Planet-hosting star

Planet-hosting stars are stars which host planets, therefore forming planetary systems. There are correlations between stars' characteristics and the characteristics of the planets that orbit them.

Most stars are accompanied by planets, though the exact proportion remains uncertain due to current limitations in detecting distant exoplanets. Current research calculates that there is, on average, at least one planet per star. One in five Sun-like stars is expected to have an "Earth-sized" planet in the habitable zone. The radial-velocity method and the transit method (the two methods responsible for the vast majority of detected planets) are most sensitive to large planets in small orbits. Thus many known exoplanets are "Hot Jupiters", planets of Jovian mass or larger in very small orbits with periods of only a few days. A survey from 2005 on radial-velocity-detected planets found that about 1.2% of Sun-like stars have a 'Hot Jupiter', where "Sun-like star" refers to any main-sequence star of spectral classes late-F, G, or early-K without a close stellar companion. This 1.2% is more than double the frequency of 'Hot Jupiters' detected by the Kepler spacecraft, for which a possible reason is that the Kepler field of view is covering a different region of the Milky Way where the metallicity of stars is different. It is further estimated that 3% to 4.5% of Sun-like stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 Earth masses.

It is known that small planets (of roughly Earth-like mass or slightly larger) are more common than giant planets. It also appears that there are more planets in large orbits than in small orbits. Based on this, it is estimated that about 20% of Sun-like stars have at least one giant planet, whereas at least 40% may have planets of lower mass. A 2012 study of gravitational microlensing data collected between 2002 and 2007 concludes the proportion of stars with planets is much higher and estimates an average of 1.6 planets orbiting between 0.5 and 10 AU per star in the Milky Way. The authors of this study conclude that "stars are orbited by planets as a rule, rather than the exception". In November 2013, it was announced that 22±8% of Sun-like stars have an Earth-sized planet in the habitable zone.

Regardless of the proportion of stars with planets, the total number of exoplanets must be very large. Since the Milky Way has at least 100 billion stars, it should also contain tens or hundreds of billions of planets.

Most known exoplanets orbit stars roughly similar to the Sun, that is, main-sequence stars of spectral categories F, G, or K. One reason is that planet-search programs have tended to concentrate on such stars. In addition, statistical analyses indicate that lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method. Nevertheless, many planets around red dwarfs have been discovered by the Kepler space telescope by the transit method, which can detect smaller planets.

Stars of spectral category A typically rotate very quickly, which makes it very difficult to measure the small Doppler shifts induced by orbiting planets because the spectral lines are very broad. However, this type of massive star eventually evolves into a cooler red giant that rotates more slowly and thus can be measured using the radial-velocity method. A few tens of planets have been found around red giants.

Observations using the Spitzer Space Telescope indicate that extremely massive stars of spectral category O, which are much hotter than the Sun, produce a photo-evaporation effect that inhibits planetary formation. When the O-type star goes supernova any planets that had formed would become free-floating due to the loss of stellar mass unless the natal kick of the resulting remnant pushes it in the same direction as an escaping planet. Fallback disks of matter that failed to escape orbit during a supernova may form planets around neutron stars and black holes.

Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, vs. 1 in 16 for Sun-like stars and only 1 in 50 for red dwarfs. On the other hand, microlensing surveys indicate that long-period Neptune-mass planets are found around 1 in 3 red dwarfs. Kepler Space Telescope observations of planets with up to one year periods show that occurrence rates of Earth- to Neptune-sized planets (1 to 4 Earth radii) around M, K, G, and F stars are successively higher towards cooler, less massive stars.