Planet-star systems are very much like binary stars, the study of which is central to stellar astronomy (primarily because it's the best way to determine masses and radii of stars). An exoplanet that has the right alignment with respect to us to periodically transit its host star is almost totally analogous to an eclipsing binary star.

Refresh your memory about how orbits work, and what quantities are observable, with this binary star applet. Make sure to read the description, below the applet.

binary orbit simulator

You can see two views of the orbits, where the inclination parameter changes the view as seen from the Earth. The spectra represent the spectra of the two stars with (arbitrarily) two spectral absorption lines in each stars' spectrum shown schematically. They move in the spectrum due to the changing Doppler shift as the two stars orbit each other. Stars have absorption lines in their spectra. Planets don't emit enough light for us to measure their spectra in the glare of their host star, so we see only one set of spectral lines (the star's). Since planets are -- basically by definition -- much less massive than stars, try setting the mass ratio in the binary simulation to 10:1 and then 100:1 (note that the masses are specified in solar mass units). What happens to the size of the (more massive star's) orbit? What does changing the eccentricity do? What about the separation (semi-major axis)? For each orbit you try, look at the privileged view, and think about where the center of mass is.

Take a look at the Earth view, and set the inclination to 90 degrees. Make sure you understand very well what is meant by the inclination angle. With i = 90 degrees, do you see how one star occasionally (periodically!) moves in front of the other? If one of those stars were a planet, it would block some of the light of the (other) star when it eclipses, or transits the star. Now try the eclipsing binary simulation, below.

eclipsing binary simulator

You might start with the inclination set to 90 degrees exactly. Check the "show lightcurve" box in the upper right. The light curve is what we measure with our telescope. Here you can set the second star's radius to be significantly less than the star's. And turn its temperature down too (binary stars are more complicated than exoplanet systems because both stars emit significant amounts of light and the sum of their un-eclipsed surface brightnesses produces the light curve we see). Then you'll be approximating an exoplanet transit.