Initially discovered by Kepler space telescope, a terrestrial planet-searching satellite built and operated by NASA, Kepler-9d is most likely a Super-Earth, with an estimated radius approximately 60% larger than that of Earth's, although its exact mass cannot be determined.

The original studies into the system first suggested that Kepler-9d might be a planet, but a follow-up investigation made by the Kepler team later confirmed that it was; the confirmation of Kepler-9d as a planet was made public with the team's paper, which was published in the Astrophysical Journal on January 1, 2011.

Kepler-9 was named for the Kepler spacecraft, a NASA satellite that aims to discover terrestrial planets in transit around, or crossing in front of, their host stars as seen from Earth.

It was recognized as a potential planet after a study into the system confirmed Kepler-9b and Kepler-9c, but follow-up studies had to be completed to verify that it was indeed a planet, and that the apparent transit event was not due to a background eclipsing binary star in the aperture of Kepler's photometer.

Kepler's team exhaustively disproved that the small transit event could have been anything but a planet, and their results were published in the Astrophysical Journal on January 1, 2011.