As the planet shrinks, its volume decreases while its mass remains constant. This means that the density of the planet increases as it shrinks, causing the gravitational pull on its surface to become stronger. This can lead to changes in the planet's geological structure, such as increased tectonic activity and volcanic eruptions. As the planet shrinks, there may also be changes in its climate and atmosphere. The shrinking surface area may result in more extreme weather patterns, with stronger winds and more frequent storms. The reduced size of the planet may also impact its ability to retain an atmosphere, leading to changes in air pressure and composition. Overall, a shrinking planet with constant mass would experience significant changes in its physical and environmental characteristics, potentially leading to a dramatic shift in its overall ecosystem and habitability.