AST 309
part 2:
Extraterrestrial Life
A new science:
Astrobiology
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Sometimes called “exobiology” and “bioastronomy”
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Literally means the study of life in the universe
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Trying to answer the age-old questions “Are we alone?”
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The discovery of life elsewhere can be regarded the single most profound
discovery in human history
A new science:
Astrobiology
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Interdisciplinary (physics, chemistry, geology, etc…)
Scientists need to learn to talk to each other!
The astronomical part is concerned with the conditions for life in the cosmos
the biological part is concerned with questions like what is life in the first
place and how did it emerge?
A new science:
Astrobiology
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The major problem: astrobiology has no gravity!
– We observe and measure gravity everywhere in the cosmos: it is truly a universal
law!
– The processes we observe for life on Earth we cannot extrapolate to the rest of
the universe
– The history of human exploration of the universe (aka science) has removed us
more and more from the center of the universe, except for one remaining issue:
life itself
A new science:
Astrobiology
Just over the past 10-20 years:
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The “astro” part discovered a large sample of exoplanets and is making
excellent progress toward finding potential habitats for alien life.
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The “biology” part discovered “alien” life-forms here on Earth: the
extremophiles can survive and thrive in conditions previously thought
impossible (this opens up a completely new parameter space for life in the
universe)
Overview
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What is life?
What do we know about life on Earth?
What about life in the Solar System (Mars)?
The Drake Equation
Life on extrasolar planets?
Extraterrestrial Intelligence? (SETI)
What is life?
• Definition is very difficult as life is a process!
• “We know it when we see it” (really true?)
• Biology uses the combination of certain characteristics as
signs of life, like metabolism, growth, reaction to stimulus,
reproduction, etc. (every single one is not enough e.g. fire
grows & breaths oxygen, crystals reproduce…)
• Darwinian Evolution is another main feature of life (even valid
for viruses)
• Physics uses entropy (level of discorder) as parameter, life is
“negative entropy” (Schroedinger) i.e. it is reducing its own
entropy at the expense of external sources
What is life?
NASA asked “How can we identify martian life?”
“I’d look for an entropy reduction,
since this must be a general characteristic of life.”
James Lovelock 1964
Life on Earth
In order to be able to find life outside our Earth, we have to
understand life in our own planet. The chemistry of life and the
different processes during the formation and evolution of the
Earth have played a crucial role.
Is life on Earth a very special thing ?
Can life spawn spontaneously elsewhere ?
There are roughly 10^11 (100 billion) stars in
the Milky Way and similar number of galaxies in the
observable Universe.
Are we that special ?
Planets, if they form at all, must form as part of the
star formation process: Planets are literally what formed
out of the “debris” that didn’t find its way into a star
These images are the Orion and Omega Nebulae: star-forming regions as observed in the visual
part of the spectrum. These regions are typical, containing hundreds to thousands of newlyformed (and forming) stars from ~ 0.1 to 100 solar masses. The gas is glowing because of
the radiation from the massive young stars. The “dark lanes” are dense regions in front of
the glowing gas; they are dark because of the dust they contain. Planets will have to form
amidst this energetic activity due to the massive stars (winds, jets, explosions), so it is not
obvious whether the formation of planets is likely.
Spectral lines in interstellar clouds:
Evidence that organic molecules form easily, even in extremely harsh environments
One promising result is that many molecules, some complex, are observed, mostly through
their spectral lines due to rotational transitions in the radio part of the spectrum. Some
examples of molecular rotational spectra, from simple to more complex, are shown below.
Glycine
Methanol
Elements of life: H, C, N, O common only here, or in our neighborhood,
but not elsewhere? Are these produced in special,rare, events?
 No, H, C, N, O are the most common elements in just about every object in the
universe.
Only the total amount of ‘metals” (heavier than He) varies, but their proportions are
amazingly constant.
 Consider composition of Sun: 75%H by mass, ~ 1% C, N, O, everything else either
helium (useless for life--inert), or much less abundant. Just about same for all known
stars!
Same is found in the gas of the interstellar medium, and in the stars
and gas of the
most
A supernova
remnant
distant galaxies.
Does it seem odd to you that the four most
abundant elements are just those elements on
which life is based, if those elements indeed
Simulation of supernova explosion (20
have special properties? The “special
milliseconds)
properties” could have been some rare element,
but no….
