You can access a lot of courses from reputable institutions via Coursera:
www.coursera.org
Many of the courses have identical content to their Undergraduate Degree programs, while others are offered purely for self-education, and you can pick and choose between multiple institutions as you wish.
A couple that I can highly recommend:
"The Science of the Solar System" by Mike "Pluto Killer" Brown of Caltech
"Astro Tech: The Science and Technology Behind Astronomical Discovery" by The University of Edinburgh
Hope this helps!