They found the answer in the sea, in coral reefs that glow in the dark.
According to scientists who study them, corals in shallow waters have found a way to guard against the worst of the sun’s rays. Fluorescent pigments in the invertebrates absorb damaging ultraviolet light, transform it, and then emit it as harmless, visible light. The instantaneous change in the wavelength of the light, from long to short, produces a brilliant show of colors, from pinks and purples to greens and reds. (The process can protect single-celled organisms that live inside the coral and supply food in exchange for shelter.)
“If such a strategy were beneficial for life on another world, it should be very likely for other life-forms to also evolve such a biofluorescent strategy,” says Kaltenegger, the director of the Sagan Institute. “If you and I would have evolved on such a world, we would probably glow, too.”
If such life were to exist on planets around Proxima Centauri, those creatures might not perish following a powerful flash. Instead, they would light up. And maybe someday, Kaltenegger says, advanced telescopes might be able to detect that glow, if it’s there.
Astronomers have discovered thousands of exoplanets in the Milky Way galaxy in the past two decades, and most of the rocky, Earth-size ones orbit stars like Proxima Centauri, known as M dwarfs. Some, such as Proxima b, even orbit in their star’s habitable zone.
The methods astronomers use to find lurking exoplanets make M dwarfs a favorite target. Their size makes it easier for telescopes to observe a faint wobble in the star, a sign that a planet could be on its way around, and the dimness helps telescopes discern a roving exoplanet against the glare. And there are so many of them: M dwarfs are the most numerous type of star in the galaxy. Our sun is surrounded.
“It’s a little bit weird, actually, that we on Earth are orbiting a star like the sun,” says Laura Kreidberg, an astronomer at the Harvard-Smithsonian Center for Astrophysics who studies exoplanet atmospheres (a beguiling thought for another time).
But the same factors that make M dwarfs excellent candidates for exoplanets might make the stars unsuited for supporting life. Because M dwarfs are dimmer than stars like our sun, planets have to huddle much closer for warmth. But the cozier these worlds are with their stars, the more they feel the effects of ultraviolet flares and winds.
Such proximity suggests that these exoplanets might be tidally locked, with one face perpetually turned toward the star and the other out to space. The illuminated side might be scorching and barren, and the darkened side frigid and icy. A potential atmosphere could be boiled away or frozen off. Kreidberg recently reported about the existence of a planet nearly 50 light-years away with a missing atmosphere, and she suspects that the M dwarf it orbits is responsible. She says other planets could hold on to their atmosphere if, like the early Earth, they have water vapor and carbon dioxide in their interior, which could break through the surface and replenish a vanishing ocean. “For every idea about how to get rid of an atmosphere on one of these planets, there’s another idea for how you can gain one back,” she says.