Radioactive decay could provide enough heat to sustain life on distant worlds without a nearby star. On Earth, life developed and evolved around the radioactive heat of heavy elements within Earth's interior, deep below its surface. The energy liberated by the radioactive decay is known to produce heat, which supports the hydrothermal ecosystems thriving on the sea floor. However, the possibility that heat from radioactive decay could support extraterrestrial life beyond our planet remains unexplored. Using high-resolution three-dimensional computer models, researchers have now assessed the potential habitability of Earth-size worlds that have only the radioactive decay of heavy elements to keep them warmed. The researchers identified four scenarios involving different compositions, which support liquid water at the surfaces of these radioactive worlds. Their results were recently published in the renowned journal Nature Astronomy.

Radioactive Decay as an Alternative Energy Source

Stars such as our Sun are powerful energy sources that drive life on Earth. While most explorations for habitable exoplanets have focused on planets orbiting stars similar to our own, the vastness of the universe suggests there could be planets located in environments devoid of stars. In such scenarios, other potential energy sources are required to sustain liquid water on their surfaces.

Radioactive elements like uranium, thorium, and potassium produce heat through radioactive decay. On Earth, these elements contribute significantly to the internal heat that propels geothermal activity, such as geysers, hot springs, and deep-sea hydrothermal vents. The researchers behind this study investigated the potential for radioactive decay to provide enough heat to support liquid water on planets without stars.

Computer Models Simulate Habitable Scenarios

To investigate this possibility, the research team employed sophisticated three-dimensional computer models. They simulated a variety of rocky planets with different compositions and interior structures. Each planet was assumed to be orbiting a distant star that did not provide sufficient heat to sustain liquid water on its own.

The models revealed four distinct scenarios where the radioactive decay of heavy elements was capable of keeping the planets' surface temperatures above freezing and potentially supporting liquid water. These scenarios comprised of:

1. Water-rich planets: Rocky worlds with abundant water bodies would likely have significant quantities of radioactive elements dissolved within their oceans. Radioactive heating from the decay of these dissolved elements would contribute to the planet's overall warmth.

2. Iron-rich planets: The presence of iron within a planet's core can enhance the production of heat from radioactive decay. Iron's propensity to conduct heat efficiently further contributes to the dispersal of heat throughout the planet.

3. Thin crust planets: Planets with less crust or thinner crust would experience reduced insulation, allowing the heat from radioactive decay within their interiors to reach the surface more easily.

4. Earth-like planets: Rocky worlds with compositions and interior structures similar to Earth also demonstrated the ability to support liquid water through radioactive decay.

The study's lead author, Professor Stephen Mojzsis from the University of Colorado Boulder, emphasized the importance of considering alternative energy sources beyond stars for sustaining life. "Our results suggest that habitable environments may arise in places we didn't previously think possible," he explained. "This expands the range of environments in which we should search for signatures of life beyond Earth."

Implications for Exoplanet Exploration

The team concluded that the scenarios they identified broaden the diversity of habitable environments we should be considering when exploring for habitable exoplanets. As exoplanet detection techniques rapidly advance, and new telescopes come online, the discovery of Earth-like planets or moons with water, iron-rich cores, thin crusts, or favorable compositions becomes more feasible. The discovery of such worlds would warrant closer examination to search for signs of life, even if they're located in remote regions of space without the warmth of a nearby star.

The results of this study offer a fresh perspective on the hunt for extraterrestrial life, encouraging researchers to consider alternative energy sources and expand their search parameters when contemplating the potential for life beyond Earth.