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University of Toronto Mississauga
J B Lester

AST 201S - Stars and Galaxies University of Toronto Mississauga Chapter 18 Summary th Essential Cosmic Evolution, 5 Edition 1. Life on Earth • Earth’s age is about 4.5 billion years • Oldest fossils date to about 3.5+ billion years, older fossils might have eroded • Earliest life was microscopic and began soon after Earth formed • Life has developed slowly since the beginning, needing a stable climate • Life is surprisingly hardy: existing in scalding volcanic pool, existing in total darkness at the bottom of the oceans • Life seems to need: source of nutrient, source of energy and liquid water • The necessities of life seem common, found many places beyond Earth - these locations can be found and understood by astronomy 2. Earth’s initial environment • Basic, simple molecules, CH4, NH 3 OH ,2plus energy from sunlight, lightening discharges or geothermal can form much more complex organic molecules - these are not life but they are associated with life • These organic molecules are wide spread – in the atmospheres of Jupiter and Saturn – in meteorites from the solar system – in interstellar gas clouds 3. Life in the Solar System • For liquid water the temperature must be in the range 0 to 100 C → range of distances from the Sun = Habitable Zone • Venus is too close to the Sun → too hot • Mars is almost too far from the Sun → too cold • There are other factors beside distance from the Sun – thickness and composition of the atmosphere traps heat (Greenhouse Ef- fect), raising the temperature – thickness of the atmosphere → atmospheric pressure changes the evapora- tion rate of water – gravitational stretching can also melt water, even beyond the habitable zone – heat from the core of a planet can also melt water continued on page 2 AST201H5S – Chapter 18 Summary page 2 of 4 • Mars is the best and closest place to search for life – surface shows many signs of water in the past – water cannot exist on Mars now (too cold, pressure too low) – no sign of life on the surface now – no signs yet of past life – search has just begun 4. Life Outside the Solar System • Other stars also have habitable zones, but they are of different sizes depending on the temperature of the star • Cool, low mass stars are most common, but – habitable zones are tiny, unlikely for a planet to be at the right distance – habitable zones are very close to the star, exposed to lethal cosmic rays from the star - unless the planet has a magnetic field to shield it • Hotter, higher mass stars have larger habitable zones, but stars have shorter lives, maybe too short for life to develop • Therefore, concentrate on stars similar to the Sun 5. Planets around other stars = extrasolar planets • First planet was discovered in 1995 • As of January 2011 there are 516 planets known • None are like Earth - most are about
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