Extraterrestrial Life

! What is astrobiology? Are bacteria commonly found in very harsh environments that could not support human life?

! What are microbial life and intelligent life? Why might it be ~10,000 times easier to find microbial than intelligent life? How long have microbes and humans lived on the Earth?

! Why does SETI search the sky specifically in radio waves? ! Is it plausible to detect TV and radio broadcasts from

extraterrestrial civilizations? Why is pi the sort of signal an alien intelligence might broadcast to try to communicate?

! Is the Principle of Mediocrity flawed? Why is life unlikely around very massive stars? What is a habitable zone?

! What Drake Equation parameters are hard to measure? Is communication with alien civilizations in our Galaxy likely?

Learning Objectives

Astrobiology !We have now found

many exoplanets!We may soon start to

find Earth-like planetsorbiting Sun-like stars

!We’ve found waterin the atmospheres of some exoplanets (e.g. WASP-39b; a “hot Saturn”)

!We can begin to ask: Are we alone?!A new initiative - Astrobiology

! The study of how life may develop and evolve on the Earth and other planets

Clarification of “Life”! In your body there are

many more microbial cells than “you” cells

!The first life on Earth was a microbe

!Microbes were the only life on Earth for about 3 billion years! Homo sapiens has existed about 200,000 years

!Microbes can live in harsh environments! there’s far more life in the ground than on it

Tubeworms in Hydrothermal Vents

Lakes beneath Europa’s icy surface???

Bacteria in Lake

Vostok???

Crytpoendoliths in Antarctic dry valleys

Lake Vostok

Hydrothermal Vents

Antarctic dry valleys

The Martian “Canals”! Giovanni Schiaparelli

saw canali (channels)on Mars in 1877

! The public thought he “saw” canals and inferred an intelligent designer! Percival Lowell the

“discoverer” of Pluto was convinced they were real Mars (1895); Mars and its Canals (1906)

! But most scientists thought the idea ridiculous (e.g. Alfred Russel Wallace, the co-discoverer of evolution; Is Mars Habitable? (1907)

The UFO Hype !Paranoia that aliens visit

Earth remains common!There is absolutely no

reliable evidence that any unidentified objects in the sky are devices of extraterrestrial origin

!But, astronomers are looking hard for evidence of extraterrestrial life! microbial life, simple

life and intelligent life

SETI !Some astronomers search

other stars for radio signals! SETI - Search for

Extraterrestrial Intelligence

! http://www.seti.org!SETI efforts are now

mostly privately financed!Congress cut NASA’s SETI funding in 1993

!You can help out!SETI@home screen saver analyzes radio

signals on your laptop or computer

The 305-meter Arecibo radio telescope

Why Look in the Radio? !We’ve broadcasted our presence

on the Earth for over 100 years! The Earth is now significantly

brighter in radio waves than the Sun

!Alien civilizations would almostcertainly also communicate using radio waves

!We’ve also broadcast a number ofdeliberate signals in radio waves!Stars emit very weakly in radio!Radio waves pass through dust

Why Look in the Radio? !Our early TV/radio signals are likely

not powerful enough to be detected! Arecibo could detect TV out to only

about the size of our Solar System ! Detecting TV out to 100 light-years

requires new technology (or a radio telescope far larger than the Earth!)

!Deliberate tight-beam signals (such as from Arecibo’s own transmitters) can reach halfway across our Galaxy!but they’re not all-directional

!World War II (tight-beam) military signals should reach ~500 light years

What are we Looking for? !There are many naturally

occurring patterns in our Galaxy! Obviously...or astronomy

would be pointless!So we search for patterns that

are difficult to recreate naturallybut that an advanced civilization might produce (deliberately or by accident)

!There are some obvious patterns the could be used to establish contact and communication!3 pulses, pause, 1 pulse, pause, 4 pulses,

pause, 5 pulses, pause, 9 pulses, pause (pi)

What are we Looking for? !There are many naturally

occurring patterns in our Galaxy! Obviously...or astronomy

would be pointless!So we search for patterns that

are difficult to recreate naturallybut that an advanced civilization might produce (deliberately or by accident)

!There are some obvious patterns the could be used to establish contact and communication!3 pulses, pause, 1 pulse, pause, 4 pulses,

pause, 5 pulses, pause, 9 pulses, pause (pi)

Life on Earth ! Is life an unlikely accident or

inevitable in typical conditions?! SETI’s Principle of Mediocrity:

! The Earth is not special astronomically, physically, or chemically. It thus shouldn’tbe special biologically, either

! So, life likely developed elsewhere! But, the assumption of mediocrity might be flawed

! We have many examples of physics in the Universe but we only have one example of life on one planet

! Perhaps having a large Moon, or a Jupiter-like planet in a Jupiter-like orbit, makes us special?

! Life requires a long-lived star...evolution took ~3 billion years on Earth but a 2 M⊙ star lasts only ~2 billion years

! Earth-like life requires planets with stable orbits, a (quite) quiet star, heavy elements for chemistry, no nearby recent supernovas, etc.

! Life also requires a planet in the habitable zone of a star (illustrated, in blue, here, for two different mass stars)

Habitable Zones

A 0.5 M⊙ star The Sun

V

E

Ma

Me

VE

The Drake Equation

!Equation for estimating the number of alien civilizations in our Galaxy with which we could communicate

N = R* x fp x ne x fl x fi x fc x L

Number of alien

civilizations

Rate of formation of stars

(~2.5 per year)

Fraction of stars

withplanets

(~1)

Planets per star

that could support

life (~0.4)

Fraction that

evolve intelli- gence

Fraction on

which life

arises

Fraction that

comm-unicate

Lifetime of

advanced civiliz-ations

Frank Drake

Easier to measure

Harder to measure

The Drake Equation

! Drake himself estimated N would be about the same as L (the number of years a typical civilization spends communicating), which is very hard to estimate

! N ~ 100,000 implies ETs on about 1 in 1 million stars ! There are ~ 1000 stars of all types within 40 light

years of us (requiring N ~ 100,000,000)! Optimistically, for 2.5 x 1 x 0.4 x 1 x 0.1 x 0.5 x L, an

inhabited planet within about 40 light years implies that L has to be more than about 2 billion years!

N = R* x fp x ne x fl x fi x fc x L

Next Time

Review Lecture