Lecture 35:Are We Alone?

Astronomy 1143

Spring 2014

Key IdeasFinding planets in habitable zone key to finding life

Liquid water critical to life on Earth – always true?

Spectra of planets can reveal the presence of life

The Drake equation • Estimate of the number of intelligent civilizations

It only takes 1 civilization and a lot of time to colonize the Galaxy

• Where are they already?• Possible solutions to the Fermi paradox

Basic Requirements for Life

Stable, Long-lived Source of Energy Energy to fuel chemical reactions

Warmth to permit liquid water

Complex Chemistry Elements heavier than H and He Carbon, liquid water, inorganics

Benign Environmental Conditions Stable, well-regulated climate

Protection from harmful UV radiation

Location for life to emerge Oceans, land masses (place to swim/stand)

The Importance of Water

Liquid water may be a requirement for life to exist on any world

Certainly true on Earth

Life needs to extract energy by chemical reactions (forming new molecules)

Many, many substances dissolve in water, so individual chemical reactions can take place

Water is liquid at temperatures where chemical reactions happen rapidly

The Habitable Zone is the region around a star where liquid water is stable on a planet’s surface

Planet too close: Runaway greenhouse effect superheats the atmosphere and vaporizes all the water.

Planet too far: Water freezes out and won’t

be liquid on the surface.

1010.1

AU

Habitable Zones for Stars

8.5 – 12.5AU

1.5 – 2.2AU

0.08–0.12AU

0.38 – 0.56AU

0.95 – 1.4 AU

Are we imaginative enough?

The big challenge: Earths are extremely faint

compared to the light of their parent stars.

The spectrum of the Earth has two components:

Combined, the Earthis about 2 billion times

fainter than the Sun.Reflection

Thermal

Reflected Sunlight

Thermal InfraredEmission

Signs of Life

Spectra from exoplanets can show if they • have an atmosphere• have oxygen in their atmosphere• have water in their atmosphere• show other features of life such as a “red edge”

Discovery of these “biomarkers” would be extremely interesting

Don’t have to travel to the planet to look for life, but do need exquisite instruments

from Beichmann et al. – NASA Origins website

Earth plants are strongly reflective in the infrared, giving their spectrum a distinctive “Red Edge”

The way plants keep cool infull sunlight.

How many civilizations are out there?

Life, of whatever kind, on other planets is fascinating. Intelligent life that we could communicate with would also be fascinating.

The Drake equation provides a useful way to think about what we know, what we don’t know, and the probability of intelligent civilizations

One reason we think intelligent life must have arisen elsewhere is the sheer number of stars

~200 billion galaxies in the visible Universe

~100 billion stars per galaxy

Total of ~2x1022 (20 billion trillion) stars

Even a chance of 1 in 1012 would yield morethan 20 billion possible sites for life.

The Drake Equation estimates the number of advanced, communicating civilizations in our Galaxy

R = rate of star formation

fp = fraction of stars with planets

ne = number of Earth-like planets per system

fl = fraction with life.

fi = fraction with intelligence

fC = fraction with communication technology

L = lifetime of an advanced civilization Frank Drake

LfffnfRN cilep

R, the rate of star formation per year, is known from extensive observations.

Age

LfffnfNN cilep

A reasonable estimate:

N 100 Billion StarsAge 13 Billon Years

R 7 stars/year

Age

NR

fp, the fraction of stars with planets, is becoming known from exoplanet searches.

Present observed fraction is

Optimistic Estimate:

fp 0.5

fp 0.15

But, this is mostly giantplanets – rocky planets may

be more common.

ne, the number of planetary systems with Earth-like planets is still unknown, but soon measurable.

Should become known in thenext decade or so.

Depends on the detaileddistribution of rocky planets

around stars.

Right now, we don’t know.

Optimistic Guess:

ne 1Allows us establish an upper limit…

fl, the fraction of Earth-like planets with life, is currently unknown and conjectural.

Some guidance from the history of Earth

Life arose within ~100 Myr of the end ofthe epoch of Heavy bombardment.

Optimistic Guess: fl 1

fi, the fraction of life that is intelligent, is even harder to guess.

Clues from Earth’s history: First life arose ~3.8 Gya

Multi-cellular life ~1.2 Gya

Cambrian explosion ~545 Mya

Land colonization ~475 Mya

Homo sapiens emerged ~100,000 years ago

Took 100 Myr for life to emerge, but ~40x longer for anintelligent species (us) to appear.

