Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 42: Life in the Universe

Key Ideas:

Life:: Extreme Life on Earth; Requirements for Life
Are We Alone?: Life vs. Intelligent Life; Drake Equation; Extraterrestrial Civilizations?; Prospects for Contact

Extreme Life on Earth

Dark Life: Bacteria thrive many kilometers beneath the Earth
Hot Life: Microbes survive in boiling geyser pools and deep under the ocean by thermal vents
On the Moon: Strep bacteria survived 3 years on the lunar surface

Requirements for Life

Energy: Warmth to produce liquid water (liquid methane?); Energy to fuel chemical reactions
Complex chemistry: Elements heavier than H and He; Building blocks for complex molecules
Protection from harmful UV: Too fast mutations may inhibit the development of complex life; Ozone layer, underwater, or underground

Life in the Solar System?

Mars: May have had liquid water and a thicker atmosphere in the past
Europa: Liquid water ocean warmed by Jupiter's tides; Protected by an outer shell of ice
Titan: Methane chemistry; Complex molecules are present

Each may meet the three requirements for life to develop

Life in the Galaxy? The Universe?

Our Milky Way galaxy is VERY BIG and VERY OLD: Over 100 billion stars; 10% may have planetary systems; Over 10 billion years old (twice the age of our Solar System); And our Milky Way galaxy is just one of billions of galaxies in the Universe

Even if the probability that life would form is incredibly small, there nevertheless must be billions of planets out there...

Life vs. Intelligent Life

Life formed within a billion years on the Earth, yet intelligent life required 4.5 billion years
There may be additional requirements for intelligent life: Long-lived, stable star (>4.5 Gyr); Chemically rich; Just the right distance from the central star (the "habitable zone")

The Drake Equation

N = R_* f_p n_e f_l f_i f_c L

N: number of advanced civilization in the Galaxy

R_*: rate at which Sun-like stars form

f_p: fraction of stars with planetary systems

n_e: number of Earth-like planets per system

f_l: fraction of Earth-like planets with life

f_i: fraction where intelligent life has evolved

f_c: fraction with communication technology

L: lifetime of an advanced civilization

Measurement and Conjecture

Only the first 3 terms of the Drake Equation can be measured by astronomers:: R_* ~ 1 per year (Sun-like stars); f_p ~ 0.1 to 0.2 from recent planet searches; n_e: the number of Earth-like planets per system may be measureable in the next 20 years
The rest are pure conjecture

What constitutes an advanced civilization?

This usually means:: Capable of communicating across interstellar distances.; Capable of interstellar travel by spacecraft; Interested in finding and communicating with other civilizations

Do we qualify as "Advanced?"

Just barely:: Only had radio communications for ~100 years; Only had limited (short-duration) manned spaceflight for ~40 years; Only sent robotic spacecraft to the edges of our Solar System in the last 10 years; May or may not yet have sufficiently sensitive radio reception technology.

Anyone Else Out There?

One very optimistic view:: n_e = 0.1 (1 in 10 solar systems have Earths); f_l = 1 (if Earth-like, life is inevitable); f_i = 1 (if life, intelligence is inevitable); f_c = 1 (if intelligence, technology is inevitable); L = 100 years (for us, so far so good)
N = 1 x 0.1 x 0.1 x 1 x 1 x 1 x 100 = 1!
And if civilizations last 1000 years, there would be 10

Extraterrestrial Visitations? No.

"Extraordinary claims require extraordinary proof"
No extraordinary proof has been offered:
Fuzzy photographs
Anecdotal accounts of visits and abductions
Claims of government conspiracies
There are unexplained sightings, but failure to explain them does not justify a leap to a truly wild explanation.

Where are they?

The extreme difficulty of interstellar travel is a plausible explanation of a lack of visits
The distances between stars are enormous
Need very large amounts of time or
Extremely large amounts of energy
The fastest spacecraft: Voyager 1 and 2
Outward bound at 15 km/s (0.005% the speed of light)
Need 80,000 years to reach the nearest stars

Relativitistic Starships

The fast route is to accelerate a starship to near the speed of light
Need to reach 0.1c to reach the nearest star in 50 years
Energy costs are enormous:
Amount of fuel increases exponentially with the acceleration time (have to push more fuel at first).
10% is the maximum efficiency for matter/antimatter fuel
But the production efficiency of such fuel is expected to be extremely low. . .
Possible given a sufficiently advanced technology?

Talk is cheap!

If you really want to bridge interstellar distances, use light to send messages.
Messages travel at the speed of light
Very low energy cost per message
What wavelengths to use?
Microwaves: 1000 - 10,000 MHz is a region of relatively low cosmic background "noise"
Lasers at visible or IR wavelengths: very few natural lasers in the sky to cause confusion.

Earth is already on the air

We have been inadvertently beaming radio signals into space for the last 80 years:
Radio broadcasts from the 1920s onward
Television broadcasts from the 1950s onward
We could detect these with current technology
Episodes of "I Love Lucy" will have already reached most stars in the solar neighborhood (~40 light years)!

Increasing radio silence

Earth's radio brightness has been decreasing
Introduction and spread of cable TV
Increased use of "directed" communication (e.g. fiber optics, beamed satellite, etc.)
Sufficiently advanced civilizations may emit less "waste radio" and become radio quiet
If a civilization wants to be found, it may have to deliberately broadcast its presence.

The Search...

SETI:
Search for Extra-Terrestrial Intelligence
A relatively inexpensive search strategy to look for radio signals from extraterrestrial civilizations
Phoenix Project of the SETI Institute
Optical and Microwave SETI at Harvard
Various smaller projects

What are we looking for?

Signals that appear "artificial"
Very narrow "bandwidth" (<300 Hz, the narrowest natural maser sources)
Pulsed signals (common way to encode information)
Highly polarized signals (another encoding scheme)
Very little frequency "drift"
So far, no detections.

Life, the Universe, and Everything

Our ability to predict the probability of life in the Universe is limited by the fact that we only have experience with one planet
Definitive identification (or not) of life in any form elsewhere in the Solar System would prove extraordinarily valuable
If intelligent life does exist elsewhere, communication seems much more likely (cheaper) than direct visitation

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.
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Updated: 2007 March 4 Copyright © Paul Martini All Rights Reserved.