Astronomers believe that Kepler-438b might have liquid water on its surface due to several factors. First, its size suggests that it could have a solid surface and not be a gas giant like some other exoplanets. Second, its orbit within the habitable zone of its star means that the planet's surface temperature could be within a range where water could exist in a liquid state. Third, the star it orbits, Kepler-438, is slightly cooler and smaller than the Sun, which may result in a more stable climate on the exoplanet. However, these are just indications, and direct confirmation of liquid water or the presence of life requires further study and observation.