# How long do you expect transit to last?

by duder1234
Tags: flux, planet, transit, transit duration
 P: 4 I have a homework problem that I am having troubles with. There are 2 parts A transiting exoplanet with a diameter twice that of the Earth orbits a sun-like star in a circular orbit of radius 1.5 AU a) How much reduction in the flux of the star occurs during the transit? Earth's diameter=Planet's radius (Rp) =8.5175×10-5 AU And because it says a "sun-like" star, I used the same values as the sun for radius Star's radius (Rs) =4.649×10-3 AU And I can use the formula $\frac{ΔF}{F}$=$\frac{R^{2}_{p}}{R^{2}_{s}}$ By plugging in the values, i got 0.03% reduction in flux b) How long do you expect the transit to last? I am stuck on this one. I was not told the impact parameter b so do I assume that the transit happens through the centre? or do I use the formula τ=$\frac{2(Rp+Rs)}{V}$ where τ= transit duration, Rp=diameter of planet, Rs=diameter of star, V=velocity Any help is appreciated, thanks in advance!
 or do I use the formula τ=$\frac{2(Rp+Rs)}{V}$
 P: 4 Thanks for the reply! I just want to check but I can get the velocity with the formula:$V_{s}$=$\frac{2πr_{s}}{P}$ where P=Period and $r_{s}$=radius of the star and the period formula being $P^{2}$=$\frac{4π}{GM}a^{3}$ M=Mass of star, a=semi-major axis