Life on Super-Earths may have more time to develop and evolve, thanks to their long-lasting magnetic fields protecting them against harmful cosmic rays, according to new research published in Science.
Space is a hazardous environment. Streams of charged particles traveling at very close to the speed of light, ejected from stars and distant galaxies, bombard planets. The intense radiation can strip atmospheres and cause oceans on planetary surfaces to dry up over time, leaving them arid and incapable of supporting habitable life. Cosmic rays, however, are deflected away from Earth, however, since it’s shielded by its magnetic field.
Now, a team of researchers led by the Lawrence Livermore National Laboratory (LLNL) believe that Super-Earths - planets that are more massive than Earth but less than Neptune - may have magnetic fields too. Their defensive bubbles, in fact, are estimated to stay intact for longer than the one around Earth, meaning life on their surfaces will have more time to develop and survive.
“While there are a lot of requirements for a habitable planet, such as a surface temperature that enables liquid water, having a magnetosphere that can protect against solar radiation for long periods of time could offer long durations of time for life to evolve,” Richard Kraus, lead author of the paper and a physicist at the LLNL, told The Register.
The key to long-lasting magnetic fields is having a liquid metallic core that cools more slowly. Earth’s magnetic field is generated by a layer of molten iron swirling around a solid iron core. Electrons in the liquid move to create electric currents that go on to power a magnetic field.
The temperature of the molten iron buried below 2,890 kilometers or 1,800 miles Earth’s surface, however, is chilling. It’ll eventually cool until it solidifies completely. At this point, its internal dynamo will cease spinning and it’ll no longer be able to support a magnetic field. Earth’s magnetic field will disappear in 6.2 billion years or so.
“When iron solidifies, it releases energy as well as lighter elements into the liquid iron, which provides the energy to power the dynamo over long periods of time. At some point the temperature of the liquid core will cool to the melting temperature, which means it will start to solidify,” Kraus explained. The iron inside of Super-Earth’s is compressed to much higher pressures than Earth, and its melting temperature is higher too.
In other words, the cores Super-Earths need to be cooled to much lower temperatures before they solidify. Their larger-sized cores also mean they lose heat at a slower rate than Earth’s too.
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“We find that super-Earth cores will take up to 30 per cent longer to solidify than Earth’s core…Owing to competing effects of stored energy versus surface area, the cores of planets smaller than Earth will solidify quickly, with the maximum time scale for solidification occurring in [Super Earths four to six times the mass of Earth],” the paper concluded.
Kraus and his colleagues were able to simulate internal conditions of a Super-Earth by studying the melting behavior of iron at pressures of 1,000 gigapascals - nearly three times the pressure of Earth’s core. The team zapped a tiny milligram fragment of iron with a series of lasers to compress it to increasingly high pressures.
The experiments showed that at 1,000 gigapascals, the melting temperature of iron is around 11,000 degrees Celsius. For comparison, Earth’s internal pressure is roughly 330 gigapascals and its core has a melting temperature of about 6,000 degrees Celsius.
“This is the first experiment to measure the melting curve of iron at pressures beyond 290 gigapascals, which means it’s the first to constrain the melting temperature of iron at the conditions of Super Earth cores,” Kraus told El Reg.
“Astronomers will use these results, as well as their observational data, to paint a better picture of what is happening in and on the surface of exoplanets.” ®