Abstract

The idea that worlds around other stars could develop and maintain environments hospitable to life, in a way like our planet, has captivated scientists for centuries. Yet, to investigate this question, we must recognize and characterize the key conditions that make a planet habitable. Earth ― the only planet on which life is known to have originated ― is unique in many ways, including the presence of abundant surface water, a large moon, a long-lived magnetic field, and plate tectonics. Yet, which of these and other characteristics are essential for its long-term habitability?

A major challenge is that habitability factors vary because they are time-dependent due to changes in the Sun’s energy and our planet’s chemical, thermal and (thereby) physical and tectonic evolution. Plate tectonics regulates interior temperatures, atmospheric greenhouse gas concentrations and surface temperatures. Subduction enables recycling of volatile elements between the surface and the mantle and is probably essential for sustaining planetary habitability. Because the questions of when, why and how plate tectonics started are debated, an improved understanding of Earth’s evolution is critically needed.

It is not necessarily obvious that key habitability factors such as plate tectonics will persist once started, a dynamo-driven magnetic field can stop and perhaps re-emerge later through inner-core nucleation, and the Earth’s axial tilt may also become unstable as the Moon is moving away. Ultimately, all planets lose their habitability, and in about two billion years when the Sun’s energy has increased by 15%, Earth will enter a moist greenhouse, followed by runaway evaporation of the oceans. An in-depth knowledge of Earth-like habitability, and how our planet sustained conditions for life’s evolution over geological timescales, is critical for identifying habitable planets orbiting other stars that potentially are, or have been, habitable around other stars.