Jan 12, 2022 8:00 AM

The Search for ET Has an X-Factor: the Evolution of Stars

To support life, a planet has to stay in its star’s “habitable zone.” But research on a nearby star shows that life-friendly zones won’t stay that way forever.

Stephen Kane was searching for stars that could host planets with warm, temperate climates hospitable to life—you know, like Earth—when he glimpsed a young red dwarf called AU Microscopii that’s “only” 32 light-years away from home.

“The star’s a complete infant, when it comes to planetary systems. That means we have an opportunity here to observe a planet at the very earliest stages of the planet evolving,” he says. So Kane, an astrophysicist at the University of California at Riverside, and his colleagues used the star as a laboratory and as a model for others like it, projecting its future life. That helped them figure out when the planets orbiting it might fall within the star’s “habitable zone”—a distance that is neither too hot nor too cold to support life. They found that the star would blaze brightly at first, then calm down and burn less intensely, so that the range of life-friendly spots would move closer toward the star by about 30 to 40 percent during the star’s first 200 million years. They published their work this month in the The Astronomical Journal.

That’s significant for Kane and other scientists, who hope to one day catch sight of a life-friendly world beyond Earth, with verdant ecosystems teeming with alien life-forms, because it suggests that a planet in a habitable spot might not stay habitable forever. For the best-case “Goldilocks” scenario, everything has to be just right, including a temperature that allows the planet to have liquid water on the surface—a prerequisite for life as we know it. (Life as we don’t know it is another story.) Other factors matter, too, like a breathable atmosphere, a stable climate, and enough protection from harsh ultraviolet radiation. Mars, for example, is in our sun’s habitable zone, but it lost its water and most of its atmosphere eons ago. Venus lies on the inner edge of the zone, but thanks to its veil of carbon dioxide, it’s blistering hot.

AU Microscopii gives scientists a glimpse at how that zone might grow or shrink over a star’s lifetime. “These red dwarf stars have a very long, very badly behaved teenage phase. It can be hundreds of millions of years before a star like this finally settles down like an adult,” says Sara Seager, an MIT astrophysicist and former deputy science director of NASA’s planet-finding mission called TESS.

Kane and his team show that since their red dwarf and other stars like it can act like teenagers for a while, a currently inhospitable world might become more amenable to life down the road. But the reverse could happen too: “A planet that is in the habitable zone now may not still be there once the star is changing,” he says.

If the host star cools down quite a bit, the planet could become too frigid for any ET’s eking out a living on it; lakes and rivers would gradually freeze. On the other hand, much older stars usually eventually heat up, so aliens who were once in a life-friendly spot could eventually see the water necessary for life boil away, as anything on their planet’s surface gets baked to death.

But maybe that doesn’t have to be the end of the story. As Jeff Goldblum puts it in Jurassic Park: “Life finds a way.”

“If you're a bacterium, you can mutate rapidly and adapt to the situation,” says Seth Shostak, senior astronomer of the SETI Institute in Mountain View, California, named after the search for extraterrestrial intelligence. Other creatures might find better conditions in caves or underground. “If you’re an intelligent, more advanced species, you can do something about it,” he says, perhaps developing technologies that reflect lots of starlight or allow you to move out—or even move the planet.

The odds of life finding the right conditions to arise at some point in a star’s life span are good around one like AU Microscopii, because red dwarf stars aren’t like our sun. They’re smaller, cooler, and live for an incredibly long time. At 4.6 billion years old, the sun is almost middle-aged, while red dwarfs last for 100 billion years. “If you're looking for ET, a planet around these stars will have billions of years of time to develop complex life. And the number of these things is so high, that makes them appealing,” Shostak says. Red dwarfs are the most common type of star, he says, while massive, short-lived, hotly burning stars are much rarer.

Stars that are smaller and cooler than the sun, like red dwarfs, have their habitable zone much closer to the star than the sun. It's the opposite for bigger, hotter stars.Illustration: Dana Berry/NASA/Kepler Mission

These stars have a complicating factor, however: They’re known for flinging out flares and harmful radiation, especially in that early, active phase that AU Microscopii is going through. If many intense flares in close succession throw high-speed clouds of charged particles at a planet, they’ll blow away its protective atmosphere, sort of like how massive storm surges can erode a beach, leaving the shoreline behind it vulnerable. And if the atmosphere goes, so does the water, which is likely what happened on Mars some 3 billion years ago.

Still, a world in the habitable zone might not turn into a rocky husk just because it gets hit by that radiation, Kane says. It might have gases stored in its interior that get vented out by geological activity, like volcanoes, replenishing the atmosphere as it’s eroded, according to his and his colleagues’ models. Even Earth underwent an early period of getting bombarded with lots of UV radiation, though it was not as intense as what red dwarfs generate. Despite those initially hostile conditions, things improved dramatically.

And that UV radiation might actually be important for life, Seager says. Some research shows that it could jump-start the chemical processes that led to the first replication of microbes, and later, to more complex organisms.

Alien life-forms could survive even if a planet heats up as the star’s life-friendly zone migrates outward. Some lost water could be restocked, perhaps by impacts from comets, argues Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell and a member of the TESS collaboration. Alternatively, little alien critters also could continue on even if the world freezes over. “When the star becomes brighter because it ages, then you could thaw that ice, and then this huge biosphere could become uncovered,” she says.

So red dwarfs remain a promising place to look for signs of ET. Using TESS’ predecessor, the Kepler Space Telescope, astronomers have already found more than 4,000 planets, with the majority of them orbiting red dwarfs. But because of the limits of that telescope’s detection capabilities, many of the worlds that have been spotted are ones that orbit extremely closely to their stars and are likely too hot to be habitable. That’s the case with AU Microscopii, where two giant planets have been discovered far too close to the star for comfort. Kane holds out hope that more sensitive telescopes will discern more worlds in the habitable zone.

But if scientists like Kaltenegger and Shostak are right, and the aliens on planets that have aged out of their star’s habitable zones are now hiding out under an icy crust or in a cave, those life-forms will be hard to detect. It’s much easier to search for telltale gases, like oxygen and methane, which escape into the atmosphere and can be spotted by faraway astronomers using the James Webb Space Telescope. That’s another advantage of the habitable zone: It’s a range of worlds that are relatively easy for us to search remotely for biosignatures or even technosignatures, like alien smog. And astronomers need to carefully choose where to point their most powerful telescopes.

That’s where Kane and his team came in, demonstrating that red dwarfs can be a stable, comfortable place for habitable worlds to orbit for billions of years—once the star’s all grown up, that is. “This paper’s interesting because they’re trying to study how a star behaves in the first tens of millions of years of its life. That would’ve been impossible to do when I was a graduate student,” Shostak says. “Understanding how stars work tells you which stars to look at for signs of life and which exoplanets we should be excited about.”