Wild Guess: fi 0.1 (is it rarer, or just take longer?)

Do we qualify as “Intelligent Life”?

You have to wonder sometimes:

Only had radio communicationstechnology for ~100 years.

Only had limited (short-duration)manned spaceflight for ~50 years.

Only sent robotic spacecraft tothe edges of our Solar System in the last decade.

May or may not yet have sufficiently sensitiveradio reception technology.

fc, the fraction of intelligent life that is capable of (or interested in) communication, is purely conjectural.

The rise of science andtechnology is very recent

cultural development.

It entails the ability (andwillingness) to make senseof the world in terms of logic

and physical principles.

Shameless and baseless optimism: fc 1

L, the lifetime of an advanced, communicating civilization, is difficult (and uncomfortable) to guess

Lower Bound: We’ve only had radio

technology for ~100 years

Upper Bounds: Next year? 100 years? When habitable zone moves past Earth in ~3 Gyr? When the Sun runs out of core Hydrogen (5 Gyr)

Only example is us…

A shamelessly optimistic guess…

N = 100 Billion starsfp = 0.5ne = 1fl =1

fi =0.1 fc =1

L =100 years (we made it this far … so far …)Age = 10 billion years

100 yr100 Billion 0.5 1 1 0.1 1

10 Gyr

50

p e l i c

LN N f n f f f

Age

With 50 civilizations, average distance is 7000 light years

The Drake Equation is open to numerous criticisms, and is not without its detractors.

Relies on many unknownquantities, and so it is heavy

on conjecture.

Doesn’t account for populationdynamics if interstellar colonization is possible.

So where are they already?

A single civilization could colonize a whole Galaxy.

Interstellar Colonization is a rapid, exponential process.

The time to colonize the entire

Galaxy is ~500 Myr even withvery modest assumptions

The Fermi ParadoxIf any civilizations have been around in our Galaxy forat least ~10–100 Myr they should have colonizedthe entire Galaxy by now.

We should have beenvisited many times over.

But, we have no evidenceof extraterrestrial visitation,

nor have we found any alien artifacts on Earth.

A number of explanations havebeen proposed …

1950 New Yorker Cartoon by Alan Dunn

UFOs Are Real?Extraterrestrial Visitations? No.

No reliable, repeatable, or reputableevidence has been offered:

Fuzzy photographs Anecdotal accounts of visits and

abductions Claims of government conspiracies

There are unexplained sightings, but failure to explain themdoes not justify leaps to truly wild explanations.

Civilizations have colonized the Galaxy, but…

They aren’t telling us… “Zoo Hypothesis”

“Sentinel Hypothesis” A “Prime Directive” (ethics)

We are too “primitive” to tell They are too advanced or

too different from us.

Many examples of “fatal impacts” between peoples inhuman history, even when well-intentioned.

Civilizations colonized us long ago

In this theory, we are actually part alien!But this flies in the face of the genetic and archaeological recordHowever, building blocks of life could have been brought on comets….

Civilizations have not colonized the Galaxy…

…because it is too expensive

…because they don’t want to

Use slow autonomous robotic probesinstead of expensive starships

Self-replicating Von Neumann machines

Bracewell “Messenger Probes”

But not a lot of precedent in human history

…because civilizations self-destruct before they can colonize the Galaxy

Human populations tend to grow faster than sustainability

Tendency toward aggressionPoor long-range planning

Irresponsibility with technology

“Doomsday Hypothesis”

Nuclear warBiological warfare

Accidental contaminationNanotechnology catastropheEnvironmental catastrophe

…because Nature can be cruel, which limits the lifetimes of even peaceful

civilizations. Possible exterminating events:

Asteroid or Comet Impact

Supernova or Gamma-Ray Burst sterilizes planet.

Ample precedent for this in Earth’sgeological history.

The Rare Earth Hypothesis posits that complex intelligent life like on Earth is extremely rare

We are a product of an extremely unlikely combinationof geological & astronomical circumstances.

We’re in the right part of our Galaxy

We are around the right kind of star

We have a relatively benign impact environment

Earth has a large moon that stabilizes its rotation

Complex life arose late, and intelligence even later

Concludes that we are the only intelligent species.

My Best Guess

Life exists on many other planets, but space travel is prohibitively hard and random communication unlikely

Earth is already getting “darker” in its broadcasts

We have sent very few deliberate messages into space toward very few locations

Nowhere close to long range space flight

Very interesting if I’m wrong