Transcript File

Life in the Universe
Are We Alone?
Illustration by Dana Berry
Standards
• Examine investigations of current
interest in science.
• Understand the scale and contents of
the universe
• Identify important questions that
science cannot answer (e.g., is there
life outside our solar system?)
Is There Life in the Universe?
• This is a question with
profound implications
for humans.
• It is, however, one for
which we have no
data.
• Earth is the only place
in the universe where
we know for certain
that life exists.
Cosmic Evolution
• There are 7 major phases in the
history of the universe:
1. Particulate
2. Galactic
3. Stellar
4. Planetary
5. Chemical
6. Biological, and
7. Cultural evolution
Arrow of Time
Cosmic Evolution
• These evolutionary stages make up the
grand sweep of cosmic evolution – the
continuous transformation of matter &
energy that has led to the appearance
of life & civilization on Earth.
• The universe has evolved from
simplicity to complexity.
Cosmic Evolution
• We are the result of an incredibly
complex chain of events that spanned
billions of years.
• Were those events random, making us
unique, or are they in a sense natural,
so that technological civilization is
inevitable?
• Are we alone in the universe, or just
one of many intelligent life-forms?
Definition of Life
• Defining life is not an easy task.
• There are no clear-cut definitions.
Definition of Life
• The following are, in general, the
characteristics of living organisms:
1.They can react to their environment.
2.They can grow by taking in nourishment
and processing it into energy.
3.They can reproduce, passing along their
characteristics to their offspring.
4.They have the capacity for genetic change
& can therefore evolve from generation to
generation & adapt to a changing
environment.
Definition of Life
• These rules are not strict.
• Stars, for example, react to the gravity
of their neighbors, grow by accretion,
generate energy & “reproduce” by
triggering formation of new stars, but
no one would suggest that they are
alive.
Case in Favor of
Extraterrestrial Life
• Assumptions of mediocrity:
1. Because life on Earth depends on just a
few basic molecules, and
2. Because the elements that make up
those molecules are (to a greater or
lesser extent) common to all stars, and
3. If the laws of science that we know
apply to the entire universe, then –
given sufficient time – life must have
originated elsewhere in the cosmos.
The Opposing View
• Maintains that intelligent life on Earth is
the product of a series of extremely
fortunate accidents – astronomical,
geological, chemical & biological
events unlikely to have occurred
anywhere else in the universe.
Chemical Evolution
• Conditions on early Earth were right for
amino acids and nucleotide bases to
form.
• These are the building blocks of life as
we know it.
• Amino acids build proteins and
nucleotide bases form genes.
Chemical Evolution
• Many experiments have been done in
which energy has been applied to a
mixture of compounds found in
atmosphere of early Earth.
• These experiments have created amino
acids and nucleotide bases, showing
that the building blocks of life can be
formed from the raw materials found on
early Earth.
Diversity and Culture
• Simple one-celled organisms such as
blue-green algae appeared on Earth
more than 3.5 Ga.
• These were followed by more
complex one-celled organisms, like
amoeba, about 2 Ga.
• Multicellular organisms, such as
sponges did not appear until about 1
Ga.
• The evolution of the rich variety of life
on our planet occurred as chance
mutations.
Diversity and Culture
• What about the development of
intelligence?
• Many anthropologists think
intelligence is favored by natural
selection and closely linked to
language.
• Cultural evolution on Earth began
around 10,000 years ago.
Life in the Solar System
• Life as we know it means carbonbased life that originated in a liquid
water environment.
• Might such life exist elsewhere in our
solar system?
The Moon & Mercury
• Lack liquid water,
protective atmospheres
& magnetic fields.
• They are bombarded by
UV radiation, solar wind,
meteoroids & cosmic
rays.
• Simple molecules could
not survive in such harsh
environments.
Venus
• By contrast, has far too much
atmosphere.
• Its dense, dry, scorchingly hot
atmospheric blanket rules it out as a
home for life, at least like us.
The Jovian Planets
• Have no solid surfaces (though some
researchers have suggested life might
have evolved in their atmospheres).
