The “Rare Earth” HypothesisRevisited

Joe Gale

The Hebrew University of Jerusalem

ILASOL December 2012

(Springer Verlag – 2000)

Ward and Brownlee – their main contentions - and critique

Why Rare? Why Not!Presence of :- Moon - not necessarily unique- Jupiter - protective effect exaggerated- Magnetosphere - “ “ “- Plate tectonics - probably not unique or necessary - Water - probably common- 2/3 of star systems are double or triple. Planet orbits (hence climates would be erratic) - even one third of all stars is a huge number.Special Events in History of Complex Life:e.g. Cambrian “explosion”. - no reason to think that such Demise of Dinosaurs events would not occur allowing dominance of mammals. Simple life possible, Complex life - number of planets in “nearby” Extremely unlikely regions of galaxy is enormous

Ancient life??

Subsurface oceansComplex Life??

Methane / Ethaneon surface. Possiblesubsurface liquid water.

Possible Life “Niches” in the extraterrestrial Solar System

Venus

Atmosphere of Venus

Planet Earth- to same scale

Mercury – may have ice at polesSee Oded Avraham, ILASOL 2012

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!).

- 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.

Conclusion – Ward and Brownlee were essentially correct –

Earth is Rare! (probably)