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 planetesimalscircular, 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”