The Big Picture: Astrobiology

The Terrestrial Planet Finder observatories was designed to detect Earth-sized planets outside our solar system. CREDIT: NASA

New academic disciplines often get cool receptions. Women's Studies and Quantum Mechanics were considered either frivolous or fictional by many when they first appeared in university catalogs. In the late 1930s, the manuscript that Grote Reber wrote describing low-frequency emission from the Milky Way — a pioneering work that broke open the field of radio astronomy — was uniformly rejected by reviewers for the Astrophysical Journal. Fortunately, the editor decided to publish Reber's paper anyway.

Astrobiology feels their pain. The field is young enough to still have vocal critics; in particular, those who think that "astrobiology" is nothing more than a hope that life will someday be discovered beyond Earth.

It's true that incontrovertible proof of extraterrestrial life is still lacking. But there are just two paths to the future: either we will eventually find biology elsewhere, or we won't. If you are among those who think that only Earth has spawned life, then astrobiology is, indeed, only useful in proving your hypothesis by enduring endless failure to reach its ultimate goal. But if it seems plausible that, among the 10 thousand billion billion other star systems of the visible universe, there are some places where the remarkable chemical interplay we call life also occurs, than astrobiology research can only speed its discovery.

A quick head count of astrobiologists at the SETI Institute tallies almost 50, and there are about 75 more across the freeway populating the cubes and labs of NASA's Astrobiology Institute at the Ames Research Center. This contingent, as impressive as it is, is merely a local condensation in a far larger cloud of researchers. Nationwide, it's reckoned there are approximately a thousand scientists who would be proud to print "astrobiologist" on their business cards.

Are this many serious and accomplished researchers toiling away with nothing more tangible than a hope that their work will eventually be justified? Of course not. The pursuit of the study of the mechanisms of life, and of its history and habitats, has produced some of the most exciting science discoveries of the past decade. Let's refine that very general statement with some specifics:

Life amongst the stars

It seems like forever ago, but in the mid-1990s we still didn't know whether planets – the cool worlds that are most conducive to life — were superabundant or scarce. In the last decade, astronomers have uncovered nearly 200 worlds orbiting nearby stars. The actual fraction of all stars with planets can only be guessed, but it certainly exceeds 5%, and could possibly surpass 90%. It's hardly a radical hypothesis to suspect that the cosmos houses more planets than stars.

Few of these extrasolar planets have been seen directly, but one of astrobiology's most daring experiments is planning to capitalize on their discovery: the launch of space-based, infrared telescopes — such as NASA's Terrestrial Planet Finder — that can not only image the faint, pin-pricks of light that mark these worlds, but analyze their spectral signatures for evidence of tell-tale gasses (such as oxygen and methane) that would signal the presence of microbial life. Just as SETI scientists pivot their antennas at nearby star systems in an effort to find intelligence, the astrobiologists are drawing up their own plans to detect life from afar – life that's tens of light-years beyond the solar system.

Conditions for life

The plentitude of extrasolar planets is not the only recent discovery to revolutionize our attitude about how conducive the cosmos might be to life. A decade ago, a "habitable" planet was one that was a simulacrum of Earth — a world that basked in the warmth of a star, with a thick atmosphere and watery, surface oceans. Other worlds might be intriguing; but other worlds were dead.

Now it seems that this presumed death may have been greatly exaggerated. In a truly eye-opening discovery, astrobiologists have learned that there are mechanisms other than starlight that can substantially warm a planet. The resonant interplay of moons in multi-satellite systems leads to an endless tug-and-squeeze that can generate more than enough heat to keep large reservoirs of water in a liquid state, and provide localized sources of energy (such as the churning vents found on ocean floors) that could fuel biology.

Moons such as Europa, Callisto, Ganymede, and Enceladus — once considered no more than dead rock — are today recast as potential sites for life. Even Titan — for which the tug-and-squeeze of tidal forces is unimportant — might occasionally have long-lived pools of liquid water under its surface, melted into existence by occasional asteroid impacts.

Indeed, the attractiveness of moons as places where life might appear has recently been extended by researchers to include putative satellites that are far larger than the familiar moons of our solar system. Such mammoth moons (perhaps the size of Earth), girdling gas giant planets orbiting other stars, might offer environments as salubrious for life as our own planet.

In other words, even very non-Earth-like worlds might host liquid water, thereby fulfilling the requirements set forth in Biology 101. Such worlds may not be capable of supporting complex life, and most terrestrial life forms would speedily perish if transported to their environs. But not all. Astrobiologists continue to make surprising discoveries about how resilient life can be. The first extremophiles, dwelling in places like Yellowstone Park, were found almost a century ago. In the last decade, we've come to learn of organisms that can thrive in rock a mile beneath the surface, or in the ink-black deeps of the oceans.

Extremophiles are considered exceptional on Earth. But they might be widespread elsewhere, simply because you can bet that the majority of worlds are tough rather than temperate.

The evolution of life

While finding life beyond our planet might offer prodigious insights into biology's workings on Earth, the reverse is also true. It's important to understand the endpoints of terrestrial life: its start, and the evolution of intelligence. Recent investigations by astrobiologists in Australia and Norway strongly suggest that life had gained a foothold on Earth more than 3.5 billion years ago, when the watery oceans of our planet were first safe for habitation. Astrobiologists are asking how this happened, and investigating the role played by the clouds of carbon compounds churned up in nearby space, and rained onto our planet while it was forming. Was this natural fertilizer essential, or merely supplemental?

The facts are that DNA and RNA are enormously complex molecules, and their appearance by chance seems, to some, improbable. But researchers have pointed to natural structures, such as clays, that could act as catalysts to speed the start of life. We still don't know for sure how life first began on Earth, but this highly fecund area of research could bear sweet fruit for astrobiology, either telling us that life is so improbable as to be nearly miraculous, or so inevitable as to be trivially common.

Equally exciting is research into how evolution produced, in the last few hundred thousand years, intelligence. Was this a mere accident that occurred on our planet — an unlikely consequence of contingency and chance? Some of astrobiology's most challenging investigations are sighted on understanding whether sentient life is to be expected often, or almost never.

The implications of these forays into unraveling life's secrets on Earth have obvious application to our search for it elsewhere. Clearly, astrobiology underlies our SETI searches; but if you think about it, you'll quickly realize that it's also the most powerful incentive for our explorations of the solar system. Why do we pay so much more attention to Mars, Europa and Titan than we do to Venus, Io, or Rhea? It's because the former worlds have the conditions that might foster and nurture the most compelling activity in the universe: life.

• SPACE.com TV: Reflections on Fermi's Paradox
• Astrobiologists to Convene at AbSciCon 2008
• Astrobiology 101: Where Teachers are the Students