Pluto and the Jovian Moons
• Are too cold for life.
• However, the possibility of liquid water
below Europa’s icy surface has caused
speculation about the possibility of life
there.
• This moon of Jupiter is a prime
candidate for future exploration & is
high on both NASA’s & the ESA’s priority
list for missions.
Mars
• Is the planet most likely to have life, or
to have had it in the past.
• It is harsh by Earth standards: liquid
water is scarce, the atmosphere is thin,
UV radiation & high-energy particles
reach the surface.
Mars
• The Martian atmosphere was thicker & the
surface probably warmer & much wetter in
the past.
• There is strong photographic evidence from
Viking Orbiter & Mars Global Surveyor for
flowing & standing water on surface in past.
• The European Mars Express confirmed
water ice at the poles in 2004.
• Opportunity reported strong geological
evidence that the area of its landing site
was once drenched in water for an
extended period.
Mars
• No life has yet been detected on Mars,
but it has not been ruled out.
Extremophiles
• There is lots of research into life in
extreme locations on Earth – called
extremophiles.
• The thought is that if there is life
elsewhere in our solar system, it will be
in the form of extremophiles.
Examples of Extreme Locations
Include:
• Undersea hydrothermal vents (or black
smokers)
Examples of Extreme Locations
Include:
Caves such
as Lechuguilla,
in NM
Examples of Extreme Locations
Include:
• Cold environments such as Antarctica and ice
caves
Alternative Biochemistries
• There may be life very different from that
on Earth.
• Some scientists suggest that maybe life
could be silicon based (instead of
carbon) and have formed in an ammonia
environment (instead of water).
• But, we know nothing about non-carbon,
nonwater biochemistries for the very
good reason that there are no examples
on Earth to study.
Alternative Biochemistries
• Note: an organism with an alternative
biochemistry was recently (Dec 2010) found
to exist in Mono Lake, CA: substitutes arsenic
for phosphorus in its cells
• This was then disproven in a study released
in 2012
Intelligent Life in the Galaxy
• At galactic distances, we have little
hope of actually detecting life
w/current equipment.
• Instead, we must ask “how likely is it
that life in any form exists?
The Drake Equation
• Is a statistical approach to whether there
is life in the Galaxy.
• Several of the factors in the equation are
a matter of opinion.
• We do not have nearly enough
information to determine every factor in
the equation.
• Its value is that it provides a framework
within which the problem can be
addressed & divides the responsibility
among different scientific disciplines.
The Drake Equation
• # of technological, intelligent civilizations
now present in Galaxy = (rate of star
formation, averaged over lifetime of
Galaxy) x (fraction of stars having
planetary systems) x (average # of
habitable planets within those planetary
systems) x (fraction of those habitable
planets on which life arises) x (fraction of
those life-bearing planets on which
intelligence evolves) x (fraction of those
intelligent-life planets that develop
technological society) x (average lifetime
of a technologically competent
civilization).
Rate of Star Formation
• Can be estimated by dividing current
# of stars in Galaxy by the 10 Ga
lifetime of our Galaxy.
• We obtain a formation rate of 10 stars
per year.
Fractions of Stars Having
Planetary Systems
• No planets like our own have yet been
discovered (would be too faint against
parent star).
• Note: The Kepler mission has
discovered 961+ exoplanets, a few of
which are Earth-like.
• Accepting the condensation theory of
star formation, it is believed that nearly
all stars form with planetary systems.
# of Habitable Planets
per Planetary System
• Temperature is the biggest deciding
factor.
• There is a zone of “comfortable”
temperature – a stellar habitable zonearound every star.
• The hotter the star, the larger the zone.
Galactic Habitable Zone
• Galactic center too violent for life
• Galactic halo has too few heavy
elements to form terrestrial planets or
populate them with technological
civilizations
# of Habitable Planets
per Planetary System
• Taking uncertainties into account as
best as possible, we get a value of
1/10.
• In other words, there is one potentially
habitable planet for every 10 planetary
systems in our Galaxy.
Fraction of Habitable Planets on
Which Life Actually Arises
• Experiments suggest that the
combination of elements into the
molecules necessary for life are likely
to occur.
