‘Super-Earth’ Exo Planets May Exist as Dead Zones

6a00d8341bf7f753ef013488d22630970c.jpg Rocky planets a few times heavier than Earth that were thought might be life-friendly may lack  a protective magnetic field that originates from an iron core that is at least partly molten.

A simulation of super-Earths between a few times and 10 times Earth's mass suggests that high pressures will keep the core solid, according to Guillaume Morard of the Institute of Mineralogy and Physics of Condensed Matter in Paris, France. Without a magnetic field, the planets would be bathed in harmful radiation, and their atmospheres would be eroded away by particles streaming from their stars.

The present-day Mars is a perfect example of a planet that lost its magnetic field. Planetary magnetic fields are created by massive molten metal currents within the planet's core.  A flowing current creates a magnetic field, even when the current is massive volumes of charged liquid metal moving under the influence of temperature gradients (convection).  But magnetic analysis of Martian sites by Berkeley researchers show that the red planet's protective field was switched off half a billion years ago.

Without the magnetic field Mars and perhaps Super Earths in the Milky Way are defenseless against the radiation that constantly pours in from space. Earth is thought to have survived the same space-bombing because of our superior size, with our dynamo maybe stuttering a little but — very importantly — not stopping.

So life would have trouble getting started on super-Earths, even if they lie in the habitable zone around their stars.

However, Vlada Stamankovic of the German Aerospace Center in Berlin reckons it is too soon to rule out molten iron cores — and magnetic fields — for super-Earths. Their interiors might get hot enough to melt iron, he says. "Actual temperatures could be much larger than assumed — we simply do not know."

About 1000+ exoplanets are now known, but most of these have been hot giants circling too close to their stars, because they are easiest to find. As techniques have been refined, smaller planets have come into view, and a recent survey by University of California astronomers, Andrew Howard and Geoffrey Marcy, concluded that about a quarter of all Sun-like stars should have Earth-size planets.

Analysis of the Kepler Mission "1000," may soon reveal Earth-size planets as well as the existence of magnetic fields and the possibility of life-bearing habitats.

Enough exoplanet research has been done so far that a cautious prediction can be made that the odds are that a life-bearing planet will orbit an M (red) dwarf star found in surveys taken within 100 light-years of Earth. Red dwarfs are much more numerous than sun-like stars, which exponentially increases the chances of being life favorable.

M dwarfs make up at least 70% of the Milky Way's stars. Their masses range from roughly half to one-twentieth the mass of our sun, but what M dwarfs lack in size, they more than make up for in longevity. Astronomers estimate that these stars can burn for 40 billion to 100 billion years, giving any habitable planets plenty of time to evolve life. (The life span of our own sun, a G-class star, is about 10 billion years.) But during at least the first few billion years of their lives, M dwarfs also sport huge magnetic fields that routinely interact with their atmospheres to create coronal mass ejections—enormous outbursts of matter from the star's highly ionized corona—and proton-rich flares.

The planet will be in the habitable, "goldilocks" zone around a red dwarf –– the zone where liquid water can remain stable on a planet’s surface. The zone will be closer to the cool red dwarf than the Earth's habitable zone to our Sun.

The profile of a planet in the habitable zone of a red dwarf includes an orbit completed in a mere two weeks, which will provide astronomers with multiple transits to enhance odds of being observed as well as being more likely to be in an orbit aligned along our Earth-bound line of sight.

Because they are much cooler than our sun, any potentially habitable planet would need to orbit them much closer than Earth does, putting it smack in the danger zone. But a new study indicates that these planets may be unexpectedly shielded from solar activity, keeping life safe.

"Overall, this is excellent news for planet hunters," says Alan Boss, a planetary scientist at the Carnegie Institution for Science in Washington, D.C., who was not involved with the study and who is part of NASA's Kepler mission to search for Earth-like planets. "This further buttresses the case that the first truly habitable world we find will likely by around a nearby M dwarf."

To find out if this solar nightmare would destroy any nearby habitable worlds, researchers led by astrobiologist Antigona Segura of the Universidad Nacional Autónoma de México (UNAM) in Mexico City turned to a computer model. The team simulated how a 1985 flare from AD Leonis (AD Leo), an M dwarf 16 light-years from Earth, would have affected a hypothetical Earth-like planet orbiting 0.16 astronomical units from the star. That's less than half Mercury's distance around the sun.

The simulation indicated that M dwarf stars are not as dangerous as feared. "When UV radiation from the star's upper atmosphere encountered the Earth-like atmosphere of our model planet, the energy resulted in a thicker ozone layer in the planetary atmosphere, providing a natural shield for the planetary surface," says astronomer Lucianne Walkowicz, a Kepler postdoctoral fellow at the University of California, Berkeley. That's because the UV radiation actually split molecules of oxygen to create more ozone than it destroyed. "Throughout most of the flare, the surface of our model Earth-like planet experienced no more radiation than is typical on a sunny day here on Earth," Walkowicz says.

The findings are especially good news, says Segura, because AD Leo is a young star—less than 300 million years old—and as a result is one of the most active M dwarfs known. The star's 1985 flare was 1000 times as energetic as a similar flare on our own sun. So the fact that the model planet's atmosphere survived such a violent event may bode well for planets around similar young M dwarfs, she says.

Young red dwarfs have awesome stellar flares that can erupt without warning and blast out lethal doses of ultraviolet radiation destroying surface life. Ocean life, however, may be safe from the UV just a few feet underwater and still extract enough light for photosynthesis.

In summary, he found that UV radiation actually split molecules of oxygen to create more ozone than it destroyed. The simulation made a thicker ozone layer in the planetary atmosphere such that the surface experienced no more radiation than is typical on a sunny day on Earth. What’s more, as the dwarf settles down to a quiescent existence, there would be very little ultraviolet light and an UV filtering ozone layer would not even be needed.

However, potentially habitable red dwarf planets may keep one hemisphere locked onto their star due to gravitational tidal forces. The resulting slow rotation may give them anaemic magnetic fields that do not block cosmic rays effectively.

The answers may be coming soon via the James Webb Space Telescope, scheduled for launch in 2014, would be used to spectroscopically 'sniff' out the exoplanet's atmosphere for chemistry that might be a by-product of organisms on the surface. If these planets develop a natural UV shield, then the discovery of an inhabited world may be no more than a decade away.

Casey Kazan via newscientist.com


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