Do there exist inhabited worlds other than our own? This question has been posed for millennia: Epicurus, in a letter to Herodotus in 305 BC, posited the existence of other worlds and predicted that they would contain “the seeds out of which animals and plants arise and all the rest of the things we see.” The sixteenth century Dominican friar Giordano Bruno wrote that “there are countless suns and countless earths all rotating round… that are no worse and no less inhabited than our earth.” Thus long before astronomers developed a detailed understanding of stars and the planets of the Solar system, philosophers speculated whether there might exist other worlds and whether we might someday come into contact with them. The answer to this big question has enormous implications: It would inform not just our scientific understanding of the formation of planets and the formation of life upon those planets, but it would directly address the uniqueness of the Earth, and it would impact each individual’s understanding of his or her place in the cosmos.
How exciting, then, that we find ourselves alive at the moment in human history when we have the technological ability to answer the age-old question: Are we alone?
My goal is to show how astronomers approach this question. The search for other life in the universe has yielded new information and benefited from new technology. Those developments, in turn, have generated new approaches. I plan to discuss the reasons water is a key element for life, describe the ways in which improved telescopes have changed our thinking about the existence of potentially inhabited worlds—and where we might find them–and explore the questions we hope to answer in the not-too-distant future.
Follow the water
The quest to find life on other worlds is (almost) always premised on the belief that life requires water, and in a liquid form. Liquid is the only state of matter in which a mixture of atoms and molecules can be present in high enough density for the chemistry of life to flourish: In a solid, each particle is held in place and unable to find other particles to initiate chemical reactions. In a gas, only a small subset of molecular species avoid condensation, and the overall density (and hence the rate of reactions) is far too low. After hydrogen (and the chemically inert helium), the most abundant atoms in our galaxy are oxygen, carbon, and nitrogen. Each of these react readily with hydrogen to form, respectively, water, methane, and ammonia. We certainly could imagine planets with lakes of methane, ammonia, or other molecules. Indeed, Titan, the largest moon of Saturn, has lakes of ethane and methane. Why are astronomers bullish on water?
The argument is largely based on four properties of water that make it special compared to the other options. First, water is liquid at a much warmer temperature (the others would all be in gaseous form on the Earth), and the rate of most chemical reactions is staggeringly temperature dependent (roughly doubling for each modest increase of 10 degrees centigrade). Said differently, lifeforms in lakes of methane would, by necessity, live at very cold temperatures, and all the chemistry of life – including those that set the stage for the first cells, and later for evolution – would proceed at a pace that was painstakingly slow. Second, water is liquid over a much wider range of temperatures. The temperature at a location on a planet changes throughout the year (think seasons), but also over thousands of years (think ice ages). Both timescales are short compared to that of evolution. Thus water offers the best chance that nascent life forms won’t find themselves quickly frozen (or left to dry out as their ocean quickly evaporates). Third, unlike most hydrocarbons such as methane, water is a polar molecule, allowing it to readily dissolve salts, and permitting a natural mechanism to form long-lived membranes (and hence cells). Finally water is peculiar in that its solid form is less dense than its liquid form. A methane ocean on a planet undergoing an ice age would quickly freeze out: Freshly frozen methane would sink to the bottom, exposing the remaining liquid and allowing it to freeze rapidly. Water ice, on the other hand, floats, providing an insulating layer and allowing lifeforms to hunker down in the liquid that is protected below, awaiting the return of warmer days.
To my eye, these are all reasonable arguments as to why we should begin our search with liquid water. I confess a worry that we are blinded by our own terrestrial upbringing, and out there somewhere an alien astronomer is lauding the benefits of methane. But for now astronomers are nearly unanimous in their belief that we should first set out to find planets with liquid surface water.
The need to search outside the Solar system
Life has dramatically altered the appearance of the Earth to would-be alien astronomers: The impressive abundance of molecular oxygen in our atmosphere, and the distinctive green color due to the chlorophyll of our forests make Earth’s life easy to spot from afar.
Several decades of imaging and, in some cases, direct exploration of the other planets of the solar system make it clear that Earth is unique in this respect. On the Earth, life can’t hide its telltale effects, yet no other body in the solar system shows so much as a hint of life’s chemical activity. We know from now-dry riverbeds that water flowed on Mars in the distant past, and maybe life hunkers there yet, but Mars never received the sort of planet-wide makeover that occurred on Earth. Yes, there is liquid water on Jupiter’s moon Europa, but it’s hidden under several miles of ice and locked away from view (and from any light that would permit photosynthesis). The hydrocarbon lakes of Titan may be home to a thriving community of methane lovers, but if so, their chemistry is so obscure and different from ours that we haven’t spotted any telltale signs yet. To us, the chemistry of Titan’s atmosphere indicates only planetary physical (and not biological) processes.
While further exploration of our neighboring worlds within the solar system is surely merited, I think the prospects are much brighter once we look to the population of planets orbiting other stars. We call these distant worlds exoplanets.
The quest for rocky worlds
On March 6, 2009, a rocket lit up the skies of eastern Florida and placed the NASA Kepler satellite safely into orbit. Dozens of ground-based efforts had already established that exoplanets were a commonplace, but these planets were mostly gas giants and unsuitable for life. The promise of Kepler was that it would be the first project with the capability to discover planets that were as small as the Earth. At the moment of launch, we truly didn’t know if such exoplanets existed. Perhaps Kepler would survey its 150,000 stars and inform us that our home planet was unique?
