptune are found across a broad host-metallicity interval (approximately −0.6 to +0.5 in [m/H]), but that the largest planets in the sample concentrate toward the metal-rich end of this range. Within that dataset the ratio of small to large planets depends on host metallicity: for stars more metal-rich than the Sun small planets outnumber large ones by roughly 3:1, whereas for metal-poor hosts the small-to-large ratio rises to about 6:1. These trends have been interpreted in terms of protoplanetary-disk physics—higher solid content accelerates the formation of massive cores and facilitates gas accretion before disk dispersal—and in terms of dynamical evolution, since many gas giants likely form at larger radii and require efficient inward migration to appear in close-in samples like Kepler’s; reduced migration efficiency in low-metallicity disks would therefore depress the observed giant-planet yield.

A subsequent 2014 study quantified the metallicity dependence across three radius-based classes (terrestrial: <1.7 R⊕; gas dwarfs: 1.7–3.9 R⊕; gas giants: >3.9 R⊕) and found that metal-rich stars show elevated occurrence rates relative to metal-poor stars by factors of ≈1.72 for terrestrial planets, ≈2.03 for gas dwarfs, and ≈9.30 for gas giants. That analysis also noted an observational bias: metal-rich stars tend to be larger, which reduces the signal-to-noise of transits for small planets; as a consequence, the reported metallicity-driven increases in small-planet occurrence should be regarded as conservative lower bounds.

Beyond planet counts and sizes, metallicity has been implicated in other host-star signatures and system properties. Several studies report enhanced lithium depletion among Sun-like planet hosts compared with similar non-host stars, though this lithium–planet connection remains contested and is not observed uniformly across stellar types. More recently (2025), evidence has emerged that short-period, small-planet systems with high mutual orbital inclinations are more frequent around metal-rich stars, suggesting that metallicity can also influence the three-dimensional orbital architecture of compact planetary systems.

Multiple stars

Stellar multiplicity increases steeply with mass: only about one quarter of M dwarfs are in multiple systems, roughly 45% of Sun‑like stars are, and the fraction rises to nearly 80% for the most massive stars. Within the multiplicity population binaries predominate, comprising roughly three quarters of multiple systems, with the remainder in higher‑order configurations (triples, quadruples, etc.). Planets are common in these environments: well over one hundred planets have been detected orbiting one member of a binary (e.g., 55 Cancri; the putative Alpha Centauri Bb remains contested), and circumbinary planets—which orbit both stars—have been directly observed (for example PSR B1620‑26 b and Kepler‑16b). Planets are also known in triple systems (e.g., 16 Cygni Bb) and at least one quadruple system (Kepler‑64), showing that planet formation and long‑term retention occur across a wide range of stellar dynamical architectures.

Kepler survey results (to October 2013) indicate circumbinary planets are not rare: seven were found among ~1,000 eclipsing binaries searched. Those discoveries revealed two notable dynamical regularities: circumbinary planets were absent around the very shortest‑period binaries (none hosted planets below a binary period of 7.4 days, despite half of binaries having periods ≤2.7 days), and detected circumbinary planets cluster close to the theoretical stability boundary. Stability analyses predict a minimum stable planetary semimajor axis of roughly two to three times the binary separation, and observed systems tend to lie near that limit.

Companion stars are also common among surveyed exoplanet hosts: a 2014 synthesis of companion searches inferred that about half of planet‑hosting stars have a stellar companion, typically within ~100 AU. This prevalence of unresolved companions introduces two practical problems. First, it can be ambiguous which stellar component a detected planet actually orbits, complicating dynamical interpretation; second, unresolved companions bias transit and radius determinations because planetary radii and orbital separations are inferred from stellar parameters. Resolving these ambiguities requires targeted follow‑up—high‑resolution imaging (e.g., speckle or adaptive optics) to detect close companions and radial‑velocity monitoring to identify spectroscopic binaries—but the majority of hosts have not received comprehensive follow‑up. Case studies such as Kepler‑132 and Kepler‑296 illustrate the issue: only with subsequent observations was it possible to assign the Kepler‑296 planets to the brighter stellar component.

Open clusters are the principal birthplaces of stars and therefore constitute a critical context for studying the origins of planetary systems. Early apparent paucity of planets in clusters led to suggestions that the clustered environment might inhibit planet formation, but a systematic evaluation in 2011 concluded that the observational basis for this claim was weak: too few and too limited surveys had been conducted to draw firm conclusions. This shortage of data partly reflects a selection effect—only a small number of suitably positioned and characterized open clusters in the Milky Way are amenable to focused planet searches—introducing availability bias into inferences about cluster planet occurrence. Subsequent detections of both giant and low-mass planets within open clusters indicate that planet formation does occur in these environments, and current evidence is consistent with planet occurrence rates in clusters being similar to those around field stars. The intermediate-age cluster NGC 6811, which hosts the planetary systems Kepler-66 and Kepler-67, exemplifies the presence of bona fide planetary systems in an open-cluster setting.