I was just wondering what is the normal amount of planets is in a star system, because in my mind 8 planets seems like a relatively large amount. So are we above average? Can we even see all the exoplanets in a star system? Also, do we know if belts of asteroids are common? Is it even likely for a star to have planets?


2 Answers 2


Best answer is that we don't know. The techniques that are used for finding exoplanets are incomplete - that is they are capable of finding planets around stars in some circumstances but not in others.

For example, finding stars using the transit detection technique is more sensitive to planets that are (i) big, (ii) close to their parent star and (iii) orbiting their star with an edge-on orientation from our point of view. The doppler techniques are sensitive to (i) massive exoplanets, (ii) those that are close-in to their parent star, (iii) favour detection when the line of sight lies close to the orbital plane.

Each detection method (and each observing strategy and telescope/satellite) has their own particular selection bias, but overall the picture is that the census of exoplanets is more complete for close-in massive planets, and very incomplete for planets that are either small or orbiting with periods much longer than 10 years. Then at very large separations and very long orbital periods (think 100s of years), direct imaging surveys are again capable of spotting massive exoplanets. Microlensing observations are also capable of detecting even small planets at large distances from their star.

The result of all this is that any figure for the observed fraction of stars with exoplanets or the number of exoplanets per star is a lower limit. The exception is where researchers try to account for the various selection effects and give a figure for the fraction of stars with certain types of planet in certain types of orbit around certain types of star.

At the moment it looks like the modal number of exoplanets per star is at least 1. In other words it seems increasingly likely that most stars have exoplanets of one sort or another.

Some facts and figures. From Kepler transit data, Petigura et al. (2013) say that 22% of Sun-like stars have "Earth-sized" planets orbiting in their habitable zone (my italics), but their plots show that the overall results for planets with orbital periods in the range 5-100 days (where Kepler is most sensitive), are that nearly 60% of stars have planets from Earth- to Jupiter- sized in this range. Cassan et al. (2012) use microlensing survey data to claim that 17% of stars have "Jupiters" at 0.5-10 au from their parent star, and that Neptune-sized or "super-Earth" sized objects orbit a further (approximately) 50% and 60% respectively. The combination of these figures also suggests multiple planets are not rare and indeed the title of their paper is "One or more bound planets per Milky Way star from microlensing observations".

The frequency of multiple planet systems is also hard to assess. All the techniques are capable of detecting certain types of multiple system, but equally all are capable of missing multiple planets even when they are there. Kepler did detect many multiple transiting systems, so they are not rare. A study by Tremain & Dong (2012) estimated a minimum fraction of 20-30% of stars that have one transiting planet will also have others. This is on top of the 15% of Kepler planetary systems where multiple planets can be directly observed (i.e where the orbital planes are very close together).

  • $\begingroup$ There are only 6 planets that can be seen with the naked eye in our own solar system. We also do not know if we have additional planets, for example, orbiting outside of the Oort cloud. We would not be able to reliably detect even a single planet in our own system if we were 1000 light years away from it. My guess is that planets are much more common than stars. $\endgroup$
    – Carl
    Sep 25, 2016 at 22:14

NASA said that on average every star has about 1 planet and that we are actually an unusual solar system. Most of the stars in the galaxy are M stars and while we know of many multi planet systems, they are far outweighed by the amount with one exoplanet orbiting its star at a distance closer than Mercury orbits the sun. Example: While Proxima Centauri has 5 exoplanets that are about Earth size, ALL of their orbits would fit inside the orbit of Mercury.

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    $\begingroup$ Adding sources and citations would improve this answer. Also Proxima Cen has so far only one discovered planet. $\endgroup$ Jul 29, 2019 at 14:11

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