Astronomers using the NASA/ESA Hubble Space Telescope observed the smallest exoplanet where water vapour has been detected in its atmosphere. At only approximately twice Earth’s diameter, the planet GJ 9827d could be an example of potential planets with water-rich atmospheres elsewhere in our galaxy.

• This would be the first time that an atmospheric detection shows that planets with water-rich atmospheres can actually exist around other stars. 
• The observation is an important step toward determining the prevalence and diversity of atmospheres on rocky planets. 
• However, it remains too early to tell whether Hubble spectroscopically measured a small amount of water vapour in a puffy hydrogen-rich atmosphere, or if the planet’s atmosphere is mostly made of water, left behind after a primaeval hydrogen/helium atmosphere evapourated under stellar radiation.