Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 43: Are We Alone?
Intelligent Life in the Universe

Readings: none (but skim Ch 30)

Personal Opinions:

Instead of spelling out the "Key Ideas" of this lecture, I'm going to start by stating four personal opinions on the question of intelligent life in the Universe that are scientifically motivated but otherwise unsubstantiated:

It is extremely likely that intelligent life has arisen elsewhere in the Universe.
There is no convincing or even compelling evidence that extraterrestrial intelligences have visited the Earth, either now or in the past.
The reason they haven't visited is readily explained by the extreme difficulty of interstellar travel.
If we ever do come into contact with extraterrestrial intelligences, it will be by receiving radio signals from them.

My intention is to try to defend these opinions in this lecture.

Basic Requirements for Life

The first question we must address is what conditions are necessary for life to exist in the first place.

Based on what we know about life on Earth, the only place we know life exists for sure, we can determine at least 3 major requirements:

Energy: Warmth to allow liquid water to exist (or liquid methane?); Energy is needed to fuel chemical reactions (metabolism)
Complex Chemistry: Elements heavier than Hydrogen & Helium; Carbon as buidling blocks for complex molecules.
Protection from harmful UV radiation: UV light can damage or break complex molecules, causing mutations that may inhibit the emergence of complex life.; Protection from UV is afforded by the Ozone Layer, underwater, or underground.

Extreme Life on Earth

In addition to the more familiar life we see around us, life on Earth is often found in surprising and extreme environments. These "extremophiles" include:

Dark Life: Bacteria that thrive many kilometers beneath the Earth or deep inside polar ice
Hot Life: Microbes surviving in boiling water in geyser pools (e.g., at Yellowstone National Park); Deep ocean life near very hot thermal vents (microbes and tube worms)
On the Moon!: Terrestrial strep bacteria survived nearly 3 years on the Lunar Surface on the underside of parts of the Lunar Surveyor III returned by Apollo 12!

In other words, life is pretty tough, so it might thrive in a broad range of conditions.

Life Elsewhere in the Solar System?

Could life exist elsewhere in our Solar System? So far we haven't found it, but some places people have suggested we look are:

Mars: Evidence it had liquid water and maybe a heavier atmosphere in the distant past. Life might have briefly arisen there, and might survive underground (like terrestrial geobacteria).; This is a big driver of present and future Mars exploration.
Europa (Icy Galilean Moon of Jupiter): One model is that it has a liquid ocean under its ice that is warmed by tides; The outer shell of ice protects it from UV radiation and cold

Enceladus (Icy Moon of Saturn)

Warm water geysers seen by the Cassini spacecraft, suggest reservoirs of warm liquid water below the ice (heated by tides).

Like Europa, shielded by the outer ice layer.

Titan (giant moon of Saturn)

Titan has a thick methane atmosphere, and liquid methane chemistry

Complex molecules are seen to be present

Maybe too cold for water-based life, but methane-based life???

What do you mean by "Intelligent"?

Most of the extreme variations on life we have seen thus far are all quite simple: bacteria and tube worms. But what about complex and intelligent life.

When most people talk about "intelligent life", this is shorthand for

Possessing a highly advanced technological civilization.
Capable of communicating across interstellar distances.
Capable of interstellar travel by spacecraft.
Interested in finding and communicating with other intelligences.

In other words, life like us...

Do we qualify as "Intelligent"?

By the previous definition, Homo sapiens just barely qualifies:

Only had radio communication technology 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.

Sheer Weight of Numbers

Question:: Why do I think that intelligent life elsewhere is highly likely?
Answer:: The sheer numbers of possible planets:

To understand this, let's do a brief cosmic census:

~200 Billion galaxies in the visible universe
~100 Billion stars per galaxy

This gives about 2x10²² or 20 billion trillion stars in the visible Universe.

Even if we assess the probability of life at one chance in a trillion, that gives 20 billion intelligent species somewhere in the Universe.

