Transcript Document
The “Rare Earth” Hypothesis Revisited Joe Gale The Hebrew University of Jerusalem ILASOL December 2012 (Springer Verlag – 2000) Ward and Brownlee – their main contentions - and critique Why Rare? Presence of : - Moon - Jupiter - Magnetosphere - Plate tectonics - Water - 2/3 of star systems are double or triple. Planet orbits (hence climates would be erratic) Special Events in History of Complex Life: e.g. Cambrian “explosion”. Demise of Dinosaurs allowing dominance of mammals. Simple life possible, Complex life Extremely unlikely Why Not! - not necessarily unique - protective effect exaggerated “ “ “ - probably not unique or necessary - probably common - even one third of all stars is a huge number. - no reason to think that such events would not occur - number of planets in “nearby” regions of galaxy is enormous Venus Mercury – may have ice at poles See Oded Avraham, ILASOL 2012 Ancient life?? Atmosphere of Venus Subsurface oceans Complex Life?? Planet Earth - to same scale Methane / Ethane on surface. Possible subsurface liquid water. Possible Life “Niches” in the extraterrestrial Solar System Kepler space Telescope (searching for extra-Solar System planets) - Launched 2009 - Studies – 145,000 “nearby” stars (~500 ly) - Detects eclipse of star by planet, when in line with Earth. - Alignment chance (Earth, planet, star) 1/215 Planets detected ( to Nov. ‘12) - 2,326 Of these: 207 Earth Size, 48 in Habitable Zones 2 orbiting Sun-like stars (Note: Earth based telescopes continue to discover planets by radial velocity perturbations of central star. e.g. an Earth size planet orbiting Alpha Centauri B.- about 3.5ly distant; a star with a surface temperature of ~5200 K) Radiation as a function of Black Body Temperature (Wien’s Law) Waveband of Oxygenic Photosynthesis Note: Sun’s surface temperature ~ 5,800oK Red Dwarf stars (about 70% of Milky Way galaxy) ~ 4000oK Caveat emptor (beware before you buy) We are limited by Astrobiology’s working hypothesis: “Life elsewhere will be similar to that on Earth”. One example of problematics: Newly found planets orbiting Red Dwarf stars, in their Habitable Zones, will receive low quantum energy photon radiation which cannot drive earth-like photosynthesis! (So, no life?!). However, there may be non Earth-like photosynthesis, operating with low energy photons at wavelengths > 800nm (< 720 nm is required to split water molecule). (So, Non-Earthlike life may be supported?!). Ernst Mayr’s argument against the chance of another Technological Civilization (from C.Sagan-E.Mayr debate. Updated-2012) (Today Archaea are added). i.e. just once in the 13.75 By universe. The Science Fiction Dream Can we send probes or spaceships to the newly found Earth-like planets, or communicate with any advanced civilization detected? Given: - A planet orbiting a “nearby” Sun-like star, at a distance of 100 ly. - A probe or space ship able to accelerate to an average speed of one tenth the speed of light. (Possible, but vastly faster than todays space probes/ships). - Human lifetime ~ 100 y; a generation time of ~ 30 y. Then: - Transit time to target – 1,000 years = 10 lifetimes or 30 generations. – not feasible - Signal (EMR) exchange with another civilization = 200 years (and they probably don’t speak English!). - Problems of energy would also prohibit transmission of signals to such distances. If there is another civilization capable of overcoming such problems: they would probably put us in a zoo (if we are so lucky!). Conclusion – Ward and Brownlee were essentially correct – Earth is Rare! (probably) - Simple Life – may be common (but still unproven, except on Earth). - Complex life, certainly technological civilizations – probably extremely rare. Moreover: - Close-up study of extra Solar Systems having Life clement conditions, And/Or two way Communication with another Civilization – are not presently feasible.