‘Pale yellow dot’ planets may be critical in the hunt for habitable worlds
Earth-like worlds with similar land-to-ocean ratios to our planet’s may be exceedingly rare. According to a new study, Earth-like planets with about 30 per cent of their surface covered by exposed continental land may make up only one per cent of rocky worlds in stars’ habitable zones, the areas around stars where liquid water can exist on the surface. Instead, roughly 80 per cent of potentially habitable worlds are completely dominated by land, and about 20 per cent are purely ocean worlds. The researchers came to this conclusion by modelling the relationship between water in a planet’s mantle and a planet’s recycling of continental land via plate tectonics. “We Earthlings enjoy the balance between land areas and oceans on our home planet,” Tilman Spohn, executive director of the International Space Science Institute in Switzerland and a member of the research team, said. “It’s tempting to assume that a second Earth would be just like ours, but our modelling results suggest that this is not likely to be the case.”
The results indicate that Earth’s ratio of land to sea (1:3) is finely balanced and that for most planets this ratio can easily tip over into mostly land or mostly sea. Spohn and his collaborator, Dennis Höning, a postdoctoral researcher at the Potsdam Institute for Climate Impact Research in Germany, concluded that the most likely time for this tipping point to occur is when a planet’s interior has cooled close to the temperature of Earth’s mantle – 1,410 degrees Celsius (2,570 degrees Fahrenheit) near the crust and as hot as 3,700 degrees Celsius (6,700 degrees Fahrenheit) at greater depths. How well subduction zones at the boundaries between tectonic plates can cycle water over land at this mantle temperature dictates whether a planet will be dominated by land or ocean.
Earth reached these conditions about
2.5 billion years ago at the end of the Archean eon, finding the delicate balance we live in today. However, over billions of years even Earth’s fine balance is unstable, although we don’t notice it because the rates of change are small, Spohn said. Other planets could have reached this tipping point much sooner. “In the engine of Earth’s plate tectonics, internal heat drives geologic activity such as earthquakes, volcanoes and mountain building and results in the growth of continents,” Spohn said. On the other hand, “the land’s erosion is part of a series of cycles that exchange water between the atmosphere and the interior. Our numerical models of how these cycles interact show that present-day Earth may be an exceptional planet.”
Spohn and Höning also considered other factors, such as how the outgassing of carbon dioxide contributes to the carbonsilicate cycle that acts as a planet’s longterm thermostat, controlling the climate over millions of years. They found that while both land and ocean-dominated planets could still be habitable, with similar temperatures if all else were equal, their life forms and climates might not be quite like Earth’s. “Their fauna and flora may be quite different,” Spohn said.
The models indicated that oceandominated planets with less than ten per cent land would likely be warm, with moist atmospheres and tropical climates, whereas land-dominated worlds with less than 30 per cent of their surfaces covered in ocean would be colder, drier and harsher than their ocean-dominated counterparts. On these land-dominated planets, cold deserts would stretch across the landmasses and vast glaciers and ice sheets would be common.