Instead, Kepler told us an astounding fact: Most stars have planets similar in size to the Earth. Our galaxy contains far more planets than it does stars.
Kepler is particularly adept at discovering planets close to their stars, but for the planets to have the possibility of containing liquid water they must lie in the so-called habitable zone: Not too close, lest the water boil; not too far, lest it freeze. Like Goldilocks, we seek planets with orbits that are just the right temperature. For a star like the Sun, finding such planets is difficult: They are so far from their star that they orbit around it only once per year, and completing only a few cycles within the 4-year baseline of the Kepler mission. Moreover, the Sun is enormous compared to the Earth, and we must push the data to its limits to eek out these signals. Still, astronomers have persevered, and the current estimate is that perhaps 10% of Sun-like stars have an Earth-like planet.
Despite what I was told in high school, the Sun is NOT an average star. Most stars are much smaller and emit much less light than the Sun. As a result, these red dwarf stars make it much easier to find Earth analogs: Instead of a one-year orbit, the habitable zone about such a star corresponds to only two weeks (the planet must be tucked in close to receive sufficient energy). Moreover, owing to their small physical size, the contrast between the planet and star isn’t nearly as great. Unlike the case for Sun-like stars, Kepler’s estimate of the rate of occurrence of Earth-like planets around red dwarfs is on much firmer footing: Last year and in subsequent work, Courtney Dressing and I found that 50% of M-dwarfs may have a habitable planet. Since red dwarfs outnumber Sun-like stars by a factor of 10, this implies with near certainly that the closest worlds (and hence the ones most accessible, at least to our telescopes) orbit these tiny little red stars, and not a solar analog. Some of my colleagues have pointed out reasons to think that life would be impossible in orbit around a red dwarf: They are notorious for emitting copious amounts of sterilizing ultraviolet light, and planets in the habitable zone would be so close that the gravity of the star would have locked the planet’s rotation, implied a permanent, scorching dayside and a frigid nightside. These arguments strike me as unduly Earth-centric. We know so little about the beginnings of life on Earth, it seems a shame to not give red dwarfs the benefit of the doubt! More importantly, regardless of the theoretical musings for or against their habitability, the red dwarfs bring an opportunity that we simply can’t afford to miss.
The fast track to an inhabited exoplanet
Now that we know of potentially habitable planets, how can we establish whether they are in fact inhabited? Other stars are simply too far to contemplate space travel, so the prospects for heading off with butterfly nets in hand are dim. The answer lies in studying the exoplanet atmosphere and searching for atmospheric biosignature gases that would point unambiguously to active lifeforms on the surface below.
The prevailing wisdom is that the study of the atmosphere of an alien Earth requires an enormous technology development, ultimately leading to the launch of very large, space-based telescope (much larger than any yet launched). Such a project would take at least 25 years and require at least 5 billion dollars. This may indeed be what is required if we want to study a true Earth-twin, in a one year orbit about a Sun-like star. But does the newly-discovered preponderance of potentially habitable planets orbiting red dwarfs present an alternate path?
A decade ago, I developed a novel method to make the first detection of the atmosphere of an exoplanet, albeit for a decidedly non-Earth-like gas giant. The work came as a surprise to most of the research community, as the prevailing wisdom was that such a study would require a fabulously large and technologically advanced telescope, capable of first taking a picture of the exoplanet separated from the glare of its star. Instead, our team waited for the moment when the planet passed in front of its star (only a small fraction of exoplanets do this). We then used the star as a flashlight: As the light from the star passed through the atmosphere of the planet, imprinted upon it were the features of the atoms and molecules present in the planetary atmosphere. We had transformed the glare of the star from foe to friend.
This method works only if such eclipse events are relatively frequent, and if the star isn’t too much bigger than the planet. Neither is true for Sun-like stars, but indeed both are true for red dwarf stars. Excitingly, the method doesn’t require the construction of a specialty telescope. While studying Earths-like planets in this way can’t be done with current facilities, a number of upcoming telescopes that are already under construction may be powerful enough for this idea to work: These include the NASA James Webb Space Telescope, scheduled for launch in 2018, and the ground-based Giant Magellan Telescope, which should see first light in 2020.
Much work remains to be done: We still need to find closer examples of the population of red-dwarf planets uncovered by Kepler, and we need to take care that the instruments on JWST and GMT are optimized for this work. But it is possible that we could undertake a meaningful study of the atmosphere of a potentially habitable planets within 5 years. The big questions posed by Epicurus, Bruno, and many others may indeed find their answer in the very near future.
- Given that any claims of the discovery of life on an exoplanet will be made based on remote sensing with no hope of a direct site visit, what would constitute a compelling level of proof? Will we always be able to concoct alternate, non-biological explanations of our measurements? Can this method unambiguously discover extraterrestrial life?
- Can we search only for life that is extremely similar to life on Earth? Would we fail to recognize the chemistry of alien life from afar?
- What will be the impact of discovering life on other planets, to yourself, to our scientific understanding, and to our society generally? Does this search merit a significant investment of public funds?