Mike Brown [photo credit: Caltech]
Mike Brown’s course, “The Science of the Solar System,” is a nine-week planetary-science class he built for his Caltech undergraduates and posted online for the public. It runs on Coursera at no charge, and Brown has also made the lectures freely available through his Caltech course site. Caltech has highlighted the class on its social media.
Brown is the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech, on the faculty in Pasadena since 1996. He is best known for discovering Eris, the finding that led astronomers to reclassify Pluto as a dwarf planet — work that earned him the nickname “Pluto Killer” in Caltech’s own experts listing. He directs the university’s Center for Comparative Planetary Evolution and, with colleague Konstantin Batygin, has argued for the possible existence of a hypothesized “Planet Nine.”
The class is organized into nine one-week sessions with graded quizzes and unit tests, covering water on Mars, the interiors of the giant planets, how the solar system’s formation is recorded in its leftover small bodies, and the search for life beyond Earth. It is built to show how the scientific process is used to investigate the planets and other bodies around us, according to Caltech. On his course page, Brown states plainly that everything is free, and that the only thing money buys is an optional printable certificate.
The reach is substantial, though the figures come from Brown and the platform. Brown’s Caltech page says more than 80,000 people have taken part and describes the class as one of the top 50 online courses of all time. Coursera’s listing shows a 4.9 instructor rating and roughly 75,000 learners.
Brown, who has won Caltech’s Richard P. Feynman teaching award and the 2012 Kavli Prize in Astrophysics, has framed the online version as outreach, calling it “the best outreach tool I’ve ever found,” according to a Caltech feature on the course. He still teaches the on-campus companion courses; Caltech’s 2025–26 catalog lists him leading its planetary-science sequence.
Those who want to take the class can enroll on Coursera’s “The Science of the Solar System” page at no cost, or access the lectures through Brown’s Caltech course site; Caltech’s social-media post notes the lectures can also be watched on YouTube. Payment is required only for the optional certificate.
Brown built his reputation on the discovery that pushed Pluto out of the planet club. The class he teaches the rest of us, it turns out, lets everyone in.
