Four hundred years ago, the Italian scientist Galileo revolutionized the science of astronomy by inventing the first telescope. People began to consider the possibility of discovering other worlds like Earth - fantastic worlds inhabited by weird and wonderful civilizations.

As technology continued to improve our telescopic vision, we sadly learned that the other planets in our solar system showed no signs of life. There seem to be no sophisticated civilizations of intelligent aliens on planets like Mars or Jupiter. Scientists began to picture our Earth as a single haven of life, floating in a vast and lonely cosmos.

In recent years however, astronomers have become more optimistic about the possibility of living worlds. More than 300 planets have been identified beyond our solar system, and some of the most recently discovered planets may be habitable.

With future astronomy missions, scientists hope that our view of the cosmos will improve yet again, and centuries after Galileo's revolutionary invention, we may finally spot a planet similar to our own.

Recently, during "The Search for Life in the Universe" symposium held at the Space Telescope Science Institute (STScI), researchers from a myriad of fields met to discuss the potential for discovering life beyond Earth.

Leaders in disciplines ranging from astronomy to astrobiology spoke about the results of current studies related to the search for life, and the prospects in store for current and future space missions.

The four days of talks covered motivations and expectations in the search for life in three different categories: "Detecting Life within 50 Astronomical Units (AU) of Earth", "Detecting Life within 100 parsecs (pc) of Earth", and "Detecting life beyond 100 pc of Earth."

The first session began with a talk by Chris McKay of the NASA Ames Research Center. McKay discussed scientific perspectives on the search for life in the universe, and he addressed the question of searching for life based on the only example we have: life on Earth. What if there are different kinds of life? How would we recognize life that has a different biochemistry than our own, that doesn't use DNA or isn't made of cells similar to ours?

McKay believes that one of the most interesting results of discovering alien life is "the possibility of a second Genesis." If we can discover life on another world, be it bacteria on Mars or messages from a planet orbiting another star, then we will know if alien life resembles us at the most basic levels. However, identifying this second Genesis could be tricky because we don't exactly know what we're looking for. McKay's advice is to, "use the Justice Potter Stewart definition."

This recalls the infamous statement by the former Associate Justice of the Supreme Court who, when dealing with cases concerning pornography, declared "I know it when I see it." We may not know what to look for at the moment, but hopefully as our knowledge and capabilities progress, we'll be able to identify alien life - even if it looks very different than the life we're used to.

Talks by Pascale Ehrenfreund (Space Policy Institute), Jeffrey Bada (UCSD) and Robert Shapiro (NYU) branched out on the current ideas about how life on Earth, and potentially life on other worlds, came to be. Modern telescopes have taught us that space is full of organic molecules, and many scientists believe that these materials could seed planets with the ingredients for life.

Comets, asteroids and meteorites could deliver materials like carboxylic acids and amino acids that were then used to build the first living cells. How this molecular soup of ingredients transformed into living organisms is unknown, but new studies are beginning to help us understand how small organic chemicals can be converted into larger, functional polymers and molecules like RNA and DNA.

Robert Shapiro discussed the concept of "metabolism-first", where the "first steps of self-organization were driven by the flow of available free energy through an appropriate chemical mixture." Ultimately, through this model, molecules would be drawn into a cycle of reactions that would evolve into more and more complex networks of reactions. Eventually, self-replicating systems would form, ensuring that the energy continued to be passed along.

The other requirement for life as we know it is liquid water. In its earliest stages, the Earth would have been too hot for liquid water to exist. But as the surface of the planet began to cool, liquid water may have been present.

Jim Kasting (Penn State University) discussed how new models are shedding light on the subject, helping scientists understand what the early atmosphere of Earth looked like, when oxygen began to fill the air, and when the Earth was covered with significant amounts of ice during glaciation periods. As of yet, no one has pinned down the exact series of events on the early Earth or the time at which life could have first survived.

Another problem in understanding how and when life emerged on Earth is the fact that ancient microbial organisms didn't leave any obvious physical traces behind when they died. Microbes can leave behind lasting chemical signatures, such as the changes they cause in the atmospheric composition of the planet, but such chemical signatures are difficult to detect.

Dawn Sumner (UC Davis) discussed how we can use our current knowledge of the ancient Earth's chemistry, a concerted search for chemical signatures left behind by microorganisms, and careful study of morphological structures in geology to identify traces of primitive life on Earth and other worlds like Mars.

Steve Squyres (Cornell University) pointed out, "Mars has been considered as a possible abode for life" in our solar system for centuries. However, from Galileo's early telescope to the Viking landers of the 70s and the controversial discovery of potential microbial fossils in the martian meteorite ALH84001, no one has provided definitive evidence that Mars was once habitable.

There is now strong evidence that liquid water was once on the surface of Mars. The MER rovers showed that water was likely present in Meridiani Planum, for instance. However, the environment would have been hostile to organisms. According to Squires, conditions there would have been "oxidizing, acidic, saline and arid."

This would have presented many hurtles for microorganisms to overcome. To answer the questions surrounding the habitability of Mars once and for all, Squires concluded that future missions will have to search for ancient biomarkers in Mars' rocks and soil.

Additional talks discussed the potential for life on moons like Europa and Titan. Ralph Lorenz (JHU Applied Physics Laboratory) argued, "Titan is more 'planet' than many planets," and added that Titan is an excellent model for studying extrasolar planets.

Titan has a thick atmosphere and a dynamic surface, with the distribution of liquid in its lakes and rivers changing over time. Titan's surface may also conceal a global ocean trapped beneath the rocks and lakes of methane identified by the Cassini-Huygens mission.

Although the next mission to moons of interest in our solar system will be sent to the Jupiter system, Lorenz argued that a mission to Saturn's moon Titan would return more information than Cassini-Huygens in just one day of operation. He is optimistic that the unique moon will provide further clues about the range of potentially habitable environments.

Speakers at the symposium also discussed our ability to detect life on exoplanets beyond our solar system. Lisa Kaltenegger (Harvard-Smithsonian CfA) presented methods for determining a planet's habitability based on spectral data received by telescopes. It all comes down to comparing the atmosphere of an exoplanet to its observable features. In doing so, researchers can decipher geological effects on the atmosphere from biological affects, ultimately identifying if a planet's atmosphere can provide a definitive sign of life.

Bill Sparks (STScI) furthered this idea by focusing on how chiral molecules affect the polarization of life. Many molecules theoretically appear in two mirror-image forms in nature, and each form of a molecules also looks different in terms of its optical activity. On Earth, biology causes only one version of a molecule, or one of its mirror images, to be present - a phenomenon known as homochirality.

If this same phenomenon occurs on other planets, extrasolar life may be detectable by examining the optical properties of extrasolar planets to identify worlds where homochirality exists. Such technologies could help us find life on planets that are too far away for us to visit directly with human or robotic missions.

These are just a handful of the topics discussed at the "Search for Life in the Universe" symposium. From our own solar system to the far, uncharted regions of the universe, today's scientists are becoming more and more optimistic about the prospects of discovering habitable worlds.

New studies of Mars have yet to rule out the potential for past or present life on the Red Planet. A new fleet of space telescopes could soon provide the first detection of a truly Earth-like world circling a distant star. With upcoming missions from NASA and other space agencies around the world, the future looks bright for the next generation of scientists who hope to prove that the universe is not such a lonely place after all.