1. E X P L O R I N G A S T R O B I O L O G Y: Is there life elsewhere in the Universe? Even if there isn’t any other life in our solar system, it probably exists somewhere out there. Given the billions of stars in our galaxy and the vast number of galaxies in the Universe, it would be surprising if life didn’t exist elsewhere. Are We Alone? Faraway Galaxies, Hubble Ultra Deep Field Image: NASA

2. E X P L O R I N G A S T R O B I O L O G Y: Is there life elsewhere in the Universe? Life Around Other Suns Life, wherever it might be, is probably found in solar systems with planets and moons. Extrasolar planets have been discovered around more than a hundred stars in our own stellar neighborhood, so solar systems must abound in the Universe at large. Each of these locations could conceivably provide a place for life to arise and thrive in one form or another.

3. E X P L O R I N G A S T R O B I O L O G Y: Is there life elsewhere in the Universe? Life’s Chemistry Life beyond Earth would most likely be carbon-based, like us. But there’s a chance it could be non-carbon-based. Some people have fancifully speculated that it could be made of silicon—an element with similar chemical properties to carbon but not as well suited to building complex molecules. Pure silicon Silicone polymer gel. Silicon-based life would use silicone polymers as its basis. Pure carbon

4. E X P L O R I N G A S T R O B I O L O G Y: Is there life elsewhere in the Universe? Rare Earth? It’s reasonable to assume that planets as well-suited to life as Earth are very rare. Most life in the Universe is probably extremely simple; multicellular organisms and complex life are almost certainly extremely uncommon. Earthrise from the Moon, Apollo 8

5. E X P L O R I N G A S T R O B I O L O G Y: Is there life elsewhere in the Universe? Evolution Takes Time Even animals as seemingly simple as jellyfish take a long time to evolve. It took almost three billion years for any multicellular organisms to appear on Earth, even under ideal environmental conditions. The first multicellular organisms might have resembled jellyfish.

6. E X P L O R I N G A S T R O B I O L O G Y: Is there life elsewhere in the Universe? Intelligent Life Intelligence is likely to be rarer than complex life. The likelihood of life achieving anything like human-level intelligence is very small. It’s only happened a few times out of all the species that have ever lived on Earth, and may require special conditions to develop.

7. E X P L O R I N G A S T R O B I O L O G Y: Is there life elsewhere in the Universe? Is Anybody Out There? It’s hard to say how common life might be in the Universe. Of all the solar systems, near and far, how many meet the basic requirements for life as we know it? What are the chances of finding complex life? Intelligent life? A formula called the “Drake Equation” was developed as a mathematical way to calculate how many intelligent alien civilizations might exist out there. What do YOU think? Are we alone? Artist’s Concept of an Earthlike Moon Image credit: NASA