Carl Sagan described Earth as viewed from space as a pale blue dot, and our first direct images of light-years distant planets will be just as minuscule. When new mega-telescopes capture their first pictures of exoplanets, we will at best see half pixels of grayish blur. Even so, investigators eager to learn whether any exoplanets harbor life might be able to find hints in those first fuzzy images. First, however, they will need to know what biosignatures would look like in data coming from worlds very different from our own.
Many teams are now focusing on finding answers. The latest entrant is Cornell’s Institute for Pale Blue Dots, officially launching on May 9 (and also just renamed the Carl Sagan Institute). The institute has been working to create a database of "fingerprints" for life that could be discerned in the light reaching telescopes from exoplanets.
That is because much of the early information about distant planets will come in the form of an electromagnetic spectrum: the wavelengths a planet radiates, either directly or via light from its star shining through its atmosphere. This spectrum can reveal the chemicals in the exoplanet’s atmosphere and, sometimes, on its surface. Earth, for example, would look green from all the photosynthesizing plants (plus blue from water, with a hint of pearly clouds) and would offer other signs of life as well. Our spectrum would reveal the presence of water vapor—a strong hint that the planet is amenable to life—as well as abundant oxygen and methane. That latter combination over time would indicate oxygen was being renewed somehow, because methane degrades oxygen and yet the oxygen does not disappear. Such a process would be an indication of life, which is one of the most likely sources of renewed oxygen in large quantities. Similar signatures on other planets—indicative of oxygen, methane, water and maybe even the green of photosynthesizing plants—would suggest they were amenable to carbon-based life like that found on Earth.
Of course, an exoplanet's spectrum could be very different from our own, making signs of life harder to parse. If the atmosphere is full of hydrocarbons, like the surface of Saturn’s largest moon Titan, it would be hard to make out anything closer to the surface through the haze. Similarly, dense cloud cover, like on Venus, would reflect light back and obscure the other gases below. Or vigorous geologic activity venting gas might obscure smaller amounts of other gases being created. And so researchers are simulating what a planet’s spectrum would look like in such cases—and many other scenarios, from a dust-covered dune world to one covered with water or circling a very dim star. “In a way, it’s a CSI for exoplanets,” Lisa Kaltenegger, an astronomer and head of the new institute, says about the burgeoning database of those simulations. Much like forensic investigators identify who committed a crime from signs like fingerprints and DNA, exoplanet researchers will be able to compare that database of spectra with real measurements from planets, working backward to see what kind of body generates that spectrum. Kaltenegger notes that she does not want to miss signs of life just because they occur on a planet bigger, smaller, hotter, colder, younger or older than Earth and the other planets in our solar system.
The most intricate atmospheric simulations available were built for Earth and incorporate details specific to its atmosphere and geography. Although they are good for weather forecasting and precise analysis researchers don’t have enough details about other planets to build something so complicated for them. Instead, Kaltenegger’s group is focusing on simpler, “one-dimensional” simulations that model the whole climate and atmosphere uniformly, as if you only had one glimpse of the whole thing. A one-dimensional model can incorporate and explore the effect of all the different types of gases, planetary structures, types of stars and life you could imagine, but treats a planet’s atmosphere as one uniform mixture; it is not tracking clouds moving over the surface but rather averaging all the water vapor in the air at once. And it is well suited to help astronomers understand the very first planetary images they will see, which will be a single point anyway.
As telescopes become more sophisticated, astronomers will gather more detailed information about an exoplanet’s properties than can be gleaned from the best instruments we have now or are currently building. Telescopes that can image the planet directly, with reflected light from its star, will fill in missing details about rotation, geography and seasons. Such information can help to reveal what features life would need to survive on a given world. A tidally locked world, with one side always facing its sun, for instance, would have very different conditions than its averaged environment might suggest—bitterly cold on one side and fiercely hot on the other—and be hospitable to different types of organisms than those that might be found on a uniformly temperate planet.
And simulation, as it gets more detailed, can reveal the unexpected: When Dimitar Sasselov, an astronomer at Harvard–Smithsonian Center for Astrophysics, helped model a planet covered entirely in water, he discovered a totally unknown form of “warm ice” at the bottom of that vast ocean where the pressure pushes the water at the ocean floor into a dense solid form, and waves might move continuously across the surface, never breaking. This scenario allows scientists to consider what features life would need to arise and persist under those conditions, even biology alien to anything we know now.
Simulation can also unmask signatures that seem to represent life but could be created by nonbiological processes. Victoria Meadows, principal investigator at the University of Washington's Virtual Planet Laboratory, says that trying to predict the signatures that might fool them has led to many of her group’s discoveries. For instance, the lab recently released a paper on four separate ways oxygen could be generated without life being involved. Knowing those, they can work out measurements that telescopes will have to take to discount those alternate causes. They can also pinpoint which spectral fingerprints would be most telling of potential life.
These simulation tools are only the first step: to identify what form life might take and what signatures those forms might provide, astronomers are partnering across disciplines and taking unfamiliar excursions into the biology lab. For instance, Kaltenegger’s group has studied the spectra of 137 microorganisms, including extremophiles that thrive in Earth's most inhospitable environs. This color catalogue provides the data that advanced telescopes would see if a planet's dominant organisms were suited for very different environments so we might recognize them from afar. Sasselov’s Origins of Life Initiative brings people from all disciplines to run experiments exploring the ingredients needed to create life and the steps by which it forms. In essence, they are asking: If we looked at the early Earth through a telescope, how would we recognize the life on it? They are also pondering how—and what—completely different life-forms might arise. “There’s going to be a lot of things we haven’t considered but we’re trying to come up with as diverse and fascinating a world as we can,” Kaltenegger says, “to make the parameter space large, to not miss signatures if we can help it.”
The Kepler space telescope revealed just how common exoplanets are by spotting the slight dimming of starlight they cause when transiting, or passing in front, of their stars. TESS, a similar mission to look for planets closer to home, will identify options bright enough to examine in more detail. When future tools, such as the James Webb Space Telescope, coming in 2018, turn their sights to exoplanets, they’ll have only a limited chance to gather the details of planetary atmospheres as their stars shine through them. Future telescopes will be able to see a dot of actual surface color. By then, however, exoplanet researchers plan to be ready. They are building a vast picture of what life can be, how it might manifest itself and how to verify that it is real—to know just what to look for in that tiny smudge of color.
“We’re in a world in which familiar is not necessarily what we see out there,” Sasselov says. “That’s the big problem as well as the big opportunity.”