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Astrobiology Primordial Soup • Life on Earth is comprised of relatively few elements. – Basic elements: carbon, nitrogen, oxygen, hydrogen – Trace elements: iron, calcium, potassium, phosphorus, ... • We observe these in compounds on other planets. – Methane, ammonia, water – Minerals with silicon, calcium, iron Recipe for Life • In 1952 Urey and Miller placed water and gases in a flask. – Simulate early planet – Spark as lightning • Amino acids were present in a week. – Building blocks of proteins Past Life • On Earth life began after about 1 billion years. – One-celled creatures for the next 2.5 to 3 billion years – Extremophiles and cyanobacteria need no oxygen • Multi-celled creatures began about 600 million years ago. Star Types • To have an Earth-like planet we need Earth’s conditions. – Assume carbon-based life • Stars can’t be too large. – Lifetime of more than 1 billion years • Stars can’t be too small. – Close planet for heat – Tidally locked with no day-night cycle Galaxy Types • Heavy elements (metals) exist in type I star populations. • Elliptical galaxies and halo clusters are poor in metals. • Metals are best in the spiral disk. Galactic Zone • The habitable zone in the galaxy may be narrow. – Too much radiation near center – Too little gas for planets far out • The best zone is 4.5 to 11.5 Kpc from the center. – Includes 20% of the Milky Way Planetary System • Planets can’t be too large or too small, too hot or too cold. • A binary system or cluster would be bad. – Complicated orbits – Large temperature changes Planetary Zone • For a single star with a system of planets there is an ideal zone of orbits. • The orbits must be in a range that creates temperatures suitable for water. too hot too cold