A few years ago, I gave a talk about astrobiology at the kick-off meeting for the DARPA Biological Technologies program office. DARPA, or the Defense Advanced Research Projects Administration, is the Department of Defense's think tank for innovative research— a unique organization that brings together scientists across disciplines to direct research on a wide variety of topics related to national security. I arrived as a sort of chaser for their day, spent worrying about problems much closer to home— and over an hour or so, told them about the incredible progress astronomers have made in searching for biology beyond our planet.
Talking about alien life is both exciting and frustrating— there's a lot say about the search for life, yet little known about life itself. For example, if you'd asked even six years ago whether small, rocky planets like the Earth were common or rare, no one could tell you. Now, thanks both to space-based projects like NASA's Kepler Mission, as well as research teams searching with Earth-bound telescopes (such as the one that recently discovered a planet orbiting our neighbor star, Proxima Centauri), we know that the Galaxy is awash in planets. There are so many planets, in fact, that when you gaze into the night sky, each star is likely the sun of another world. By and large, these planets are worlds roughly the size of Earth-- a tantalizing hint that, while we have yet to find life elsewhere, the potential real estate abounds.
Amazingly, the majority of planets exist around the suns you cannot see: tiny red stars, whose feeble shine makes them invisible to the unaided human eye. These little red stars (known as M dwarfs) are extremely populous, comprising 70% of all the stars in our Galaxy— but they were also once the pariahs of planet hunting. Astronomers had any number of reasons to dismiss them as hosts for habitable worlds: their meager energy output meant that planets would have to orbit so close to them, they would become locked by tidal forces— one side in perpetual day, the other in perpetual night. Even if planets did exist around them, surely those planets' atmospheres would freeze and collapse! Or if they escaped that fate, it was thought that energetic flares from their parent stars would shower the unsuspecting planets with high energy ultraviolet and X-ray radiation, sterilizing the surface. Even if some biology survived, in their very red light, most beyond the scope of human vision in the infrared (energies of radiation we experience as heat, which we can see only with night vision goggles), photosynthesis would be limited— or impossible! Not only would life not thrive, it wouldn't even have a fighting chance.
In 2005, however, a small workshop was held at the SETI Institute, with the intent of reevaluating the decades of planetary party-pooping— and I, as a particularly lucky young graduate student, got to attend. Over the week, we examined these assumptions in a fresh light, eventually concluding that none of those supposed showstoppers are really all that show-stopping. While it's true that planets around M dwarfs are likely affected by tides, they may also remain habitable in spite of those effects, and here on Earth, plants can use red light for photosynthesis. Even those pesky flares didn't seem so bad: several years back, my colleague Antigona Segura and I used computer models combined with stellar data to show that even a large flare might not be all that detrimental to habitability: much of the energetic ultraviolet light is filtered by the planet's atmosphere. In any event, evolution can be very enterprising when it comes to protecting organisms in high-radiation environments: some high altitude plants produce their own protective waxes, and more vulnerable critters can just hide underwater, which provides an effective shield from UV radiation. In the years since that SETI workshop, myriad teams have worked hard to make the question of habitability on planets around M dwarfs answerable with nuance and detail, rather than a simple yes or no.
While our understanding of what these alien environments might be like has grown by leaps and bounds in the past decade, many unknowns remain. Stellar activity— the catch-all term for flares, as well as the energetic particles that stream from magnetic stars in ribbons and blobs— still poses a compelling threat to planetary habitability. Even if planets are partly protected from the effects of stellar activity by their atmospheres (or their own magnetic fields), the resulting chemistry in the planet's atmosphere may drive its composition towards poisonous, rather than pleasant— at least for life as we know it. More confounding, stellar activity can also erode planetary atmospheres with time, washing over them and gradually scraping the atmosphere away. If a planet doesn't have an atmosphere, there's no pressure to maintain liquid water on its surface— no matter whether it is the right distance from the warming glow of its star to be considered habitable, or not. Right now, we lack the tools to test whether large numbers of planets have atmospheres or not, although new insights will come from the James Webb Space Telescope in the near future.
If atmospheres of planets around M dwarfs do survive, there is still the question of irradiation from stellar flares. Proxima b, the roughly Earth-sized planet recently announced around our neighboring star, is an interesting case in point.
As data was being gathered for the planet’s discovery, another team of astronomers was studying flares from its small red host star, Proxima Cen (http://www.ifweassume.com/
These unknowns wind into the wooded future, promising paths for both concrete scientific research, and imaginative thought. After all, if 70% of all stars have planets that are largely incapable of hosting life, it makes a big difference for whether life in the universe as a whole is common, or rare. A favorite thought experiment of mine is to consider the implications of challenging planetary environments on our chances for recognizing— or communicating with— intelligent life beyond our own world. I imagine a universe filled with rocky planets around little red stars, and on days I'm feeling optimistic, I imagine the atmospheres of these worlds have survived. Global oceans protect surface life from the vagaries of stellar irradiation, and intelligent (even technologically advanced) life might be more akin to the dolphins of our own planet. What would the relationship of this underwater life be to the sky, and to its place in space?
Fermi's paradox (despite being neither Fermi's, nor a paradox http://blogs.
Astronomy, space travel, and the will to communicate with worlds beyond our own are not only part of a scientific quest, they are the outgrowth of cultural values— values we hold as a species that is both capable of viewing the stars, and of contemplating our place amongst them. What would we learn, if we could ask dolphins about their conception of the universe? Do they want to build crafts that will ferry them (or their robotic avatars) outside the bounds of natural habitability, like humans have done to explore both space and the sea? Or is the ability to actually see the night sky ultimately tied to the desire to know whether these island planets also hold life? What about the planets out there that might be shrouded in haze, obscuring the stars from their inhabitants, even if they aren't underwater? And what if all the planets around M dwarfs are just barren rocks, devoid of atmospheres, where any life might remain sealed under ice caps like the surface of Jupiter's moon, Europa?
The difference between science fiction and science itself is that fiction is content to imagine and dream— but science lives to dive beneath the waves, and figure out the answer. While scientists are often loathe to say that we live in a special time, in some sense we do: we stand at the dawn of knowing that the universe teems with worlds, but not yet knowing if we are alone. At this moment, we must be keenly cognizant of how far we have to go. Otherwise, our assumptions about the completeness of our search, the universality (or not) of the values we hold, and our inability to communicate even with species we share the same swimming space with, will blind us to the possibilities— and limitations— of what we might come to know about life in the universe.
Do Dolphins Dream of Space Travel? Little red stars offer big questions in the search for intelligent life
Dr. Lucianne Walkowicz is an astronomer at The Adler Planetarium in Chicago.