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Life in the Universe Life in the Universe Life on Earth Is there life beyond Earth? Many astronomers of the past have suggested that life existed elsewhere. Great for science fiction plots, but at present there is no undeniable evidence that aliens have been here. Kepler – thought there were inhabitants on the Moon. Herschel – claimed life existed on nearly all the planets. Lowell – thought he saw canals on Mars. Before searching for life elsewhere in the universe we must first look at what conditions are necessary for life on Earth. © Sierra College Astronomy Department 2 Life in the Universe Life on Earth What is necessary for Life to exist? While animals need moderate temperatures and abundant oxygen, simpler life can live under much more extreme conditions and locations (extremophiles). Underground, high and low temperatures. There seem to be 3 basic requirements. A source of nutrients Energy to fuel the activities of life Liquid water (the biggest constraint) © Sierra College Astronomy Department 3 Life in the Universe Life in the Solar System To look for life elsewhere, we need to search for places where the basic necessities of life exist – the habitable worlds. This eliminates most of worlds in our solar system. Moon and Mercury are barren and dry. Venus too hot for liquid water. Jovian planets are gaseous. This leaves Mars and a few of the moons orbiting the Jovian planets, notably Europa. © Sierra College Astronomy Department 4 Life in the Universe Life in the Solar System Mars Percival Lowell thought he saw canals on Mars, but we are quite confident now that there are no civilizations on Mars. Nevertheless, we have good evidence the liquid water once flowed on the Martian surface. Today it contains subsurface ice which could be heated to form areas of liquid water underground. © Sierra College Astronomy Department 5 Life in the Universe Life in the Solar System Missions to Mars, looking for life The Viking missions took soil samples and looked for chemical changes that could be attributed to biological processes. 3 experiments suggested that life may be present, but also ordinary chemical reactions could have caused the same results. A fourth experiment found little organic material, the opposite of what one would expect if life were present. © Sierra College Astronomy Department 6 Life in the Universe Life in the Solar System The Mars Phoenix mission detected water under the surface, though soil was basic and may have trouble harboring life (perchrolates) Pathfinder, Spirit and Opportunity studied the Martian conditions to see if life might have existed. The Mars Express orbiter detected methane gas. Methane should disappear within a few centuries due to chemical reactions. So, something is supplying Mars with methane. It could come from comet impacts, volcanoes, or life. Volcanism seems to be the most likely candidate. © Sierra College Astronomy Department 7 Life in the Universe Life in the Solar System Martian Meteorites One meteorite which landed in Antarctica 13,000 years ago and found in 1984 was clearly of Martian origin. Inside the meteorite were complex organic materials and structures which looked like nanobacteria , very small bacteria which have been discovered on Earth. These structures can also be made by chemical and geological means. Contamination from being on the Earth may also explain the presence of organic materials. © Sierra College Astronomy Department 8 Life in the Universe Life in the Solar System Life on Europa Europa has enough tidal heating to possibly form a subsurface ocean underneath its icy crust. Life there could form like the “black smokers” on Earth. Larger life forms could exist in the vast oceans, but energy sources are limited and this would tend to limit the size of any life there. © Sierra College Astronomy Department 9 Life in the Universe Life in the Solar System Life on Ganymede, Callisto and Titan Ganymede and Callisto might have subsurface oceans, but their internal heat is small and liquid water would not be terribly abundant. Titan has no native liquid water, but an abundance of organic materials. Could life evolve from the lakes of methane? Water might be brought in from comets, but this would eventually freeze. © Sierra College Astronomy Department 10 Life in the Universe Life Around Other Stars Beyond the Solar System Where in the Galaxy might we find life? Since technology might allow us to obtain surface pictures or spectra, we restrict ourselves to considering extrasolar planets with habitable surfaces. So far all detected extrasolar planets (except maybe one or two) are gaseous giants and are unlikely to have surface life. However, they may be surrounded by moons which may support life. © Sierra College Astronomy Department 11 Life in the Universe Life Around Other Stars Constraints on Star Systems A star must be stable and live long enough to allow a planet to develop life. Stars greater than a few solar masses are ruled out (but this is only about 1% of all stars). A star must allow stable planet orbits. Binary and multiple star systems are much less likely to have this – about 50% of all star systems. © Sierra College Astronomy Department 12 Life in the Universe Life Around Other Stars A third constraint is a planet must form in the habitable zone. This is a region where a terrestrial type planet would have the right surface temperature for liquid water to exist. Stars less massive than the Sun have smaller zones. A star like the Sun (or more massive) would have the largest zone. Even if we restricted our search to Sun-like stars, we would still have to consider billions of stars in our Galaxy. © Sierra College Astronomy Department 13 Life in the Universe Life Around Other Stars Finding Habitable Planets Two upcoming missions may be able spot Earth sized planets. Kepler will look for transits of planets across other stars. The Space Interferometer Mission (SIM) may be able to detect Earth sized planets. A decade or so from now, the Terrestrial Planet Finder (TPF) or something like it may be able to image extrasolar planets. Infrared spectra from future telescopes can look for signatures of life © Sierra College Astronomy Department 14 Life in the Universe Life Around Other Stars Rare Earth? Some feel that an Earth type planet (with its complex type of life) is rare: Galactic constraints Too close to the galaxy’s center and the rate of supernovae are too great. Too far from the center and “metal” content is too low. This leaves about 10% of the galaxy’s disk that might be habitable. A stellar system needs a Jupiter-like planet to sweepout and deflect meteors that might wipe out life on Earth. Climate stability Plate tectonics and the carbon dioxide cycle. Earth’s large Moon keeps axial tilt relatively stable. © Sierra College Astronomy Department 15 Life in the Universe Life Around Other Stars Rare Earth? Counterarguments to the Rare Earth Hypothesis The above conditions may not affect the creation and advancement of complex life as much as we think. There may be other overlooked conditions and processes that could assist the creation and advancement of complex life. © Sierra College Astronomy Department 16 Life in the Universe The Search for Extraterrestrial Intelligence What About “Intelligent” Life beyond the Solar System? SETI (Search for Extraterrestrial Intelligence) is trying to find signs of alien communication How many civilizations out there? The (modified) Drake Equation suggests the number of civilizations we might be able to contact: # of civilizations = NHP × flife × fciv × fnow where NHP is the number of habitable worlds flife is the fraction of these world which actually have life fciv is the fraction of these worlds which have interstellar communications fnow is the fraction of these worlds which have a civilization at the present time It is hard to know exactly what any of these numbers are at the present time. © Sierra College Astronomy Department 17 Life in the Universe Interstellar Travel If one is restricted to going no faster than the speed of light, then interstellar travel will be difficult. In any event, vast new energy sources must be used to propel a ship. Hydrogen scoopers. Nuclear bombs or nuclear power Matter-antimater. © Sierra College Astronomy Department 18 Life in the Universe Interstellar Travel If there is advanced alien life out there, why haven’t we seen them (Fermi Paradox)? We are alone and there is no other advanced life out there. Civilizations are common, but no one has colonized the galaxy because Technology prevents a widespread travel. The desire to explore is unusual. Civilizations destroy themselves before they can colonize the stars. There is a galactic civilization, but it has not revealed itself!!! © Sierra College Astronomy Department 19 THE END