We’ve discovered nearby habitable planets, but we lack the science to say whether they’re inhabited.
By Michael White
The recent discovery of seven Earth-like exoplanets, orbiting a small star only 39 light years away, has scientists hoping that they finally have a realistic shot at answering one of our biggest questions: Is there other life out there? The planets, which orbit the star TRAPPIST-1 (named for the telescopes, not the beer), are roughly Earth-sized, rocky, and cool enough to hold liquid water — conditions that could possibly support life. And, importantly, the planets are close enough to be observed relatively easily. “Here, if life managed to thrive and release gases similar to that we have on Earth, then we will know,” one of the astronomers on the team that discovered the planets told the New York Times.
But to find life on the TRAPPIST-1 planets, or any others outside our solar system, isn’t going to be so easy. While our chances of detecting alien life are now better than they’ve ever been, the truth is that such a discovery is almost certainly still out of reach, because we lack both the technology and scientific knowledge to make it. Before we can point to a planet 230 trillion miles away and say definitively that life exists there, we’ll need to make more scientific progress.
“Our inference of the existence of life on another world will be probabilistic, an estimate of our confidence that life is the only reasonable explanation of the atmospheric chemistry of an exoplanet.”
For the foreseeable future, scientists won’t be able to make high-resolution images of the surface of such planets, much less send out a robot lander that can collect and analyze samples, the way the Curiosity rover does on Mars. In other words, we’re not going to be able to look directly at alien planets and actually see life, the way we see it around us here on Earth. Instead, scientists hope to infer the presence of life from gases present in an exoplanets’ atmosphere. Such gases, called “biosignature gases,” are ones expected to be produced only by living organisms, and not by non-living, geological processes. If we detect these gases in the atmosphere of a potentially habitable planet, then we have evidence that life exists there.
Detecting biosignature gases is the only realistic way in which we’ll find life outside our solar system in the near future. But there are some major challenges to overcome before a biosignature gas ever becomes a smoking gun for alien life. The first is that we haven’t yet built the technology we need to confidently measure biosignature gases in the atmospheres of distant planets. To detect these gases, astronomers measure the light that passes through a planet’s atmosphere as the planet orbits across its sun. As the light travels through the atmosphere, some of it is absorbed. Because different gases absorb light at different wavelengths, scientists can tell what’s in a planet’s atmosphere by measuring the wavelength the light passes through.
Unfortunately, none of the ground or space-based telescopes we have today were designed to do this for distant planets, though astronomers have, in some cases, been able to make due with existing telescopes. As planetary scientists Sara Seager and Wiliam Bain wrote in a paper on biosignature gases, “we are severely limited as to the underlying raw data we can collect from something as small as a shell of gas a few hundred kilometers thick around a faint planet tens of trillions or more kilometers away.”
If we’re going to find alien life on the TRAPPIST-1 planets or elsewhere, we’re going to need a new generation of telescopes carrying better instruments to measure the light passing through exoplanet atmospheres. One of these, the NASA/European Space Agency James Webb Space Telescope, which has been in development for decades, is slated to launch next year. This telescope will have much more light-gathering power than the aging Hubble telescope, the current premier space telescope. Also slated to launch next year is NASA’s Transiting Expolanet Survey Satellite, which is designed to find more than a thousand new planets by searching much deeper into space.
A more substantial boost in our capabilities will come almost a decade from now, when NASA plans to launch the Wide-Field Infrared Survey Telescope (WFIRST). WFIRST was not initially designed to image distant planets (it’s main mission is to study the universe’s mysterious dark matter), and so it’s advanced planet-imaging instruments are a late addition. To help WFRIST get a better look at distant planets, scientists are talking about building a new technology called a “starshade,” a giant screen that will float thousands of miles away from the WFIRST, and block out the direct light of distant stars. This will allow the instruments on WFIRST to better see the planets themselves. It’s much like putting your hand in front of your face to help you see better when someone shines a flashlight directly at your eyes.
Aside from the need for new telescopes, the other challenge we face is that scientists don’t yet know which biosignature gases will give the best evidence for life. The trick is to find a gas, or combination of gases, whose presence can only be due to the presence of life. On Earth, that gas is oxygen, which makes up 21 percent of the Earth’s atmosphere and is produced by photosynthesizing organisms. Without life, Earth’s atmosphere would have almost no oxygen. But that’s not necessarily true of other planets that might be dominated by different geological processes. One thing that astronomers have learned from their growing catalog of more than 3,000 extra-solar planets is how little we actually know about the surprisingly diverse kinds of planets that exist in the universe. And if there is life out there, we can expect it to be just as diverse, producing atmospheric biosignatures unlike anything we’re familiar with on Earth. Seager and Bain, who study the chemistry of potential biosignature gases, argue that, because of such uncertainties, we’re unlikely to be able to point out a single planet and confidently say, “Life exists there.” Instead, we should expect that “our inference of the existence of life on another world will be probabilistic, an estimate of our confidence that life is the only reasonable explanation of the atmospheric chemistry of an exoplanet.”
These challenges may sound daunting, but we shouldn’t be discouraged. To be on the cusp of actually finding alien life in the universe is an amazing achievement, the result of centuries of steady scientific progress since Galileo first aimed a telescope at the sky. Within the decade — assuming we continue to launch new telescopes and improve our understanding of biosignature gases — we may very well have observed enough planets to confidently say that some of these planets are not only habitable, but actually inhabited.