Habitability: Earth to
Universe
But could we recognize life?
Plan of Presentation
What ‘Our Kind’ of life needs to form and
thrive
Types of Planets?
Our Solar System
Other Planetary
Systems
Other Stars
Galaxies
The Universe
Requirements for ‘Our Kind’ of Life
Right Elements: ¡SPONCH!
Carbon, hydrogen, oxygen,
nitrogen, sulfur, phosphorus
Trace elements, like iron (Fe)
& magnesium (Mg)
Stable Environment
Right Temperature
Liquid Solvent (Water)
Energy Source
Light
Chemicals
Element Distribution (¡SPONCH!)
in the Solar System
Distributed by volatility!
O N C H are very volatile (gases / 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); Ceres?
A
b
u
n
d
a
n
c
e
¡SP
ONCH!


M
V
E
M
J
S
Stable Planetary Environment
Planets have near-circular
orbits
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.
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
temperature.
Tidal lock = same
face to sun (sort-of)
Why Liquid? Why Water?
Why Liquid?
Allows easy movement of
molecules to/from reactions
Allows creation of complex
molecules
Why Water?
H2O is abundant in solar
system, and is versatile.
Liquid ammonia (NH3) and
hydrocarbons (methane, ethane)
are possible.
Where in the Solar System could
we find liquid water?
Europa
Earth: Certainly.
Mars: Certainly in the
past, maybe now
Europa: Liquid water
ocean (blue) under ice;
rocky mantle, metal
core.
Venus: ? Possibly, in
its distant past.
Ceres: Dwarf Planet, Ex-asteroid
Avg. diameter 942 km
Equatorial - 975 km
Polar - 909 km
Density = 2.07 gm/cm3 mix rock + water
Polar flattening suggests
rotating ‘fluid’ glob (ice
or water shell over rock.
Maybe liquid water,
depending on how much
internal heat from
radioactivity.
Enceladus: Moon of Saturn, with
water vapor plumes
Plumes erupting from
Enceladus.
Known to be water
vapor by its
temperature, just about
0°C
Titan - Another
solvent!
Dense atmosphere, thick
cloud cover. Orange color
is hydrocarbon smog.
RADAR imager on
Cassini spacecraft shows
river beds and lakes on
surface.
Lakes probably made of
liquid hydrocarbons, like
methane (CH4) and ethane
(C2H6).
lake
land
Planets around Other Stars?
Do they have Stable Environments?
Planets are common Hundreds now known!
Most known are not good.
Hot gas giants, or
Highly elliptical orbits
(unstable environments).
Very tough to detect planet
like Earth - small and far
from star.
New Extra-solar Planets
Water vapor! On Hot Jupiter HD 189733b
4.5 million km from star (Mercury is 70 mil km)
2.2 day orbit, surface T 700°C (1000K)
Habitable? Gliese 581 C - Super-Earth
Red dwarf star (M-type), 50 times dimmer than sun
• C is 15 times closer to G581 than Earth to Sun.
Half again as big as Earth, 5 times more massive
Surface T at 0 - 40°C. !!! Surface G is 2.2 x Earth.
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
radiation.
Variable stars, flare stars
don’t provide stable
environments
Giants, supergiants
White dwarfs are remnants
after star explosions.
Multiple Stars?
Most stars are double, triple or
more.
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?
¡SPONC ! 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
survive?
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."