Why are abundances of elements so universal?
Hydrogen has been around from the beginning (universe
was originally only fundamental particles, including
protons = hydrogen (rest was electrons, photons, …, no
other elements)
Carbon, oxygen produced in red giant stars, which later
explode as supernovae. All stars become red giants, but
only massive stars produce supernovae; massive stars
are rare (~ 1% of stars). So why is there carbon and
oxygen everywhere?
Nitrogen  the Earth’s atmosphere is mostly nitrogen.
Important? (Yes: Nitrogen doesn’t react well with oceans,
rocks, so our atmosphere is stable. But weird
part to this: If not for the nitrogen cycle, involving bacteria,
the nitrogen would have disappeared long ago.
Abundances of elements vs.
atomic number
All planetary systems formed as part of the star formation process
The standard model of the
formation of the sun is that
it collapses under gravity
from a proto-cloud
Because of rotation it
collapses into a disk.
Jets and other mechanisms
provide a means to remove
angular momentum
So we had our Earth forming by so-called planetesimal
accretion at 1 AU from our young Sun
Life on Earth
Life is a self-sustained set of chemical
reactions based on
with carbon (C) as the key chemical
element
The atomic structure of carbon allows for
the formation of long chains of C-C
chemical bonds on which other elements
can be attached, giving a wide range of
chemical properties.
Life on Earth
Life is a self-sustained set of chemical
reactions based on
+ carbon as the key chemical element
and
+ water as the key solvent
Water comprises ~70% of the mass
content of living organisms and constitutes
the medium on which biochemical
reactions take place.
Life on Earth
All known life on Earth is
DNA-based => we all share
a common ancestor!
Life on Earth
How and where did life form
in the beginning?
Life on Earth
How and where did life form
in the beginning?
Nobody knows!
What about Mars?
Viking lander
What about Europa?
What about Titan?
What about Enceladus?
What about Earth-like planets around other stars?
The Drake Equation
Where should we search for extraterrestrial life? How should we search?
What is required to have life? Complex life? Life we could communicate with?
The “Drake Equation” simply organizes these supposed requirements into separate
factors, a sort of list of possibilities for our consideration.
We want to estimate the likelihood that there are stars with planets with life that
developed into complex “intelligent” technological forms that might be
sending or receiving signals.
What we really want is the total number of them, because that tells us how far we
might have to search.
The Drake equation assigns a symbol for each one of these key factors, representing its
probability of occurrence, and multiplies all of them together. It is not something that is
actually solved, or that you will have to work with except to see a few basic things.
The Drake Equation:
At the end you should see this “equation” as a map of our class
topics:
N = N*
fpl
nhab
Stars ? Planets? Habitable
planets?
fL
fC
Origin Complex
of life? life?
fT
L/T
Intelligence, Lifetime
technology? of civilization
The Drake equation is just a symbolic way of asking what the
probabilities are that a sequence of events like those below
(and more) might occur in other planetary systems.
Our place in the Galaxy
The disk is ~100,000 l.y. across, Sun is about 30,000 l.y. from center.
Think about times for communication at the speed of light!
Clearly we can only search for life among the nearest stars, and for that to be
successful, it must be the case that a significant fraction of all stars must have life,
“N” must be very very large
Now estimate number of planets with life in our Galaxy
(not number with intelligent, communicating life)
If we leave out fi and fc (i.e. assume they are unity—all life forms develop our kind
of intelligence and technology and try to communicate), we are calculating the
number of life-bearing planets in our Galaxy at any given time (like now). We
know there has been life on our planet for 3 billion years, so take L = 3 billion.
Let’s be optimistic about fP (0.1), nP (1), and fL= (0.1). Then
Nlife ~ 1011 x 0.1 x 1 x 0.1 x (3 billion/10 billion) = 300 million
300 million planets with life in our Galaxy! That’s roughly1 out of 1000 stars. This
means that the nearest life-bearing planet might only be 10-100 light years away,
close enough that in the future we may be able to detect such planets and obtain
their spectra (that is the primary goal of astrobiology space missions for the next
decade).
This result is a major reason for exerting most of our effort toward
detecting signatures of biochemistry in the spectra of planets orbiting
nearby stars. You will be reading and hearing a lot about “biosignatures” in
this class soon!
