In astronomy, transit probability is the geometric chance that an exoplanet’s orbit is aligned closely enough with our line of sight to pass directly between its host star and Earth. Because we can only detect an exoplanet via the Transit Method if its orbital plane is viewed nearly “edge-on,” calculating this probability helps astronomers evaluate how many planets they are missing and which targets are worth targeting for follow-up observations. The Core Formula (Circular Orbits)
For a planet on a simple circular orbit, assuming the planet’s radius is much smaller than the star’s radius ( ), the a priori transit probability ( Ptrcap P sub tr end-sub ) is expressed mathematically as:
Ptr≈R*acap P sub tr end-sub is approximately equal to the fraction with numerator cap R subend-sub and denominator a end-fraction R*cap R sub * end-sub is the radius of the host star.
a is the semi-major axis (the distance from the planet to the star).
If we account for the physical size of the planet to include “grazing transits” (where only a portion of the planet clips the edge of the star), the equation expands slightly:
Ptr=R*+Rpacap P sub tr end-sub equals the fraction with numerator cap R sub * end-sub plus cap R sub p and denominator a end-fraction Key Factors Influencing the Probability
Leave a Reply