Artist’s impression of “Super-Earth” planet LHS1140b orbiting a red dwarf star just 40 light-years away in the constellation of Cetus. The potentially rocky planet is 1.4 times the size and seven times the mass of Earth, and sits within the star’s habitable zone. Researchers have calculated that billions of the stars in the Milky Way have one to three planets in the “Goldilocks zone,” where there is the potential for liquid water and where life could exist. Credit: M. Weiss/CfA
(Download Image)Studies Provide New Insights into the Formation of ‘Super-Earths’
The odds that giant Earth-like planets outside our solar system could support life just got a little better.
Recent studies by LLNL researchers and their colleagues suggest that the early-stage formation of these "super-Earths" could result in the presence of liquid water near the surface, as well as a magnetic magma ocean at the core. Along with an atmosphere and a mild climate, liquid water and a magnetic field to protect against radiation are considered necessities for the evolution of life as we know it.
As astronomers discover more and more Earth-like planets in the "Goldilocks zone" of other solar systems—in orbits where it’s not so hot that water boils away, and not so cold that it’s perpetually frozen—scientists are increasingly intrigued by the possibility that the evolution of some of these terrestrial planets could result in conditions conducive to life.
Extrasolar planets are too far away to detect the presence of water or a magnetic field directly, but laboratory shock-compression experiments using high-energy lasers allow researchers to make inferences about the planets’ "melting curve"—how their primordial liquid magma oceans might cool during the early stages of their evolution.
Using the OMEGA laser facility at the University of Rochester, the researchers measured the thermodynamic properties of samples of enstatite, or magnesium silicate (MgSiO3), a common mineral found in rocks on Earth and in the mantle of terrestrial exoplanets. The experiments exposed the samples to pressures of about 230 gigapascals (2.3 million Earth atmospheres) until they liquified, then boosted the pressure to more than 380 GPa to determine their equation of state and other characteristics.
"We were able to measure some fundamental properties about how the liquid evolves with temperature and pressure," said LLNL physicist Dayne Fratanduono, lead author of a Physical Review B paper reporting on the new research that was published online on June 21. "We were then able to say some things about super-Earths—how shallow their melt lines are or how steep their melt lines are. Which then, as these large planets cool, gives you some indication of how the magma ocean would evolve, whether or not you may have a solid or a liquid core, and which types of super-Earths may support life and which ones may not."
Rapid Solidification
Contrary to simulations and previous studies, the findings suggest that "complete freezing of a deep silicate magma ocean could occur over a potential temperature range of only a few hundred degrees," the researchers said. "Having such a small range of potential temperature that separates a mostly liquid from a mostly solid planet," they said, "would imply that the planet would solidify rapidly, trapping the water that will be required to facilitate prebiotic chemistry on a potentially habitable super Earth."
"We think that in very large planets (three to 10 times the mass of Earth), for a very small temperature change the magma might freeze at once," said Fratanduono, "and as a result would be more likely to capture water close to the surface."
Magnetic fields are essential for life because they deflect harmful charged particles from the sun and cosmic rays that constantly bombard a planet and prevent the solar wind from eroding the atmosphere (see "Probing the Possibility of Life on ‘Super-Earths’"). "To generate a magnetic field," Fratanduono said, "you could have a magma ocean that is magnetic. You need a certain specific heat, and the specific heat for enstatite is high enough that a convecting magma core may be able to support a magnetic field.
"It (this research) is a tiny piece of the puzzle," he said. "That’s what we as scientists try to do—fill in the missing pieces. It’s pretty exciting, when the Kepler space mission is constantly discovering new super-Earths almost daily. It’s an exciting time."
Joining Fratanduono on the paper were LLNL colleagues Marius Millot, Rick Kraus, Peter Celliers, and Jon Eggert, along with Dylan Spaulding of the University of California, Davis, and Rip Collins of the Laboratory for Laser Energetics at the University of Rochester.
