1. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Planet Survey So how do we go about looking for life beyond Earth? The first step in the search for extraterrestrial life is to locate as many planets and moons as we can—in our solar system and elsewhere. Venus and Mars at sunrise, from orbit Image: NASA

2. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Close to Home In an effort that began in ancient times, the major planets, moons, and small bodies that orbit our star—the Sun—have been located. Left: William Herschel, discoverer of the planet Uranus Right: A telescope made by Galileo Gallilei, discoverer of Jupiter’s four major moons

3. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Extrasolar Planet Search There are billions of stars out there. Of course not all stars have planets, but many do. Between 1995 and 2004, 108 extrasolar planetary systems— with some 123 planets—were located in our stellar neighborhood. Artist’s View of Planetary System in Globular Cluster M4 NASA and G. Bacon (STSci) – STSci-PRC03-19a

4. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Seeing the Unseen Finding extrasolar planets is difficult, because they are so distant and dim that their parent star obscures them. They can only be detected indirectly, through their effects on their star’s motion and appearance. Artist’s View of Planet Around Star Gliese 876 NASA and G. Bacon (STSci) – STSci-PRC02-27

5. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Likely Locations The next step is to narrow the search to planets and moons in so-called “habitable zones” where life could theoretically exist. Information about geology, atmosphere, composition, history, and relationship to the rest of its solar system is required to determine if a world might be hospitable to life. High-resolution 3D Images from ESA Mars Express Orbiter Images: ESA

6. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Unsuited for Life Some solar systems are probably not suitable for any kind of life. Some factors that make a system unlikely to harbor life are: stars whose temperature and light vary in intensity. multiple star systems high energy radiation from the star or surrounding areas very old, very young, very hot, and very cool stars Artist's Concept, Evaporating Planet HD 209458b Image: NASA

7. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Potential Homes To qualify as a potential home for life as we know it, a location must have liquid water, temperatures between -4°F (-20°C) and 266°F (130°C), an energy source, and organic molecules. Image: NASA Earth From Low Orbit

8. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Just Right For liquid water to exist, a planet needs a source of heat such as tidal forces or heat from its sun. If the heat comes from a star, the planet has to be in a “Goldilocks” position, just the right distance from the star to allow for liquid water. Sunrise, Nepal

9. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Viable Energy Source An energy source might include sunlight for photosynthesis or chemicals for chemosynthesis. Organic molecules could include amino acids, carbohydrates, or methane, which can all be produced by atmospheric and geologic processes. Hydrothermal Vent and Chemosynthetic Bacteria Image: NOAA

10. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y The Signature of Life A potentially hospitable planet might display detectable biosignatures, such as visible signs of life, quantities of organic molecules, or atmospheric gases resulting from metabolism. Credit: Defense Meteorological Satellite Program Night View of Earth

11. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Finding Habitable Worlds Since the 1960s, spacecraft like Viking, Mariner, Voyager, Galileo, Mars Global Surveyor, the Mars Exploration Rovers, and the Cassini/Huygens Mission have been providing detailed information about the planets in our solar system. Mariner 2 Cassini-Huygens Mars Exploration Rover Artist’s Concepts Images: NASA

12. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Bridging the Gap But what about distant worlds? Physical probes, even if we decided to send them, couldn’t reach the vicinity of other stars for many, many years. Investigation of extrasolar planets will have to be done at a distance, with future space telescopes like the Terrestrial Planet Finder. HD 209458, a planetary system 150 light years from Earth. It would take an impossibly long time for a probe to get there!

13. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y Terrestrial Planet Finder The Terrestrial Planet Finder space telescope mission will provide us with the closest look we’re likely to get of extrasolar planets. Artist’s Concept, Terrestrial Planet Finder Image: NASA

14. How do we look for life? E X P L O R I N G A S T R O B I O L O G Y A Different Approach The Search for Extraterrestrial Intelligence (SETI) is an alternative approach to the search for life. It searches the sky not for planets and biosignatures, but for detectable signs of intelligent life—such as radio signals and other forms of communication. Stay tuned! The radio telescope at Arecibo, Puerto Rico Goldstone Radio Telescope, California