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Habitability: Earth to
But could we recognize life?
Plan of Presentation
What ‘Our Kind’ of life needs to form and
Types of Planets?
Our Solar System
Other Planetary
Other Stars
The Universe
Requirements for ‘Our Kind’ of Life
Planet with Stable Environment
Right Elements: ¡SPONCH!
Carbon, hydrogen, oxygen,
nitrogen, sulfur, phosphorus
Trace elements, like iron (Fe)
& magnesium (Mg)
Liquid Solvent (Water)
Right Temperature
(Energy Source
Chemicals )
Stable Planetary Environment
Planets have near-circular
No huge ellipses with hot
and cold times
Sun is stable
Sunspot cycle is really minor
Not many asteroids or comets
Not very many Dinosaur-killers!
Few nearby stars
Near pass would disrupt planet orbits.
Explosion would fill solar system with lethal radiation.
Element Distribution (¡SPONCH!)
in the Solar System
Distributed by volatility!
O N C H are very volatile (gasses / ices)
• Burned mostly out of inner solar system by the Sun
• Some carried back in by comets and asteroids
S P are not volatile - stay with rocky solid
Good ¡SPONCH! mix in Venus, Earth, Mars; plus
Titan (Saturn); Europa, Callisto, Ganymede (Jupiter)
Why Liquid? Why Water?
Why Liquid?
Allows easy movement of
molecules to/from reactions
Allows creation of complex
Why Water?
H2O is abundant in solar
system, and is versatile.
Liquid ammonia (NH3) and
hydrocarbons (methane, ethane)
are possible.
Temperature in a solar system
Heat mostly from star, decreases away from it.
Yellow zone = liquid water at planet’s surface.
Stars get hotter as
they age, so yellow
zone moves out.
Planet rotation
smooths out
Tidal lock = same
face to sun (sort-of)
Planets around Other Stars?
Do they have Stable Environments?
Planets are common Hundreds now known!
Known ones not good.
Hot gas giants, or
Hghly elliptical orbits
(unstable environments).
Very tough to detect
planet like Earth - small
and far from star.
What sort of Star?
Only some main sequence cool stars (types F, G, K,
M ) are suitable
Hot, large stars (O, B, some
A) explode too soon.
Hot stars (O, B) make too
much deadly ultraviolet
Variable stars, flare stars
don’t provide stable
Giants, supergiants
White dwarfs are remnants
after star explosions.
Multiple Stars?
Most stars are double, triple or
Some have planets! (HD88753)
Planet orbits are stable only near
a star or far from them all.
A multiple star system is as
bad for life as its worst star.
And … multiple stars have more
restricted habitable
zones, and more variable
planetary environments.
Imagine our solar system with a
small star in place of Jupiter!
Young or Old Stars?
¡SPONCH ! form in stars
during normal burning
Most important trace
elements (like Fe) also
Heavier elements form in
star explosions
Star explosions release
these atoms as dust
More star generations,
more ¡SPONCH!
Younger, multi-generation
stars more likely to have
solid planets!
Where in the galaxy can life
Far from the core
Intense radiation from
its huge black hole
Too many stars, will
disrupt planets’ orbits
In from the rim
Rim stars tend to be
older, poor in SPONCH
Outside galaxy arms
Too many stars, will
disrupt planets’ orbits
Why is our Universe Habitable?
The Anthropic Principle
The Universe seems right for life
Space is ‘flat’
Atom stability is right
Gravity works as 1/r2
How to explain these coincidences?
If intelligent life weren’t possible, we
wouldn't be here to think about it.
Our the universe is designed for life.
"If we weren't here, the universe
couldn't exist."