According to a team of American researchers from Penn State University, there could be far more habitable planets in our universe than we previously believed. They came to this conclusion by highlighting the fact that tectonic plates – long assumed to be a necessary condition for supporting life – may not actually be necessary.
When researching potentially habitable planets, scientists look for biosignatures of atmospheric carbon dioxide. On Earth, this increases the surface temperature due to the Greenhouse Effect, but carbon also moves between the subsurface and the atmosphere via natural processes. “Volcanism releases gases into the atmosphere, and then through weathering, carbon dioxide is pulled from the atmosphere and sequestered into surface rocks and sediment,” explains Bradford Foley, the main author of the study, due to appear in the next issue of Astrobiology. “Balancing those two processes keeps carbon dioxide at a certain level in the atmosphere, which is really important for whether the climate stays temperate and suitable for life.”
The majority of volcanoes on Earth are found on the borders of tectonic plates. This is why researchers believed that these plates were indispensable for supporting life. Note also that subduction – when a plate is pushed deeply into the subsurface by a collision plate – may also contribute to carbon cycling, pushing the volatile gas into the mantle.
On planets without tectonic plates, the crust takes the form of one giant and spherical plate that floats on the mantle. There seems to be far more of these planets than there are ones with tectonic plates, and in fact the Earth is the only planet confirmed to possess tectonic plates.
The researchers in this instance created a computer model of a planet’s life cycle. They examined the quantity of heat that a planet’s climate could retain, depending on its initial heat budget, or on the quantity of heat and heat-producing elements present during its formation. Certain heat-producing elements produce heat as they decompose. On Earth, decomposing uranium produces thorium and heat, and decomposing thorium produces potassium and heat.
After having carried out hundreds of simulations to vary the size and chemical composition of a planet, the researchers discovered that planets without tectonic plates could effectively maintain the conditions necessary for liquid water for billions of years. At the highest extreme, they could support life for up to 4 billion years – which is the life cycle of the Earth to date.
“You still have volcanism on stagnant lid planets, but it’s much shorter lived than on planets with plate tectonics because there isn’t as much cycling,” write the researchers. “Volcanoes result in a succession of lava flows, which are buried like layers of a cake over time. Rocks and sediment heat up more the deeper they are buried.”
The researchers found that at sufficiently raised heat and pressure levels, that carbon dioxide could effectively escape from rocks and reach the surface.