How likely is it for a planet to transit its star?

Do planets transit all stars for us? If not, how likely is it for a planet to transit?

No, very few planets transit their host stars. When astronomers find a planet transiting they are very lucky guys and gals! This is because there are many more ways for a planet's orbit to miss our line of site than for planets to intersect our line of sight.

Consider the animated gif attached. There are many possible viewing angles where you'd never see the planet cross in front of its host star. Only one viewing angle lines up so that the planet and star can eclipse each other. I snatched these images from the astronomy education materials on http://astro.unl.edu/classaction/ which are totally fabulous by the way!

viewing angles

Planets that are very far from their host stars (like Neptune is for our Solar System) are even less likely to align than close-in ones. The formula, for those who want it, is approximately Probability = (R* / a) where R* is the stellar radius and a is the semi-major axis of the planet's orbit ("orbital radius" for circular orbits).

December 2011, Everett Schlawin (more by Everett Schlawin) (Like this Answer)

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