Although ocean worlds are swimming in what is thought to be a key ingredient for life—water—their lack of land may limit how much of it they can host. Water covering the surface interacts with carbon dioxide in the atmosphere in ways that can turn chilly planets frigid and make warm ones even hotter. New research published online before print in the Monthly Notices of the Royal Astronomical Society shows that Earth-sized water worlds are habitable only in a very limited range of temperatures—from about 0°C to 127°C. Anything outside that range, which tends to occur on planets that are in a “Goldilocks zone” of 102 million to 140 million miles away from their stars (Earth is about 93 million miles away from the sun), could be devastating for life as we know it. When temperatures are low, the ocean dissolves an increasing amount of carbon dioxide, a potent greenhouse gas, further reducing the planet’s temperature. At higher temperatures, less of the gas is absorbed, and the ocean releases more carbon dioxide into the air, contributing to a runaway greenhouse effect. Although the results are based on calculations of Earth-sized worlds surrounding sunlike stars, the researchers say the process would be similar for larger worlds and stars. They also say that a similar cycle would take place with other greenhouse gases, such as methane. With such a narrow range for habitability, ocean planets may not make as much of a splash as we thought when it comes to welcoming life.