Red or "M" dwarfs, the most common type of star, making up about 75% of all stars in the Milky Way, are prime hunting grounds in the search for Earth's Twin. Extrasolar planets were discovered orbiting the red dwarf Gliese 581 in 2005, about the mass of Neptune, or sixteen Earth masses. It orbits just 6 million kilometers (0.04 AU) from its star, and is estimated to have a surface temperature of 150 °C, despite the dimness of the star.
But the Kepler planets will be too far away –- hundreds or thousand of light-years -- for any follow-up observations to be able to determine if they are inhabited. All we will have from the Kepler data is planet mass, diameter, orbital period, and parent star type. The Earth clones will forever remain a blip on the exoplanet radar when it comes to determining true habitability.
But enough exoplanet research has been done so far that a cautious prediction can be made that the odds are that the 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.
Image Credit: NASA, The stunning image at top of page features spiral galaxy NGC 5792 a typical spiral, almost edge-on, with blue spiral arms and some dust features, what is cool is its neighbor, a red dwarf that is actually part of the Milky Way.