What about Dead Zones -places where we know life cannot likely survive? (from Ward & Brownlee's textbook "Rare Earth")
early universe -- galaxies too young to have metals (elements like C,S,N,O etc) for formation of Earth-size inner planets, energetic
quasar and supernova activity
globular clusters -- contain many old stars, but too metal-poor, solar-mass stars have evolved
to giants that are too hot for life on inner planets, stellar encounters perturb outer planet orbits
elliptical galaxies -- stars are too metal-poor, solar-mass stars have evolved into too-hot giants
small galaxies -- most stars are too metal-poor
centers of galaxies -- energetic processes impede complex life
edges of galaxies -- many stars are too metal-poor
planetary systems with ``hot Jupiters'' -- inward spiral of giant planets drives inner planets
into central star
planetary systems with giant planets in eccentric orbits -- environments too unstable for higher life, some planets
lost to space
future stars -- uranium, potassium, and thorium are perhaps too rare to provide sufficient heat to drive
plate tectonics
Habitable Zones -- Location! Location! Location! (Ward & Brownlee)
habitable zone or HZ (or ``comfort zone'')
is region where heating from central star provides planetary surface temperature at which water ocean does
not freeze or boil -- THIS ASSUMES LIQUID SURFACE WATER IS REQUIRED FOR LIFE (which we know may not be true on other worlds).
width of HZ around star depends on how Earth-like planet must be to be habitable
HZ moves out in time because as stars evolve on the main sequence, they become brighter and hotter
(Sun is now 30% brighter than it was 4 billion years ago; 1-3 billion years from now, greenhouse
effect will cause Earth to be more like Venus)
CO2-silicate (or ``CO2-rock cycle'') cycle acts as regulating thermostat
``greenhouse effect'' due to CO2 increases
Earth's surface about 40 degrees C
as planet warms, increased
weathering
(physical and/or chemical breakdown of rocks and minerals)
removes CO2 from atmosphere, cooling Earth
water reacts with CO2 to produce carbonic acid
then CO2 + CaSiO3 (Calcium silicate rock) = CaCO3 (Calcium Carbonate, limestone!) + SiO2 (Silicon oxide, quartz!)
carbonates and quartz get buried as sediments, mostly in ocean basins
when Earth is too cool, weathering and CO2 removal decrease, while continual atmospheric
build-up of volcanic CO2 leads to warming
without plate tectonics and liquid water, system does not work efficiently
also, does not work well on planets without land surfaces
remarkable negative feedback system widens CHZ and complicates efforts to determine its precise
boundaries because ``CO2-rock cycle'' is not perfectly understood on planetary scale
still, estimated width of CHZ suitable for surface-dwelling, water-dependent life is
0.95 to 1.15 astronomical units
being within HZ is not requirement for life, just for animal life?
animal habitable zone might require narrower HZ -- where, say, wheat or rice could be cultivated to
feed several billion people
microbial habitable zone may encompass entire Solar System and extend temporally from soon
after planet formation to today
planets can be ejected out of HZ's as planetary systems in general are not necessarily gravitationally
stable for billions of years
ejected planets would lack light and warmth
leading to surface temperatures like
liquid helium
radioactive interior could allow
deep subsurface biosphere to survive -"mole world"
extremophiles (like those in the Columbia Basalts) might make it
microbial life on moons of ejected giant planets might survive and even evolve as
flexing of interior continues due to tidal effects of planet (differential gravity -- leading to tidal heating: "Europa world")
habitable zones in other star systems
HZ location determined by star's brightness, which depends on size, type, and age
star 50% more massive than Sun enters red giant phase after only 2 billion years --->
outward migration of HZ is much faster and HZ has shorter duration
stars more massive than Sun radiate more ultraviolet (UV) light, which breaks bonds of most biological molecules
and heats atmosphere past escape velocity
95% of stars are less massive than Sun, for example, common M stars are 10% of Sun's mass
planets of low-mass stars would need close orbits for liquid water to exist
but, gravitational tidal effects would then introduce synchronous rotation (where planet
spins only once on its axis each time it orbits star, like Earth
tidally locks Moon) ---> atmospheric
freeze-out due to cold dark side
most stars are in binary or multiple star systems
planets might not be able to form in first place
if planets form, what happens to CHZ's?
orbits affected
insolation (stellar energy received by planet) would vary
evolution of two or more stars would cause CHZ's to recede or even disappear
variable star, neutron star, and white dwarf systems even less habitable
open cluster stars too young, globular cluster stars too old (metal-poor) and concentrated
habitable zones in Galaxy
inner limit of HZ defined by high density of stars, dangerous supernovae, and energy
sources in central region of Milky Way
gamma-ray repeaters -- examples of neutron stars -- that have magnetic fields more than a million times stronger than
most powerful magnets in laboratories on Earth,
and 10 to 100 times stronger than observed magnetic fields of normal neutron stars
emit dangerous X-rays and charged particles too
outer limit of HZ not defined by energy flux, but by type of matter
concentration of heavy elements lower in outer parts of Milky Way because rate of star formation is lower
elements heavier than helium needed for life, for Earth's solid/liquid metal core that includes
radioactive material (metal core produces shielding magnetic field and radioactive heat fuels plate tectonics)
habitable zones in time
``habitable zone'' of Universe in time began only after first 2 billion years,
because elements like carbon, oxygen, nitrogen (and 23 other building blocks of life)
and uranium (heating Earth's core) created in centers of stars
early galaxies and elliptical galaxies today have few metals --
planets that form around metal-poor stars may be too small to retain oceans, atmosphere,
and plate tectonics
Sun is unusually metal-rich (richest out of 174 well-studied stars)
so far, extrasolar planets all seem to orbit metal-rich stars
