We have discovered quite a number of exoplanets to date. The Kepler spacecraft has examined 150,000 stars and found 1,059 exoplanets.

We know that Kepler, as well as all other exoplanet searches to date, can only find planets that cross in front of their star. That means that a large percentage of existing planets will not be detected. In addition, a lot of planets may be too small for the sensitivity of our detectors.

From these facts, has anyone calculated the probability of a star having a planetary system? I am guessing it is likely quite close to 1, i.e. all stars have planets, but I have not seen anyone figures for it.


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There are several different estimates, based on several factors. The article One or more bound planets per Milky Way star from microlensing observations (Cassan et al. 2011) suggests that your estimate that the probability being close to 1, suggesting that

One or more bound planets per Milky Way star from microlensing observations

If you take particular types of exoplanets as examples - such as, according to the Keck Observatory article One in Five (sun like) Stars Has Earth-sized Planet in Habitable Zone (2013), through a combination of monitoring and modelling, they have determined

Accounting for missed planets, as well as the fact that only a small fraction of planets are oriented so that they cross in front of their host star as seen from Earth, allowed them to estimate that 22 percent of all sun-like stars in the galaxy have Earth-size planets in their habitable zones.

  • $\begingroup$ Thanks, @Amaterasu. It looks as if my guess was in the ballpark. I have a couple of others as well, e.g. life will be everywhere, but intelligent life will not be found elsewhere in the galaxy. I do not expect an answer to them soon though. $\endgroup$
    – hdhondt
    Jan 4, 2014 at 10:48

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