This is one of humanity’s most fundamental and persistent questions. The answer has always seemed, until recently, elusive and out of reach. But this is beginning to change.
Biologists have been deepening our understanding of what kinds of environments life might need to flourish, and have found terrestrial life appearing pretty much everywhere they’ve looked, including in some of the most inhospitable places one might imagine. Meanwhile, astronomers have been looking more deeply, and with better instruments, into the universe around us, and have discovered that planets appear to be quite common in the cosmos, as are the chemical ingredients we know that life seems to require here on Earth.
Among the surprising environments in which scientists have found life on Earth, the mid-ocean vent ecosystems might be the strangest. Deep under water, on the ocean floor along the seams between continental plates (much like seams on baseball), extremely hot and mineral-laden water heated by magma from within the Earth's upper mantle (known as the asthenosphere) belches from vents, similar to the geysers in Yellowstone National Park. These vents are deep enough beneath the waves that not only is sunlight completely absent, but temperatures far exceeding the boiling point of water at normal atmospheric pressure are possible. With no sunlight, pressures as high as 200 times that found at sea level and temperatures approaching 400 degrees centigrade, you might be excused for thinking that life couldn’t exist in these environments. Indeed, many have been surprised to learn that not only is life in such a hostile environment possible, it thrives. These hydrothermal systems have been found throughout the Earth’s oceans and support ecosystems of unusual creatures we call “extremophiles,” life forms that live in what we would consider extreme environments. In the absence of sunlight, many of these extremophiles derive energy from the chemical interactions between the hot, mineral-rich water emitted by the vents and the surrounding rocks and colder seawater, in a process we call “chemosynthesis.”
Undersea vents are not the only places where extremophiles have been found: Life has been found deep under the Antarctic ice cap, in lakes with conditions that would be toxic for most common microbes, and even deep within the bedrock beneath our feet. As we try to understand what kinds of life might be possible on other worlds, rather than looking at other planets or moons for examples of what might be possible, organisms right here on Earth provide a window into the kinds of alien worlds that are possible. Such discoveries are shaping our understanding of just what constitutes life and what kinds of environments can be called “habitable,” and suggest that life might be quite common in the universe.
As we look at planetary and other bodies and ask ourselves whether life might be possible on them, we must be mindful of the variety and adaptability of the life we have found here on Earth. In the case of Mars, we see a planet that shows signs of volcanism, has a dynamic atmosphere and has a history that includes periods during which liquid water has been present on the surface. Data collected by orbiting spacecraft, landers and rovers have given us conclusive evidence that in the distant past, perhaps billions of years ago, Mars was wetter, warmer and had a thicker atmosphere than it has today. While this tends to suggest that Mars might have been capable of supporting life, it does not tell us that it ever actually did. We do not thoroughly understand how life’s earliest and simplest processes develop, nor do we know how often life develops given the right environmental conditions. Current research aims to give us a better understanding of whether Mars might have had oceans or seas, and if so, how long they existed before the planet became as cold and dry as it is today.
Farther from the Sun than Mars, we find Jupiter and its moons. The most interesting of these is perhaps Europa, which is one of the four moons discovered by Galileo more than 400 years ago. Europa is a moon made predominantly of water ice and an almost nonexistent atmosphere. Scientists generally agree that there is a large liquid ocean beneath the icy surface of Europa, and this is why it is seen as a promising target for research as we look for signs of life on other worlds. With gravitational “tidal forces” acting on the moon, there may be enough heat generated inside Europa to support a liquid ocean 20 to 100 miles beneath the ice. If this is in fact the case, we may not need to rely on science fiction to tell us what sorts of life may exist there: Our own hydrothermal vents offer us a valuable window into the possibilities.
Our search for life does not end with our solar system. Since the discovery of the first planets around other stars in the early 1990s, scientists have discovered at least 538 planets outside our solar system. The launch of NASA’s Kepler mission in March of 2009 marked the beginning of a new chapter in our search for other worlds. Kepler orbits the sun, following Earth in its orbit. The observatory is designed to stare continuously at more than 100,000 stars in a section of the sky near the constellation Cygnus, the swan. If planets do orbit any of those stars, some of them will occasionally pass in front of the stars themselves, causing a very slight decrease in brightness. The cameras onboard Kepler are designed to measure the brightness of those stars so precisely that they can detect such a planet crossing, or “transiting,” its host star. At the time of this writing, Kepler has confirmed the detection of 15 new planets, but hundreds of potential candidates await confirmation with additional observations.
Pretend for a moment that we were looking at our solar system from hundreds or thousands of light years away. To see Earth pass in front of the Sun once, we would need to stare at the Sun for about a year. For the sake of scientific accuracy, we would want to see multiple transits of Earth before we could be very sure of our observations. Kepler has yet to complete its second year of operation, and so many scientists believe that the next few years of Kepler’s observations are likely to be the most fruitful.
The search for life continues. As we learn more about how life has evolved here on Earth, and the surprisingly wide range of conditions under which it can thrive, we begin to search the universe for comparable conditions. We believe, based on the available evidence, that such conditions may be common. And yet, for the moment, we are still stuck without an answer to that fundamental question: Are we alone? Stay tuned.Join Fiske Planetarium and Sommers-Bausch Observatory for Astronomy Day on Saturday, April 23, from noon to midnight. This is a free event for the community with activities, shows and prizes to celebrate the discoveries of hundreds of extra-solar planets. For more information about Astronomy Day and the science in the column, visit http://fiske.colorado.edu or call 303-492-5002.