Astronomy 1021 Chapter Notes - Chapter 19: Rare Earth Hypothesis, Galactic Habitable Zone, Axial Tilt
28 May 2018
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Astronomy 1021
Chapter 19: Life in the Universe
Life in the Solar System
•Search is for habitable worlds- contain all the necessities of life
•Water is key ingredient we look for- Mars: water may exist right underneath the surface, indicated
by dark streams on surface
oJupiter’s moons- Europa and Saturn’s moons- Encylladus and Titan- both have ice water
and titan has liquid methane and ethane
•Life on Exoplanets: previous considerations- circumstellar habitable zone (neither too close or too
far from star so temp is enough for liquid water to exist), volcanism, plate tectonics, magnetic
field (w/o this, atmosphere thins)
•“Rare Earth Hypothesis”- earth sized planets are common; some are in habitable zone; life on
Earth formed as soon as it could VS rare Earth hypothesis- Earth is rare, life followed from a
series of ‘lucky’ events, three further possible constraints
•1. Galactic Habitable Zone- analogous to circumstellar habitable zone around a star, there may be
an ideal zone around the centre of the galaxy; the entire Solar System is in the galactic zone
oThis is important b/c father away from the galactic centre less metal available, so can’t be
farther out b/c won’t have enough material
oCounter- Mass of Earth is 100000 times less than MSun; so don’t need a lot of rocky
material, just a little bit to form rocky planet like Earth
oIf closer to the galactic centre, those who favour rare hypothesis say, no possibility of life
b/c surrounded by lots of stars and at any point one of them could go supernova and kill
your planet
oCounter- even if could be more exposed to radiation, you could a thick atmosphere that
protects you, or organisms that live at the bottom of the ocean who are protected
•2. Jupiter and Impact Rates- no Jupiter means extended bombardment period; this is b/c Jupiter
migrated inward and scattered the debris outwards which formed the Oort Cloud, Jupiter is the
reason why material got cleared;
oJupiter protects us from comets and asteroids, so its placement is important
oMost Jupiter-sized planets are closer to their stars; there are still Jupiter sized planets at
Jupiter’s distance, but need some more time to observe (P Jupiter ~ 12 years)
•3. Moon and Climate Stability- earth’s relatively large Moon perhaps kept our axial tilt stable
oImportant b/c Variation in axial tilt would have lead to large swings in seasons