Planetary Requirements for Life

Some astrophysical factors that influence which kinds of stars could have life-sustaining planets:

Long-lived, stable star

Good stars: F, G, and some K stars: last >3 Gyr, long enough to allow life time to evolve intelligence.
Bad Stars: O, B & A stars: These are all short-lived and hot, and so have high UV radiation output (damaging to organic molecules)
M stars: small and dim, often have powerful flaring (bad radiation environment)

A Stable orbital environment

Exclude most binary star systems from consideration

Metals (heavy elements)

Need metals to make rocky planets
Need carbon for complex molecules
Need a metal-rich star in a chemically evolved environment

On the latter point, there is some observational indication that those stars around which we have detected planets so far are all relatively metal rich - slightly more so than the local average.

The Drake Equation

One way of assessing the odds of finding intelligent life in our Galaxy was proposed by radio astronomer Frank Drake in 1961:

The terms in the Drake equation are:

N = number of advanced civilizations in the Galaxy

R_* = rate at which suitable stars are formed

f_p = fraction of stars with planetary systems

n_e = number of Earth-like planets per system

f_l = fraction of Earth-like planets that have life

f_i = fraction where intelligent life has evolved

f_c = fraction with communication technology

L = lifetime of an advanced civilization

If we could assign numbers to each of these, we could estimate how many intelligent civilizations (by our criteria) we would expect in our Galaxy.

Measurement & Conjecture

Only the first three terms of the Drake Equation can be measured by astronomers:

R_* is about 1 suitable (F, G, and K-type) star per year in the Milky Way galaxy.
f_p is at least 0.1-0.2 from recent searches for planetary systems, but still rather uncertain.
n_e may be measurable in the next few years (various ground-based and space-borne projects proposed)

All of the other terms in the Drake Equation are purely conjectural (and highly controversial) at the present time, and at least for the forseeable future.

Shameless Optimism

Let's take our situation here on Earth at face value and make the following very optimistic assumptions:

If a solar system forms, an earth like planet forms 1 in 10 times (n_e=0.1)
If such a planet forms, life inevitably arises (f_l=1)
Given enough time (~4.5 Gyr), intelligent life will inevitably emerge (f_i=1)
All intelligent beings will eventually develop radio technology and curiosity about other intelligences in the Universe (f_c=1)
We did it in at least 100 years and haven't blown ourselves up, yet, (L=100 years)

Putting these all together gives:

Shamelessly optimistic Drake Equation estimate

Or about 2 intelligent civilizations in the Galaxy. Good thing it is at least one if you count us.

The only way we can increase the count is to increase the number of earth-like planets expected, increase the number of solar systems, or increase the lifetime of advanced civilizations (I've already maxed-out most of the conjectural terms, which would probably excite controversy among my biologist friends). These increases are plausible, but only the first two are observables.

The bottom line is, if you take the Drake Equation at face value, the answer may be "life is rare in our galaxy".

Extraterrestrial Visitations? No.

Has the earth been visited by extraterrestrials, either recently or in the past? I believe the answer is a firm "No".

My guiding principle in evaluating this question is the maxim "Extraordinary claims require extraordinary proof."

None of the "proofs" offered to date qualify as "extraordinary" or convincing:

Fuzzy photographs
Anecdotal accounts of visits and abductions
Claims of government conspiracies (Roswell, Area 51, etc.)

Are there unexplained sightings? Yes. But admitting they are "unexplained" does not mean we are justified in leaping to truly wild explanations. On careful examination, most "unexplained" phenomena eventually have rather mundane explanations. No claim of extraterrestrial visitation has ever withstood even modest scrutiny.

The whole UFO business is, in my opinion, nothing more than a singular failure of imagination. The claimed visitations all have an unmistakable ring of the familiar about them (especially given the intensive saturation of our popular culture by Hollywood depictions of extraterrestrials and their vessels). If a truly "alien" civilization did visit us, it would probably be totally different than our expectations (if you had never seen or had described to you an elephant, could you have imagined one?).

The claim that our ancestors may have been visited (and even instructed) by extraterrestrial visitors is the reprehensible "Ancient Astronauts" nonsense that borders on a form of racism. I completely dismiss this offensive notion on the grounds that it shamelessly belittles our ancestors and the considerable achievements of their cultures. In the words of astronomer Frank Shu, it "represents a form of grave robbing".

