What is the official IAU name of the exoplanet 55 Cancri e?

The official IAU name of the exoplanet 55 Cancri e is Janssen.

The planet orbits 55 Cancri A, a Sun-like star about 41 light-years away in the constellation Cancer. It was discovered through the radial-velocity method in 2004, making it one of the earliest known super-Earths. The name “Janssen” honors Zacharias Janssen, a Dutch spectacle maker traditionally associated with the invention of the telescope.

55 Cancri e is extremely close to its star: it completes an orbit in roughly 17.7 hours. Its mass is about eight times Earth’s, and its diameter is approximately twice Earth’s. Because of its tight orbit, it is intensely hot and is not considered a likely home for life as we know it.

A common mix-up is treating “55 Cancri e” and “Janssen” as different planets. The former is the scientific designation; the latter is the IAU-approved proper name. The star’s other planets retain designations such as 55 Cancri b, c, d and f.