Other Worlds, Other Tectonics, Other Life
Planets are warmed by their sun. Planets also has their own internal warmth that drives local volcanism and tectonics and helps keep water liquid and not frozen. Scattered around the Milky Way are stars that resemble our own sun—but a new study is finding that any planets orbiting those stars may very well be hotter and more dynamic than Earth and not due to their suns. That’s because the interiors of any terrestrial planets in these systems are likely warmer due to their radioactive composition than Earth—up to 25 percent warmer, which would make them more geologically active and more likely to retain enough liquid water to support life.
The preliminary finding comes from geologists and astronomers at Ohio State University who have teamed up to search for alien life in a new way.
They studied eight solar twins of our sun—stars that very closely match the sun in size, age, and overall composition—in order to measure the amounts of radioactive elements they contain.
They searched the solar twins for elements such as thorium and uranium, which are essential to Earth’s plate tectonics because they warm our planet’s interior. Plate tectonics helps maintain water on the surface of the Earth, so the existence of plate tectonics is sometimes taken as an indicator of a planet’s hospitality to life.
Of the eight solar twins they’ve studied so far, seven appear to contain much more thorium than our sun—which suggests that any planets orbiting those stars probably contain more thorium, too. That, in turn, means that the interior of their terrestrial planets are probably warmer than ours.
For example, one star in the survey contains 2.5 times more thorium than our sun, said Ohio State doctoral student Cayman Unterborn. According to his measurements, terrestrial planets that formed around that star probably generate 25 percent more internal heat than Earth does, allowing for plate tectonics to persist longer through a planet’s history, giving more time for life to arise.
"If it turns out that these planets are warmer than we previously thought, then we can effectively increase the size of the habitable zone around these stars by pushing the habitable zone farther from the host star, and consider more of those planets hospitable to microbial life," said Unterborn, who presented the results at the American Geophysical Union meeting in San Francisco this week.
"At this point, all we can say for sure is that there is some natural variation in the amount of radioactive elements inside stars like ours," he added. "With only nine samples including the sun, we can’t say much about the full extent of that variation throughout the galaxy. But from what we know about planet formation, we do know that the planets around those stars probably exhibit the same variation, which has implications for the possibility of life."
About 50% of the heat given off by the Earth is generated by the radioactive decay of elements such as uranium and thorium, and their decay products. That is the conclusion of an international team of physicists.
"The core is hot because it started out hot," Panero said. "But the core isn’t our only heat source. A comparable contributor is the slow radioactive decay of elements that were here when the Earth formed. Without radioactivity, there wouldn’t be enough heat to drive the plate tectonics that maintains surface oceans on Earth."
The relationship between plate tectonics and surface water is complex and not completely understood. But researchers are beginning to suspect that the same forces of heat convection in the mantle that move Earth’s crust somehow regulate the amount of water in the oceans, too.
"It seems that if a planet is to retain an ocean over geologic timescales, it needs some kind of crust recycling system, and for us that’s mantle convection," Unterborn said.
On Earth, most of the heat from radioactive decay comes from uranium. Planets rich in thorium, which is more energetic than uranium and has a longer half-life, would run hotter and remain hot longer, he said, which gives them more time to develop life assuming they are the right distance form their sun.
As to why our solar system has less thorium, Unterborn explained:
"It all starts with supernovae. The elements created in a supernova determine the materials that are available for new stars and planets to form. The solar twins we studied are scattered around the galaxy, so they all formed from different supernovae. It just so happens that they had more thorium available when they formed than we did."
For further information see Radioactive Heat.
World Core image via Wikipedia.