It takes more than four years for its light to reach us, but Proxima Centauri is one of our closest neighbors. The star orbits in the constellation Centaurus, visible in the Southern Hemisphere, but is itself too faint to see with the naked eye. Proxima isn’t like our sun; it is smaller, dimmer, and cooler. These suns are prone to frequent flares of ultraviolet radiation, which can be bad news for planets orbiting too closely.
Some scientists think the bursts could strip away entire atmospheres and boil off oceans. But others think these conditions, as ferocious as they might be, could actually give rise to life.
That’s the hope of Lisa Kaltenegger and Jack O’Malley-James, who chat often about alien life over coffee at work—a typical office discussion for a pair of astronomers at the Carl Sagan Institute at Cornell University. Because Proxima has a planet, maybe even two. The known planet, Proxima Centauri b, is about the same size as Earth, and might be rocky like it, too. It resides in that magical slice of solar systems, the habitable zone, where conditions are not too cold or too hot for liquid water to burble on the surface.
Potential life on Proxima b—on any planets around other stars—probably won’t resemble the kind on our planet, Kaltenegger says, but earthly beings are the only blueprints we have. So the astronomers wondered, What happens here when ultraviolet radiation from the sun smacks into life-forms on Earth?
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.
Imagine if astronomers found a planet around an M dwarf that kept its atmosphere long enough for water to pool on its surface. Kaltenegger envisions a world where fluorescent organisms teem in a shallow, transparent ocean wrapped in a thin, wispy atmosphere. Kaltenegger and her team’s research, published earlier this month, suggests that a fluorescent signal would be easier to detect beneath a mostly cloud-free sky. Below, the clear water would teem with organisms unburdened by ultraviolet radiation—in fact, shaped by it. The prospect of this alien world, Kaltenegger says, “counterintuitively makes highly active flaring stars good places to look.”
Other astronomers who work with exoplanets say the idea of glowing alien corals on other worlds is an intriguing one. Biofluorescence on Earth—which comes not only from marine creatures, but from vegetation and other sources—is too small to detect from great distances. “In order for the signature to be detected, it would have to be relatively strong—stronger than is typical for biofluorescent life-forms on Earth—and global,” says Jens Hoeijmakers, an astronomer at the universities of Bern and Geneva in Switzerland. “Even then, this signature would be very hard to detect using instruments that will be available to astronomers in the near future.”
Astronomers would need to rule out other sources of fluorescence, including nonliving ones. “We have fluorescent rocks that make for nice demonstrations of fluorescence due to minerals they contain,” says Michael Latz, a biologist at the University of California at San Diego who studies bioluminescence, a glow that arises from chemical reactions unrelated to radiation exposure. “And you have probably seen the fluorescence of fibers in clothing. So biofluorescence is not the exclusive source of fluorescence.”
Kaltenegger’s study is another entry in the growing list of potential signatures to consider in the search for life beyond Earth. The suggestions range from the Earth-inspired (familiar molecules in atmospheres, light from photosynthetic plants) to sci-fi fantasies (giant structures that harvest energy from stars, blast shields that protect civilizations from lethal cosmic rays).
Barring the surprise invention of warp speed, human beings will never visit Proxima Centauri b, where the sky, if it has one, would be a brew of purples and oranges instead of our familiar blues. It will be years before a new generation of telescopes, now under construction, can take a closer look at the planet and other rocky worlds around flaring stars. Perhaps they might detect a strange soft glow from life molded by another star, a sign that we have neighbors, and that they left the lights on.