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

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 angle animation

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).

This page updated on July 18, 2015.

About the Author

Everett Schlawin

Everett's research focus is extra-solar planets or exoplanets. These are the planets far, far beyond Neptune and Pluto, which orbit other star systems. He observers exoplanet atmospheres to learn about their composition. The colors of an absorbing exoplanet atmosphere tell astronomers what the atmosphere is made of, so he uses spectrographs to split up the colors of these star-planet systems and infer which gases make up its atmosphere. He also is building a new infrared spectrograph to go on the Blanco Telescope in Chile with the TripleSpec 4 team.

Everett's Website: http://astro.cornell.edu/~schlawin/

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