| Scientists don't yet know whether our solar system harbors alien organisms. But they do know it's rich in the ingredients for life. Earlier this month, researchers reported that NASA's Curiosity rover had found complex organic molecules — essential components of the chemistry that powers all cells on Earth — in Martian rocks from the bottom of a dried up ancient lake. And today, scientists announced that they'd detected similar compounds in the plumes of ice and water vapor that stream from the south pole of Enceladus, a tiny moon of Saturn with a thick ice crust, a saltwater ocean and a rocky core that's heated by Saturn's gravitational push and pull. On Earth, many of these molecules are associated with microorganisms called chemotrophs, which pull energy from chemicals in their environments, rather than radiation from the sun. A spacecraft equipped with modern instruments could conceivably fly through the plumes of Enceladus in the next few decades and detect lipids, amino acids and other compounds created by these microbes — assuming such molecules are there to be found. "The next mission, without any science fiction aspect, could really test if there is life or not," planetary scientist Frank Postberg, the lead author of the Enceladus study, told me. Often when people hear the word "alien," they think of tailed, toothy monsters or little green men. But many astrobiologists think the first life beyond Earth we discover will be microbial, the kind of creature that could thrive in the dark, watery depths of a world like Enceladus. After all, single-celled organisms arose relatively quickly on this planet — scientists looking at rocks from the tumultuous first billion years on Earth have detected several features that look like signs of ancient life. Those microbes pretty much dominated the planet for the next 2 billion years. Animals didn't evolve until about 500 million years ago. And tool-using, fire-starting, bipedal hominins have been around for less than one tenth of 1 percent of Earth's history. It might actually be a good thing if we find microbes first, Mary Voytek, NASA's senior scientist for astrobiology, told me. Despite what you may have read in "The Andromeda Strain," it's unlikely that an alien bacterium could hurt us — at least, not at first. Pathogens need to evolve with their hosts, she pointed out, so it would take time for a new microbe to exploit our bodies and overcome our immune systems. Besides, fewer than 1,500 of the estimated millions of microbe species on Earth are capable of making humans sick. What would it mean if we discovered alien microbes — in the ocean of Enceladus or elsewhere? Psychologists have investigated the issue, and found that most people would react positively to the news. Most of all, detecting organisms on one other world increases our chances of finding more. It's just a matter of statistics. As theoretical physicist Jim Al-Khalili wrote in the introduction to his book "Aliens," there are so many stars circled by so many planets, that "unless the Earth is astonishingly and unjustifiably special, the universe should be teeming with life." — Sarah | 
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