'Pale blue dot' planets like Earth may make up only 1% of potentially 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% of their surface covered by exposed continental land may make up only 1% of rocky worlds in stars' habitable zones, the areas around stars where liquid water can exist on a planet's surface. Instead, roughly 80% of potentially habitable worlds are completely dominated by land, and about 20% are purely ocean worlds, the study found.

The researchers came to this conclusion by modeling the relationship between water in a planet's mantle and a planet's recycling of continental land via plate tectonics.

Related: How plate tectonics, mountains and deep-sea sediments have maintained Earth's 'Goldilocks' climate

"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 in a statement. "It is tempting to assume that a second Earth would be just like ours, but our modeling 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, which is 2,570 degrees Fahrenheit (1,410 degrees Celsius) near the crust and as hot as 6,700 F (3,700 C) 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, and our planet found 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 (a greenhouse gas) contributes to the carbon-silicate cycle that acts as a planet's long-term 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 unlike Earth's.

"Their fauna and flora may be quite different," Spohn said.

The models indicated that ocean-dominated planets with less than 10% land would likely be warm, with moist atmospheres and tropical climates, whereas land-dominated worlds with less than 30% 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.

Spohn and Höning's results differ slightly from those of other research teams, however. For example, a study by Evelyn MacDonald of the University of Toronto found that for tidally locked worlds, the more land there is, the greater the average surface temperatures in general, Space.com previously reported. And perhaps the most famous study of land planets, led by Yutaka Abe of the University of Tokyo in 2011, found that land planets can remain habitable across much wider distances from their star than water worlds can and that they don't freeze over as fast because there is less water for ice and snow. However, Abe's study, along with others, agrees with Spohn and Höning's conclusion that land-dominated planets would be far more common than Earth-like or water-rich planets. 

Consequently, instead of looking for Carl Sagan's quintessential "pale blue dot," astronomers should be searching habitable zones for "pale yellow dots."

The results were presented at the European Science Congress, which took place in Granada, Spain, from Sept. 18 to 23, and the findings are described in the team's conference abstract.

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