Massive Earth-like planets that have both continents and oceans may be better at harboring extraterrestrial life than those that are water-only worlds. A new study gives hope for the possibility that many super-Earth planets orbiting distant stars have exposed continents rather than just water-covered surfaces.
Researchers typically expect super-Earths to exist as water-worlds because their strong surface gravity creates relatively flattened surface geography and deep oceans. But the new study found that super-Earths with active tectonics can have exposed continents if their water is less than 0.2 percent of the total planetary mass.
"A planet could be ten times wetter than Earth and still have exposed continents," said Nicolas Cowan, a planetary scientist at Northwestern University and co-author on the new paper. "That's important for what the planet looks like and how it ages."
Cowan and Dorian Abbot, a climate scientist at the University of Chicago, built the model in the study. The model uses Earth as a starting point in defining how a planet's water distribution could end up balanced in a steady state between the surface oceans and the mantle, which allows the researchers to calculate whether a super-Earth is likely to be a water-world or not.
The movement of tectonic plates on Earth transfers water continuously between the surface oceans and the mantle. Ocean water enters the mantle as part of deep-sea rocks when one tectonic plate slides under another and goes down into the mantle.
"Earth is the only known planet with plate tectonics, a deep water cycle, etc., so it's a good place to start," Cowan said. "On the other hand, if it turns out that Earth's deep water cycle in nowhere near a steady-state, then our conclusions are way off the mark. "
Water in the mantle can re-enter the ocean when volcanic activity splits the planet's crust at mid-ocean ridges. The loss of the crust causes a drop in pressure that leads the underlying mantle rock to melt and lose volatiles such as water. (An additional twist is that super-Earths with their stronger gravity could have greater seafloor pressure that suppresses the mantle's loss of water, so that more of the planet's overall water remains in the mantle.)
There are other uncertainties that could make a big difference in the model's accuracy in predicting a super-Earth's likelihood of having dry continents. One unknown is the amount of water hidden deep within Earth's own mantle; Cowan and Abbot cite estimates of one to two oceans worth of water. Another factor is whether or not super-Earths have tectonic processes. If the researchers' assumptions about either factor are wrong, that would change their model's calculation of the "water-world boundary," which represents the mathematical model's dividing point between water-worlds and worlds with dry continents.
"If some of our input parameters are wildly off, then the actual water-world boundary might differ by an order of magnitude," Cowan said. "No matter how you cut it, though, the water-world boundary is unlikely to be as damning as previously thought."
The debate over super-Earths will continue until space missions begin collecting hard data on how much water exists on such planets. A space telescope with an interior coronagraph or exterior starshade could block the blinding light of distant stars to get a peek at orbiting planets. But no active space telescopes can currently do the necessary work of mapping the surface of super-Earths.
""At the very least, you'd need a space telescope with a mirror a few meters wide, coupled to a starshade tens of thousands of kilometers away," Cowan explained. "NASA is mooting this idea, but it is not the next priority." One space telescope that could fit the bill would be NASA's Wide-Field Infrared Survey Telescope (WFIRST) — a planned 2.4-meter telescope with an instrument for imaging exoplanets. The $1.6 billion mission remains up in the air until NASA can squeeze it into the budget, but Cowan expects that WFIRST could get off the ground by the mid-2020s or 2030s. If so, that would bring researchers one step closer to understanding whether super-Earths truly work like our own world.
Research by astronomers at the Harvard-Smithsonian Center for Astrophysics in 2012 showed that if Earth had been slightly smaller and less massive, it would not have plate tectonics - the forces that move continents and build mountains. And without plate tectonics, life might never have gained a foothold.
"Plate tectonics are essential to life as we know it," said Diana Valencia of Harvard University. "Our calculations show that bigger is better when it comes to the habitability of rocky planets."
Plate tectonics -the movement of huge chunks, or plates, of a planet's surface- are crucial to a planet's habitability because they enable complex chemistry and recycle substances like carbon dioxide, which acts as a thermostat and keeps Earth balmy. Carbon dioxide that was locked into rocks is released when those rocks melt, returning to the atmosphere from volcanoes and oceanic ridges.
"Recycling is important even on a planetary scale," Valencia explained.
Valencia and her colleagues, Richard O'Connell and Dimitar Sasselov (Harvard University), have examined the extremes to determine whether plate tectonics would be more or less likely on different-sized rocky worlds. In particular, focusing on "super-Earths"-planets more than twice the size of Earth and up to 10 times as massive.
"It might not be a coincidence that Earth is the largest rocky planet in our solar system, and also the only one with life," said Valencia.
"There are not only more potentially habitable planets, but MANY more," stated Sasselov, who is director of the Harvard Origins of Life Initiative.
In fact, a super-Earth could prove to be have volcanic "rings of fire" that could span the globe while the equivalent of Yellowstone Park would bubble with hot springs and burst with hundreds of geysers. An Earth-like atmosphere would be possible, while the surface gravity would be up to three times that of Earth on the biggest super-Earths.
Sasselov observed that although a super-Earth would be twice the size of our home planet, it would have similar geography. Rapid plate tectonics would provide less time for mountains and ocean trenches to form before the surface was recycled, yielding mountains no taller and trenches no deeper than those on Earth. Even the weather might be comparable for a world in an Earth-like orbit.
"The landscape would be familiar. A super-Earth would feel very much like home," said Sasselov
The Daily Galaxy via Astrobio.net and Harvard-Smithsonian Center for Astrophysics
Image credit: David A. Aguilar (Harvard-Smithsonian CfA)