[stevenwoo]

Maybe you mixed up the dates slightly?

K2-18b (name of the exo-planet) orbiting star K2-18 was discovered in 2015 but the water was not detected until 2019.

[billforsternz]

Sorry, didn't notice the reply in a timely way. I went back to the library, the book was "The Universe" by Andrew Cohen. It was indeed K2-18B, discovered by Kepler in 2015. It says that the water was discovered by Hubble later, so presumably 2019.