I am looking for an API or dataset that has information about the known exoplanets. The properties of this dataset would include things like mass, orbital period, atmosphere, temperature, etc.
I'm not sure whether it's duplicative but here's another link.
(Disclaimer - I am the Sr. API Strategist for GSA)
The NASA Exoplanet archive also has a bulk download feature.
More generically, the astronomical community has a number of standardized APIs managed by the International Virtual Observatory Alliance (IVOA):
Some things that people might consider 'astronomical' the science community calls 'planetary', and their primary system in the United States is the Planetary Data System, and their standards are the basis for the International Planetary Data Alliance (IPDA) :
... of course, you then have to find the appropriate endpoint to talk to.