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Magnetic Fields May Protect Exo-Earths After All
Astronomers fear that Earth-like planets around dwarf stars cannot be protected by magnetic fields. Now they may be forced to change their minds
Back in September, astronomers in the US announced the discovery of an Earth-like planet orbiting a red dwarf star in the constellation of Libra. This planet called Gliese 581 g is about 3 times the mass of Earth and makes an orbit around its star every 37 days at a distance of only 15 million kilometres. That’s closer to its star than Mercury is to ours. However, because this star is smaller and cooler than ours, the temperature is probably about right for liquid water.
(That’s if Gliese 581 g exists at all. Since then, other astronomers have failed to see any sign of it.)
But the presence of liquid water is not the only condition that astrobiologists think necessary for complex life. Another is the presence of a magnetic field strong enough to protect the surface from the ravages of high energy particles from space, which would otherwise break apart the molecules of life.
Physicists think that Earth’s magnetic field is generated in the planet’s liquid metallic core by a dynamo effect. Two factors are crucial in keeping this going, the convection currents that generate a magnetic dipole, and the Earth’s rotation.
This leads to a problem as far as planets around dwarf stars are concerned. Astronomers have calculated that planets that form close to dwarf stars soon become tidally locked to their parent. That means that the same side of the planet constantly faces the Sun, like the Moon orbiting Earth.
The thinking is that when this happens, the dynamo effect cannot be strong enough to generate a magnetic field. So even if Gliese 581 g has liquid water, the surface may not be well enough protected from high energy particles to allow the formation of complex chemistries.
Today we get a new perspective from Natalia Gómez-Pérez and pals at the Carnegie Institution of Washington in DC. They’ve taken a closer look at the kinds of cores that exoplanets might have and conclude that they could be less dense if they contain lighter elements than ours and significantly larger too (as a proportion of the entire planet). That would allow the formation of a significant magnetic dipole.
“We predict that planets close-in to the star may have significant magnetic fields that allow a magnetosphere to be present,” say Gómez-Pérez and pals.
So that means conditions on Gliese 581 g may be even more conducive to life than originally estimated. Provided, of course, that it exists at all.
Ref: arxiv.org/abs/1011.5798: Predicted Dynamos For Terrestrial Extra-Solar Planets And Their Influence In Habitability.