Detecting Life In Space: The Red Edge
The universe's "most interesting star" just started acting up again.
Known as KIC 8462852, or the WTF star (for "Where's The Flux"), this sun became famous a few years ago for its strange short-lived dips in light output. Its behavior was weird enough that astronomers added "alien megastructures" to the long list of possible explanations for WTF star's changes in brightness. My twitter feed was lighting up this weekend as WTF star started, once again, to show a pronounced diminution.
The excitement over WTF star is, ultimately, about finding evidence for life on other planets. But even without alien megastructures (which are probably not going to be the explanation), WTF star highlights the remarkable range of ways astronomers might be able to find evidence for life on distant planets.
Since we'll be staring hard at thousands of these worlds over the next few decades, each kind of life detection method is exciting in its own right. That's why today I want to introduce you to "The Red Edge" (which, also, just sounds pretty cool).
The story starts with one of space exploration's greatest tragedies.
Back in 1986, the Space Shuttle Challenger exploded during launch, killing the entire crew. The loss was devastating for so many reasons, but one of its consequences was that the Galileo mission to Jupiter was left without a launch vehicle. The plan had been for it to ride a shuttle into space, where a powerful booster would then send it on to a half-billion-or-so-mile ride to the solar system's largest world. In the aftermath of the Challenger disaster, NASA scientists were forced to launch Galileo using a less powerful booster, placing it onto a complicated trajectory that used the gravity of a number of planets to slingshot the spacecraft out to Jupiter. In particular, Galileo first headed inward to Venus and then back to Earth — twice — picking up speed with each gravity "assist." It was during these Earth-return fly-bys that scientists came to the novel idea of viewing our planet as if it were an alien world.
The idea was, in part, Carl Sagan's. As Galileo passed by Earth, it trained its instruments on our planet and recorded the light it reflected from the sun. In this way, scientists could run an experiment to see if Earth's light "signal" could reveal the presence of life.
Of all the features the scientists found in the Earth-light, one of the most important came in the form of a sharp change in our planet's reflectance just past the wavelengths of red light. Reflectance means how much of the sunlight falling on the planet gets bounced back into space. Low reflectance means much of the light is being absorbed, while high reflectance means the opposite. The Galileo data showed that Earth's reflectance depended strongly on the light's color (meaning its wavelength). Most of the light in the visible spectrum was strongly absorbed. But around the wavelength of red light (past a half of a millionth of meter) the reflectance shot up. This wall in the reflectance curve was known from remote sensing studies and was called The Red Edge. In a paper titled "A Search for Life on Earth From The Galileo Spacecraft," Sagan and his co-authors highlighted the planet's absorption and reflection properties as one of four critical pieces of data that confirmed Earth was, in fact, a living world.
But what causes the Red Edge and why might it be a cosmic life-detector? The answer lies within even the most lowly houseplant.
Almost all of the world's vegetation will strongly absorb light in the blue and green parts of the spectrum. Much of this is due to photosynthesis, since these are the wavelengths of light plants absorb to make sugars and power their metabolism. At longer wavelengths, the leaves have no need to absorb sunlight, so they've evolved to reflect this light back into space. So what Galileo was seeing as it flew by Earth was the combined absorbing power of an entire planet's worth of vegetation.
Of course, Sagan and other astronomers knew that what works for Earth may work for alien worlds, too. Maybe we could detect Red Edges in the spectra from other worlds. If so, wouldn't that be proof of alien life and, even better, of full alien biospheres?
Astronomers are now deep in their "exo-planet" revolution. That means they're learning how to probe the light from these worlds in ways that, in principle, mimic what Galileo did for Earth. The crucial difference, of course, is distance. These planets are light-years away. That means we get just a minute fraction of the light they reflect from their stars. Astronomers must be extremely clever in how they go about their searches for exoplanet Red Edges. That's a challenge they're already working to overcome.
One early example of of these efforts is a well-regarded paper from 2005 by Sara Seager and collaborators titled Vegetation's Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants. The team laid out arguments for how the Red Edge might be detected in light from other worlds. They also made the important case that evolution on any planet might lead to some kind of feature like the Red Edge. As they put it, " ... while extraterrestrial 'light-harvesting organisms' have no compelling reason to display the exact same red edge feature as terrestrial vegetation, they might have similar spectroscopic features at different wavelengths than terrestrial vegetation." Thus a Red Edge of some kind, may, perhaps be universal.
For years now, movies and TV have taught us that the only way to find life in the universe was to directly detect purposeful signals from intelligences on those worlds. With the advent of the exo-planet revolution, a new era has opened. The WTF star raised the issue that we may detect intelligence by the artifacts they leave. Even more important, and more likely, is that we might find evidence of less-evolved forms of life by the imprints they leave on the planet as a whole. The lesson of the Red Edge is that life — even plants — can change planets in ways that might be visible from across the depths of space.
Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4
Copyright 2021 NPR. To see more, visit https://www.npr.org.