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The Search for
Extraterrestrial Life
Calvin College, December 1, 2006
Larry Molnar & Loren Haarsma
Physics & Astronomy Department, Calvin College
(Cartoon by Bob Thaves, Frank & Ernest)
Where to look: Earth-like Planets
Habitable planets should be ...
• Orbiting a star big enough to keep you
warm, small enough to last a long time
• Big enough to retain an atmosphere, small
enough to not be engulfed in a hydrogen
envelope
• At just the right distance from the star to
have liquid water
Our planet system as a model
• Circular orbits
• Giant planets confined to the outside
– Jupiter’s orbit: 12 y = 3143 d; 5.2 AU; T=120 K
• Terrestrial planets spanning the habitable zone
– Earth’s orbit: 1 y = 365 d; 0.0003 MJ; T = 280 K
– Mercury: 88 d, 0.39 AU, 100 – 700 K
All good properties.
All inevitable consequences of our formation
scenario.
Stages of planetary formation
1. gas cloud (3D)
2. solids condense, fall to midplane (2D)
3. accretion of planetesimals
circular, coplanar orbits yield low speed
interactions
4. planets assemble from planetesimals
But could you see Earth from
another star?
• Mass
– Tiny compared to Sun: little effect on Sun’s
position or speed
• Light
– Next to much larger, brighter object
• And we want more than a detection
– Spectrum tells atmospheric composition
• 
It Never Hurts to Look Anyway
• Effects on the star:
– radial velocity: favors massive, nearby planets
– position offset: favors massive, distant planets (with
patience)
– timing: favors massive, nearby planets (requires a
star that pulses)
• Photometry
– favors large, aligned, or hot planets
• Imaging
– favors distant, large, hot planets
• Microlensing
– catches everything, but only briefly
What works
• Radial velocity: 169 systems (20 multiple
systems, 197 planets total)
– one published this fall coauthored by a Calvin grad
• Microlensing: 4 planets
• Transits:
– 4 known in general (1 by discovery)
– 0 in 47 Tuc
– 16 candidates in latest HST survey
Implications
• Eccentric orbits: little room left for Earths
• Small orbits:
– again keeping Earths from the habitable zone
– hot objects
• observable by thermal emission 
• may evaporate! 
• Implications for formation scenario
– obviously missing rich variety
• Needed: firm statistics
– how many nondetections are there?
Conclusions
• Early work not immediately encouraging
for extraterrestrial life
• Rapid progress, observationally &
theoretically, underway
– all conclusions are very tentative
Extraterrestrial life has not yet
been discovered
• Should we expect that God made life on
other planets?
• How did God create life on Earth?
• Is abiogenesis possible? If so, how?
• How likely is abiogenesis?
• If we did find extraterrestrial life, what
would be the consequences?
Theological question: Should we expect
that God made life on other planets?
A few theologians insist: YES
A few theologians insist: NO
Most theologians say: no compelling reason
either way.
Theological question: Should we expect that
God made life on other planets?
“Scripture strongly implies that no intelligent life exists
elsewhere…. Earth was created to be home for
creatures made in God the Creator's image. It was on
Earth that the first human couple rebelled against its
creator and brought the cosmos under His curse…. The
second person of the holy trinity incarnated on Earth
alone, took on human nature, died for the sins of those
with whom He has the kinsman-redeemer relationship,
then ascended to the right hand of God the Father…. It
would therefore seem hard to reconcile intelligent life on
other worlds with the doctrine of the incarnation. It would
also seem odd for God to create microscopic life on
other planets, but we should not be dogmatic on this.”
Jonathan Sarfati, www.beliefnet.com
Theological question: Should we expect that
God made life on other planets?
“It is, of course, the essence of Christianity that
God loves man and for his sake became man
and died. But that does not prove that man is
the sole end of Nature. In the parable, it was the
one lost sheep that the shepherd went in search
of: it was not the only sheep in the flock, and we
are not told that it was the most valuable - save
in so far as the most desperately in need has,
while the need lasts, a peculiar value in the eyes
of Love.”
Dogma and the Universe, C.S. Lewis
Theological question: Should we expect that
God made life on other planets?
“I can’t see why the same Son of God, who has
existed for all time, can’t take on the form of other
creatures once they reach the stage where they can
communicate with God.”
Russell Stannard, Science & Wonders, p74.
