edX is an online repository of educational courses covering a wide variety of subjects delivered by a number of recognised universities. All of the courses are free to take; if you choose to, you can pay to be assessed and receive a certificate.
I've compiled a list of those that may be of interest to the community:
Alien Worlds: The Science of Exoplanet Discovery and Characterization
Exoplanets: Exploring the mysteries of exoplanets - planets around other stars.