• Therefore, given time, life is likely
Fraction of Life-Bearing Planets
on Which Intelligence Arises
• Appearance of well-developed brain
unlikely if only by chance.
• Some say if evolution plays a part,
intelligence is inevitable, given
enough time.
• Others argue that there is only one
known case of intelligent life.
Fraction of Planets on Which
Intelligent Life Develops & Uses
Technology
• This is unknown.
• Some scientists say that species on
other planets will probably always rise
to fill the technological niche.
Average Lifetime of a
Technological Civilization
• This is totally unknown.
• Humans on Earth are our only
example.
• Humans have been technological for
about 100 years.
The Drake Equation
• If we go with the optimistic value for all
factors, then we end up with:
# of technologically intelligent
civilizations now present in our Galaxy
= average lifetime of a technologically
competent civilization in years
The Drake Equation
• Thus, if civilizations typically survive for
1000 years, there should be 1000 of
them currently in existence in our
Galaxy.
• If they live for a million years, we would
expect a million advanced civilizations,
and so on.
• If the life expectancy of a civilization is
only a few thousand years, it is unlikely
to have time to communicate with its
nearest neighbor (communications
can’t go faster than the speed of light).
Meeting Our Neighbors
• Let’s assume there are 1000
technological civilizations in our
Galaxy.
• Given the size & shape of our Galaxy,
they would be ~30 pc (or 100 l. y. )
apart.
• Therefore, any 2-way communication –
using signals at the speed of light would take at least 200 years.
Meeting Our Neighbors
• One obvious way to search for ET life
would be to travel far outside our solar
system.
• This may never be a practical possibility.
• At a speed of 50 km/s (fastest probes
today) the round trip to a distance of 30
pc would take 600,000 years (just to get
to the nearest star would take 50,000
years).
• Going the speed of light is currently
beyond our technology.
Meeting Our Neighbors
• We have already launched IS probes.
• Pioneer 10 launched in the 1970’s and
is now well beyond the orbit of Pluto,
on its way out of the solar system.
Meeting Our Neighbors
• Replica of plaque mounted on board Pioneer 10. The important
features of the plaque include a scale drawing of the spacecraft, a
man, and a woman; a diagram of the hydrogen atom undergoing a
change in energy (top left); a starburst pattern representing various
pulsars and the frequencies of their radio waves that can be used to
estimate when the craft was launched (middle left); and a depiction
of the solar system, showing that the spacecraft departed the third
planet from the Sun and passed the fifth planet on its way into outer
space (bottom). All the drawings have computer-coded (binary)
markings from which actual sizes, distances, and times can be
derived. (C. Sagan)
Meeting Our Neighbors
• The Voyager probes were launched in
1978 with similar information.
• Although incapable of reporting back
to Earth, scientists hope any
encountered civilization could unravel
the contents using the universal
language of mathematics.
Meeting Our Neighbors
• Aside from practical problems,
communication may not be a good
idea.
• Any encountered civilization would be
more advanced (since we are so
technologically young).
• The most advanced civilization may try
to dominate the others (remember
example of European explorers and
their domination of “primitive” cultures).
Radio Communication
• Is a cheaper & more practical
alternative to direct contact.
• Radio can travel through dusty IS
space.
• Radio telescopes on Earth listen
passively for radio signals emitted by
other civilizations.
The Water Hole
• But, at what frequency, among the
many that make up the radio part of the
spectrum, should we listen?
• Basic arguments suggest a wavelength
of 20 cm.
• Hydrogen (H) atoms radiate at 21 cm &
hydroxl (OH) radiates at 18 cm.
• Together, these form water (H2O), the
substance out of which life as we know
it was formed.
The Water Hole
• Therefore, researchers have proposed
this interval between 18 & 21 cm
(called the water hole) as the most
likely place to search.
Radio Searches
• A few are now in progress in and
around the water hole.
• One of the most sensitive &
comprehensive searches for ET
intelligence was Project Phoenix
(known to many by its acronym SETI),
carried out during the late 1990’s.