Red dwarf star systems: Promising for life, but watch the apocalyptic flares

Red dwarf stars have several virtues that make them potential homes for Earth-like planets, but a new study suggests they also produce the largest solar flares ever seen.

Note from the Hubble Space Telescope: Looking for a home outside the solar system and considering that tempting-looking planet around an "old" red dwarf star? Check for flares from that star before you buy.

Astronomers surveying 210,000 red dwarf stars toward the Milky Way's center have found that a relative handful of the oldest among them are sending flares of hot gas hurtling into the so-called habitable zones of those stars – an activity that has typically been associated with the youngest red dwarf stars.

That does not bode well for any otherwise-attractive planet that might be orbiting in that zone, suggests Adam Kowalski, a graduate student in astrophysics at the University of Washington in Seattle.

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These stars were generating flares "much larger than the largest solar flares we've ever seen," says Mr. Kowalski.

He was part of a team conducting the survey, an effort driven in large part by the potentially rich hunting grounds red dwarfs present as locations for planets that might be hospitable for life. He presented the results during a briefing at the American Astronomical Society's winter meeting in Seattle, which runs through Jan. 13.

The trials of Gliese 581G

The interest is more than academic. Last September, a team of astronomers led by Steven Vogt of the University of California at Santa Cruz announced the discovery of two additional planets – including a possible Earth-mass planet, Gliese 581G – orbiting a red dwarf known as Gliese 581.

The team calculated that their candidate for an Earth-mass-planet was orbiting Gliese 581 in the star's narrow habitable zone – a region of space around the star where liquid water could remain stable on a planet's surface. Gliese 581 appears some 20 light-years from Earth in the constellation Libra.

It was a difficult detection, however, requiring data combined from major observatories in Hawaii and Chile. Two weeks later, Francesco Pepe, with the Geneva Observatory, reported that his attempt to provide independent confirmation of the discovery yielded no evidence for either new planet.

For now, Gliese 581G has been relegated to the scientific limbo known as "unconfirmed" discovery.

Rich planet-hunting grounds?

Similarly, red dwarfs – stars with less than half the mass of the sun – have had mixed success working their way onto some astronomers' lists of stars that might host habitable planets.

On the plus side, they are ubiquitous; astronomers estimate that such stars comprise roughly 75 percent of all the stars in the Milky Way. In addition, they burn faintly and slowly compared with a more massive star like the sun. The 4.6 billion years it took for life to evolve on Earth would amount to barely a blip on a red dwarf's time line – measured in trillions of years. The sun's estimated life time? About 10 billion years.

But a red dwarf's faintness also represents a potential Achilles' heel for habitable planets. A red dwarf's habitable zone is very close to the star, by some estimates between 10 million and 33 million miles out. (By contrast, the sun is 93 million miles from Earth.) And a red dwarf's habitable zone is narrow, perhaps only 400,000 to 4 million miles wide, depending on the mass of the star. The habitable zone in our solar system spans roughly 37 million miles.

What a flare would do to life

Some astronomers have argued that, among other downsides, a planet in a red dwarf's habitable zone would be vulnerable to massive flares and other outbursts from the star, bombarding it with high-energy particles, X-rays, and ultraviolet radiation at levels that would disrupt biological activity on Earth's surface.

Until now, however, many thought such activity occurred only for the first 1 billion or 2 billion years of a red dwarf's existence. Thus, if an Earth-mass planet formed beyond the reach of the flares, and migrated inward with time, the planet could take up its final position in the habitable zone after the young star calmed down.

But the stars the team studied were some 10 billion years old, suggesting that energetic flaring continues, even if less frequently.

Kowalksi cites one "megaflare" as particularly noteworthy.

"I literally have nightmares about this flare," he says. The star's brightness in ultraviolet light – damaging to fragile biological molecules such as DNA – increased by more than 20,000 percent during the event's peak.

Struck by such a flare, an atmosphere with ozone in it would lose it within two years of the event, with effects lasting up to 50 years. That would render any forms of life on the surface vulnerable to additional flares that would be likely to occur within that period.

During the survey, the team spotted 100 flares over a seven-day period in visible wavelengths that brightened their stars by up to 10 percent – a far larger increase than those generated by the sun's flares.

The team posits that one reason the flares are so powerful is that the stars are part of binary systems and their magnetic fields are interacting in ways that boost the energy the flares release.

Could life survive?

Still, all may not be lost in any extraterrestrial real estate search involving red dwarfs.

Three years ago, NASA's Astrobiology Institute held a workshop for astronomers and astrobiologists at the Seti Institute in Mountain View, Calif., that gave red dwarfs a fresh look.

In a summary of the meeting's discussions, which looked at everything from geological and atmospheric processes on such a planet to the radiation environment it would encounter from the star as well as from deep space, the group concluded that potential hardships imposed by a planet's close proximity to the star could, under the right conditions, still allow for microbial and even plant life on the planet's surface – a conclusion the group said "must be treated with caution."

They also suggested that these hardships also imply that the appearance of life could be a blink-and-you-miss-it affair, particularly given the long lifetime calculated for red dwarfs.