Where are they?

A plausible explanation for the lack of visits is that interstellar travel is extremely difficult.

The distances between stars are enormous:

Need very large amounts of time, or
Extremely large amounts of energy

The fastest human spacecraft: Voyager 1 and 2

Outward bound at ~15 km/sec (0.005% c)
Would take ~80,000 years to reach the nearest stars.

Relativistic Starships

The "fast route" to interstellar travel is to accelerate a starship to near-light speed:

Need to reach at least 0.1c to reach the nearest stars in 50 years

The energy costs are simply enormous:

The amount of fuel required increases exponentially with the time spent accelerating (you have to burn more fuel at the start, and then to slow down again)
The best fuel, matter/antimatter annihilation, is only ~50% efficient at best (if you use anti-protons, there are other annihilation channels than photons that are poorly-suited for propulsion)
The production efficiency of such fuel is extremely low (it takes us a lot of energy to make a small number of anti-protons in a particle collider).

It would require a maximum committment of time and resources on the part of any sufficiently advanced civilization to attempt even local interstellar travel. If the estimates of the relative paucity of such civilizations (the Drake Equation above) are correct, then the mean distance between such civilizations in the Galaxy would be far larger than the typical distances between the stars, making matters worse.

Even so, a sufficiently advanced civilization might decide they can pull it off. If so, the trick appears to be that you need to be patient and travel light.

My guess is that if our Solar System is ever visited by an extraterrestrial starship, it will be a surprisingly small robotic craft on a one-way trip (saves enormously on life-support, food, and crew wear-and-tear).

Note: This is where we will pick up the topic on Friday, March 10

Talk is cheap!

If you really want to bridge interstellar distances, use light (electromagnetic radiation) to send messages.

Messages travel at the speed of light.
Very low energy cost per message.

What wavelengths to use?

Microwaves at 1000-10,000 MHz frequency. This is a region of relatively low cosmic background signal (also known as "The Water Hole").
Lasers at visible or infrared wavelengths. There are very few naturally occuring lasers in the sky, so they would stand out.

Earth is already on-the-air...

We have been inadvertently beaming radio signals into space for the last 80-odd years:

Regular AM radio broadcasting started in the 1920s
Television broadcasts started in the 1950s.

It is a somewhat sobering thought that episodes of "I Love Lucy" will already have reached most of the stars in the solar neighborhood (stars within ~50 light years).

Radio silence?

Curiously, the earth's "radio brightness" has been decreasing:

Introduction and spread of "closed channel" cable TV transmission.
Increased use of "directed" communications (optical fibers, low-power distributed radio, beamed high-frequency satellite up/downlinks) instead of "broadcast".

It may well be that sufficiently advanced civilizations emit less "waste radio" and so become "radio quiet" as they advance.

If a civilization wants to be found, it may have to purposely broadcast its presence with that end in mind.

The Search...

This brings us to SETI, the "Search for Extra-Terrestrial Intelligence".

SETI is a relatively inexpensive strategy to search for radio signals from extraterrestrial civilizations.

Phoenix Project of the SETI Institute
Microwave and Optical SETI at Harvard.
A number of smaller projects.

Much of the funding for these efforts is private.

What are we looking for?

One persistent problem is to ask what kind of signals will clearly be from an intelligent source, rather than naturally occuring.

Some properties of radio signals that might mean they are "artificial":

Narrow "bandwidth" (<300 Hz, the narrowest naturally occuring maser sources)
Pulsed signals (a common way to encode information)
High polarization (another way of encoding information)
Very little frequency "drift"

So far, there have been no detections, but the searches continue and will likely expand over the next few years.

What if we detect something?

Interesting question. What do you think?

I am again beholden to Prof. Barbara Ryden, whose own lecture on this topic for her Astronomy 162 class was a direct source of inspiration and whose approach I adopted.

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Lecture 43: Are We Alone? Intelligent Life in the Universe