“If intelligent beings were found elsewhere in the
universe, (they) couldn’t compromise the special
relationship already existing between God and
human beings any more than a young couple
compromises their love for their first child after
having a second. The first child may feel slighted for
a time, but the parent’s love nevertheless remains
steadfast.” Hal N. Ostrander, associate dean of
Christian theology at Southern Baptist Seminary
Theological question: Should we expect that
God made life on other planets?
Steven J. Dick,
Many Worlds: The New
Universe,
Extraterrestrial Life, and
the Theological
Implications
(a collection of essays
from a variety of
viewpoints, from a 1998
Templeton Foundation
conference)
What is needed for life to exist?
Probably:
• Carbon (and other elements)
• Liquid water
• Steady energy source
• Stable conditions over a long time
Where might we find those
conditions?
• Near a moderate-sized, stable, third-generation
star neither too close nor too far from the
galactic center.
• A planet like Earth, in the “habitable zone” of
the star for the right temperature range, big
enough to have an atmosphere and plate
tectonics, not so big as to be a “gas giant.”
• Or possibly a moon like Europa, a rocky/icy
moon with a steady source of tidal energy.
How did God create life on Earth?
Miraculously?
If so, then whether or not we should
expect that there is life elsewhere is
mostly a theological question.
Through natural processes?
(abiogenesis)
If so, then we can also address this
question scientifically.
Is abiogenesis possible? If so, how?
The best current hypothesis: (could be wrong)
1. Gather raw materials (H20, CO2, N2, CH4, etc.)
2. Small organic molecules, up to and including
amino acids
3. Short chains of amino acids
4. Short chains of RNA
5. Longer RNA strands and other polypeptides
which can catalyze chemical reactions
6. Proteins and DNA
At some point during steps 3-5, cell walls form.
How likely is abiogenesis?
Hypotheses for “improving the odds”:
• Clay substrates
• Evaporating ponds near volcanoes
• Hydrothermal vents
• Deep underground
How likely is abiogenesis?
Life on Earth exists under extreme
conditions of temperature, pressure,
salinity, pH.
But this doesn’t tell us if life can get started
under extreme conditions.
How tight are the environmental constraints
for abiogenesis?
How “lucky” do you have to be under those
conditions (or: how much time would it
take, under those conditions)?
The “Rare Earth” hypothesis
•
•
•
Do we need gas giants like Jupiter and
Saturn to shield the Earth from asteroid
and comet bombardment?
Does Earth need a large moon to
stabilize its rotational axis?
Because the sun’s brightness increased
by 25% over its lifetime, is the “habitable
zone” for liquid water very tightly
constrained?
The Drake Equation:
N = R* x fp x ne x fl x fi x fc x L
N = the number of civilizations in our galaxy with which we
might expect to be able to communicate at any given time.
R* = the rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = average number of planets that can potentially support life
per star that has planets
fl = the fraction of the above that actually go on to develop life
fi = the fraction of the above that actually go on to develop
intelligent life
fc = the fraction of the above that are willing and able to
communicate
L = the expected lifetime of such a civilization
How do we assign probabilities?
Probability calculations depend heavily
on the assumptions made.
Those with strong worldview preferences
(pro or con) often uncritically make
assumptions which favor their view.
Scientifically: a very hard problem and an
open question.
Two facts which might constrain
assumptions:
1. We haven’t been contacted yet.
2. First life on Earth appears fairly early.
(Somewhere between 3.8 and 3.0 Gya)
And another hypothesis which calls into
question earlier assumptions:
Panspermia – the idea that life on Earth
came from space.
If we found extraterrestrial microbial
life, what would be the
consequences?
If we found extraterrestrial intelligent
life, what would be the
consequences?
References:
Astrobiology: A Brief Introduction, Kevin Plaxco & Michael Gross
“The Beginnings of Life on Earth,” Christian deDuve,
www.amsci.org/amsci/articles/95articles/cdeduve.html
“The Origins and Early Evolution of Life,”
www.chemistry.ucsc.edu/~deamer/home.html
“The Origins of Life,” Albrecht Moritz
www.talkorigins.org/faqs/abioprob/originoflife.html
“Astrobiology: The Study of the Living Universe,” Christopher Chyba
and Kevin Hand, Annu. Rev. Astron. Astrophys. 2005. 43:31-74.
“Astrophysics in 2005,” Virginia Trimble, Markus J. Aschwanden, and
Carl J. Hansen. Chapter 7, “Astrobiology”