Illustration is “zoom-in” on the region
of our Galaxy from which we can
plausibly detect life, or signals from
communicating civilizations.
The reason for searching for
signs of life from only nearby
stars is not only a matter of
communication times.
We are also interested in
detecting signs of life in the
spectrum of light emitted from
a planet’s atmosphere.
In order to find such
“biosignatures,” you need an
extremely large telescope,
and for the planet + star to be
as nearby as possible, both
in
order to maximize the light
received.
A timeline for the very early history of the Earth
Another way of looking at the sequence of the required events
that are (symbolically) represented by the Drake equation:
Habitable planets
Origin of life on planets
Development of
complex life
Drake Equation
Factors that determine the likelihood of life, intelligence, and
technological civilizations in our Galactic neighborhood.
 Stars
The only thing everyone agrees on is that, to get life, you need a
planet, and a planet orbiting a star (nearly all probably do). So
first we need the number of stars in our Galaxy, which is about
1011.We denote this “N subscript star” or N*, so
N* = 1011 stars
The Sun is an average star. We’ll see which stars might be best for life later. See
picture above—young stars are very active: Dangerous radiation environment.
Average separation of stars is a few light years.
(Compare with size of Galaxy: about 100,000 light years;
nearest other galaxies ~ millions of light years distant.
This means that if there are only 106 (a million) communicating
civilizations in the Galaxy, the average one will be too distant for two-way
communication. (Think about this.)
The planet factor
Life almost certainly requires complex
molecules. Complex molecules
require planets. Why?
Molecules can only react and survive at temperatures
like those of planets. This is ~ room temperature ~ 300 degrees Kelvin (300K).
Temperatures of stellar surfaces are ~ 3,000-10,000 K: too hot for molecules,
water, anything we think you need for life. It all vaporizes to a simple gas of atoms.
Why are planets so much cooler than stars? (Important for rest of
course)
The Drake equation question: What is the probability that a star has a planet?
Or, what fraction fpl of stars have planets? In equation form, we could write
N(planets) = N* times fpl .
This just says: number of stars with planets in the Galaxy is the number of stars times
the fraction of them that have planets.
Evidence: Observations of extrasolar planets show that giant planets (like Jupiter,
or even Neptune).
But how about Earth-like (much smaller, rocky) planets? With
oceans? One may be found this semester! Kepler mission)
Planets: Can’t have life without them
Artist’s conception of an
extrasolar planet orbiting
A faint red parent star
Planets of our Solar System
Planets: Can’t have life without them
Artist’s conception of an
extrasolar planet orbiting
A faint red parent star
Planets of our Solar System
OOPPSS….
What controls a planet’s surface temperature?
The temperature of a planet’s surface is mostly controlled by it’s
distance to its parent star, and its parent star’s luminosity, because
that determines how much energy it receives.
The illustration below shows the Sun as it would appear from Pluto:
Way too cold for liquid water (but plenty of water ice)
=> Pluto is far outside the “habitable zone.”
The “Habitable Zone” (HZ)
around different stars
• Conditions just right to allow liquid surface water on a
rocky planet.
The galactic habitable zone
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There may also be a
preferred time and location
within the galaxy for
habitable planets to exist
Stars that are too close to
the center of the galaxy are
subject to frequent nearcollisions and more
supernovae and gamma ray
bursts
Stars that are too far out in
the galaxy (or that evolved
too early in its history) may
be too metal-poor
Fortunately, though, the
GHZ is probably large
compared to the local solar
neighborhood
Ref.: Lineweaver et al., Science (2004)
What about Earth-like planets around other stars?
Terrestrial Planet Finder:
Kepler:
Searching for Earth-like planets
and so-called “bio-signatures” in
their atmospheres
ELT (42m)
What about Earth-like planets around other stars?
Search for ExtraTerrestrial
Intelligence (SETI):
•How do we search for ETI?
•What has been done?
•What have we found?
Main issues of Astrobiology:
• Understanding the origin of life on Earth
• How likely is it for life to appear under
conditions similar to Earth’s?
• How common are planets like Earth’s
• How likely is it for intelligent life to evolve?
• How likely is it for a civilization to survive
over stellar lifetimes?