Lecture 21 Extraterrestrial Life
Are We Alone?
Humans have speculated throughout history about life on other worlds.
• it was assumed by many scientists & thinkers of the 17th
& 18th Centuries that life was present on other worlds
• and widely accepted by the public at the turn of the 20th
• scientists became more sceptical once we began to
explore the planets
Recent advances in astronomy and biology have renewed interest.
• discovery of extrasolar planets indicate that planetary
• indications that liquid water can exist on other worlds
• organic molecules are found throughout the Solar
• geological evidence suggests life on Earth arose as soon as
• discovery that living organisms can survive in the most
This interest has spawned a new science called astrobiology.
o the study of life in the Universe
This new discipline includes:
o chemical evolution
o the origin and evolution of life
o planetary biology and chemistry
o the formation of stars and planets
o space science
o expansion of terrestrial life into space
Questions in extraterrestrial life
Is there intelligent life in our Galaxy?
... in the Universe? ...in the solar system?
Can we communicate with other forms of life?
When will we communicate with these other forms? How?
Is there any other form of life in our Solar System?
... in out Galaxy?
Was there life in any form in the past in our Solar System?
How did life originate on our planet?
o spontaneously? (Endogenous) o carried to Earth by something? (Exogenous)
Could we recognize other forms of life?
It is easy to produce the building blocks of life
•Miller and Urey(1953) experiment
•this simple experiment produces amino acids – the basic building blocks of life
as we know it...
•but not so easy to turn them into life
Clues to growth of amino acids?
clustering of millions of amino acids into droplets, which grow, divide
fossils ~2Gyr old (left) compared with modern blue-green algae
oily droplets made by exposing freezing mixture to strong UV light produce cell-
life exists under very harsh conditions
•boiling water at right and rich sulphur clouds in ocean depths
•but water is a common factor The Murchison Meteorite: arrow points to organic matter => chemical
evolution has occurred elsewhere
From organic compounds to life: not an easy step
o exploration of the Solar System has revealed no sign of large life
o we must search for microbial life
o Mars: the best candidate for extraterrestrial life?
some reasons why Mars is the best candidate to host such life:
o Mars was apparently warm & wet for some periods in its distant
o conditions, similar to early Earth, made it possible for life to evolve
o had the chemical ingredients for life
o has significant amounts of water ice
o pockets of underground liquid water might exist if there is still
Viking Lander (1976)
We have searched for life on Mars.
Viking scooped up soil and ran tests
o looked for products of respiration or metabolism of living organisms
o results were positive, but could have been caused by chemical reactions
o no organic molecules were found
o results inconsistent with life
this is not the final word.
o Viking sampled only surface soil
o took readings at only two locations
o life could be elsewhere or underground
o rocks ejected by impact from Mars have been found in Antarctica.
o analysis of one revealed...
o age of 4.5 billion years old
o landed on Earth 13,000 yrs ago
o contained complex organic molecules & chains of crystals similar
to those created by Earth bacteria
also found fossils of nanobacteria.
o recently discovered on Earth
o they have DNA, but are they life?
big news, but since then...
o structures seen could also be formed by chemical & geological
o Earth bacteria have been found living in the meteorite o CONTAMINATED!
Possibly Life on Jovian Moons
beneath its icy surface, Europa may have an ocean of liquid water.
tidal heating keeps it warm
possibly with volcanic vents on the
conditions may be similar to how Earth life arose
life need not only be microbial
Ganymede & Callisto may also have subsurface oceans, but tidal heating is
Titan has a thick atmosphere and oceans of methane & ethane.
water is frozen
perhaps life can exist in liquids other than water
pockets of liquid water might exist deep underground.
and in 5Gy – it will be warmer...
Life in other planetary systems...
what are the requirements on a planet for it to produce life as we know it?
the planetary system must be old enough for life to have evolved
the star must survive long enough for life to evolve
development of planetary crust – planet has to cool
composition of planet?
stability of life-sustaining conditions – e.g. no very large variations in
surface temperature over ? 500 million years
large single Moon provides climate stability
Which Stars make Good Suns?
which stars are most likely to have planets harboring life?
must be long lived and old enough that life could arise in a few x 10 y
this rules out massive O & B main sequence stars
they must allow for stable planetary orbits
o this rules out binary and multiple star systems
they must have relatively large habitable zones
• habitable zone is the region where large terrestrial planets could have
surface temperature that allow water to exist as a liquid
Width and location of habitable zone depend on the type of star
•in the solar system the habitable zone is from inside Earth’s orbit to just beyond
•this will change as the sun evolves
need both habitable zone and time...
•if life takes 10 yr to evolve => best bet is stars 3MSun
•but below 1M Sunsize of habitable zone shrinks rapidly
Planets and binary systems?
a planet in a binary star system must have a stable, uniform orbit to maintain the
right conditions for life
Have we detected habitable planets around other stars?
NO...current technology is insufficient, because an earth-like planet
o is too small to be resolved or for us to notice its gravitational pull on
its parent star
o would be lost in the glare of its parent star
Recent launch of the Kepler mission which will...
o measure the light curves of stars to look for transits of Earth-sized
o measure planets orbits to determine if they are in the stars habitable
zone Detecting habitable planets around other stars
In the next decade, NASA plans to launch Terrestrial Planet Finder.
o an interferometer in space
o will take spectra and make crude images of Earth-sized
spectrum of a planet can tell us if it is habitable.
o look for absorption lines of ozone and water
Earth-like Planets: Rare or Common?
expectation (models, some observations) is that Earth-like planets are
o billions of stars in our Galaxy have at least medium-size habitable
o theory of planet formation indicates ter