In February, NASA's Spitzer Space Telescope discovered the first system of seven Earth-sized planets orbiting a single star called TRAPPIST-1. The space community was abuzz with the discovery, which added this red dwarf star to a growing list of ultra-cool stars hosting possibly habitable planets.
When scientists are seeking planets that may be friendly to life, they're usually looking for three characteristics—a rocky surface, about the mass of Earth (for gravity's sake), and an orbit that is within its respective star's "goldilocks zone," where there's enough energy for liquid water, but not so much that the atmosphere begins to boil away.
But a new ten-year study of 10,000 red dwarfs from the Galaxy Evolution Explorer (GALEX) mission, a space telescope NASA launched in 2003 to observe 10 billion years of intergalactic history in ultraviolet light is not up to snuff.
GALEX's original purpose was to limn how galaxies and larger universal structures evolve and change. But scientists at MAST examined its data to study red dwarfs.They found that frequent solar flares from red dwarfs, which are typically cooler and smaller than our Sun and emit energy in infrared, might blow exoplanets' chances at habitability away.
"Flares [can] strip away atmosphere altogether by blowing it off the planet. It happens more often to larger, Jupiter-like planets—but it could happen to smaller Earth-sized planets, too," said Scott Fleming, astronomer and archive scientist at the Mikulski Archive for Space Telescopes (MAST), the official archive for all space missions NASA operates (going back to the 1970s).
Like our Sun, red dwarfs emit solar flares—sudden emissions of energy exploding from the Sun's surface with the force of one billion megatons of TNT. Some last for hours and put out massive amounts of energy, while others (called micro- or nano-flares) last only seconds or less, with less energy emitted. We on Earth don't really worry about the Sun's micro-flares, because they dissipate long before reaching us. And the likelihood of major flares reaching us is remote at best.
However, the relative proximity of planets like those orbiting TRAPPIST-1 means that unlike Earth they experience micro-flares at point-blank range.
The Hubble, Kepler, and Spitzer telescopes—which are all in space—and telescopes on the ground aren't equipped to monitor flares lasting only minutes or less. So Fleming and his team at MAST turned to GALEX because it's equipped with the astronomical equivalent of a high-speed shutter camera. Although the GALEX spacecraft itself was decommissioned in 2013, much of its data remains unexamined.
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"Project gPhoton is a collection of GALEX data that contains high-sampling measurements. With it, we sliced datasets down to a few seconds, the scale at which micro-flares happen. Of the 10,000 targets identified as red dwarfs, we've found more than a hundred flares," Fleming said. "The frequency of small flares with [red] dwarfs is large. That was expected."
This is significant because it means red dwarfs might regularly bombard rocky planets with enough micro-flares to rival radiation levels of the large solar flares described above. "If that much radiation makes it down to the surface, it can break DNA bonds," Fleming told me. In other words, even bacterium would have difficulty forming, let alone surviving long enough to evolve into more complex life-forms. The question becomes, he continued, what will prevent flares from irradiating an unsuspecting exoplanet's surface?
Hint: It's the stuff we breathe. Radiation levels depend on the atmosphere's thickness and chemical content. If you have lots of oxygen, then the planet probably has an ozone layer, which helps absorb radiation. Although space telescopes like Hubble have observed "compact" atmospheres around planets orbiting TRAPPIST-1, we won't begin to know the composition of their atmospheres until the James Webb Space Telescope begins its survey of potentially habitable planets next year.
But don't lose hope. Red dwarf stars aren't out of the game yet. "We're absolutely not stating that planets orbiting [red] dwarfs are uninhabitable," Fleming declared. But the threat to life by radiation from high-frequency flares adds a serious proviso that scientists must confront, before jumping to interstellar conclusions.