Life in the Universe

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Life in the Universe
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24.2 Life in the Solar System

• Our goals for learning
• How do we find/identify Life?
• Could there be life in the Solar System and where
  would it be?

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Finding life in the Universe

• Solar System -close enough to visit, active
  exploration possible.
• Look for signs of life in likely places
• Other star systems -too far away to visit,
  passive exploration dependant on naturally
  received data only.
• Finding stars with planets (preferably
  Earth-like), identifying right materials for
  habitability, looking for signals from
  intelligence.

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What Does Life look Like?

• Only one known example- Earth!
• We use Earths fossil record and environmental
  niches to gain an idea of where, what and how
  life exists. (Terrestrial analogues)

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Brief History of Life on Earth

• 4.4 billion years - early oceans form
• 3.5 billion years - cyanobacteria start releasing
  oxygen.
• 2.0 billion years - oxygen begins building up in
  atmosphere
• 540-500 million years - Cambrian Explosion
• 225-65 million years - dinosaurs and small
  mammals (dinosaurs ruled)
• Few million years - earliest hominids

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Necessities for Life

• Nutrient source
• Energy (sunlight, chemical reactions, internal
  heat)
• Liquid water (or possibly some other liquid)
• Stable environment for life to form (e.g. not
  devastated by volcanoes or impacts every 5
  minutes)

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Looking For Life in the Solar System

• Look for water and the other necessities
• Examine environments that life can flourish
• (extremophiles)
• Examining fossils to recognize past life
  (identifying fossil bacteria)

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Life in Unexpected Places(Extremophiles)

• Extreme cold
• Extreme heat
• Extreme pressure
• No sunlight
• Low water content

Poison tolerant Salt tolerant Acid
tolerant Alkali tolerant Radiation tolerant
Lives in minerals No sunlight No oxygen
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1. Searching for Life on Mars

• Mars had liquid water in the distant past, still
  has subsurface ice possibly subsurface water
• Energy sources solar, possible old volcanic
  vents
• Carbon compounds from meteorites

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In 2004, NASA Spirit and Opportunity Rovers sent
home new mineral evidence of past liquid water on
Mars. Also looking for evidence of micro-fossils
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2. Jovian Ice Worlds
Evidence for global subsurface ocean on Europa
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• Ganymede, Callisto also show some evidence for
  subsurface oceans.
• Relatively little energy available for life, but
  still
• Intriguing prospect of THREE potential homes for
  life around Jupiter alone

Ganymede
Callisto
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3. Titan?

• Surface too cold for liquid water (but deep
  underground?)
• Liquid ethane/methane on surface
• Lots of hydrocarbons
• Relatively stable, energy sources could be a
  problem.

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What have we learned?

• Identifying Life
• The most common and longest life forms are
  single-celled. They arose at least 3.85 billion
  years ago on Earth and occur in a variety of
  environments that complex life cannot endure.
• What are the necessities of life?
• Nutrients, energy, and liquid water
• Could there be life In the Solar System?
• Evidence for present or past liquid water occur
  on Mars, Europa, possibly Ganymede and Callisto.
  Mars is the easiest to explore and exploration is
  ongoing.
• Any life on Mars would be simple and small though

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24.3 Looking for Life Around Other Stars

• Our goals for learning
• Are habitable planets likely?
• Hunting for Planets
• Are Earth-like planets rare or common?

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Are habitable planets likely?
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Habitable Planets

• Definition
• A habitable world contains the basic necessities
  for life as we know it, including liquid water.
• It does not necessarily have life.

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• Constraints on star systems
• Old enough to allow time for evolution (rules out
  high-mass stars - 1)
• Need to have stable orbits (might rule out
  binary/multiple star systems - 50)
• Size of habitable zone region in which a
  planet of the right size could have liquid water
  on its surface.

Even so billions of stars in the Milky Way seem
at least to offer the possibility of habitable
worlds.
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The more massive the star, the larger the
habitable zone higher probability of a planet
in this zone.
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Finding them will be hard

• Recall our scale model solar system
• Looking for an Earthlike planet around a nearby
  star is like standing on the East Coast of the
  United States and looking for a pinhead on the
  West Coast with a VERY bright grapefruit
  nearby.

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Hunting For Planets

• Have to use light coming to us- no interstellar
  exploration (
• Direct Pictures or spectra of the planets
  themselves
• Indirect Measuring the effects of planets on the
  properties of their parent stars.
• (Stellar wobble, Doppler shift effects,
  brightness changes during transits/eclipses)

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Stellar Wobble

• Sun and Jupiter orbit around their common center
  of mass
• Sun therefore wobbles around that center of mass
  with same period as Jupiter

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Astrometrics (Stellar Wandering)

• Suns motion around solar systems center of mass
  depends on tugs from all the planets
• Astronomers around other stars that measured this
  motion could determine masses and orbits of all
  the planets

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Astrometrics (Stellar Wandering)

• We can detect planets by measuring the change in
  a stars position on sky
• However, these tiny motions are very difficult to
  measure (0.001 arcsecond)

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Doppler Technique

• Measuring a stars Doppler shift can tell us its
  motion toward and away from us
• Current techniques can measure motions as small
  as 1 m/s (walking speed!)

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Transits and Eclipses

• A transit is when a planet crosses in front of a
  star
• The resulting eclipse reduces the stars apparent
  brightness and tells us planets radius

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Finding Planets

• Current technologies cannot detect a planet as
  small as Earth, most exo-planets found so far are
  Jupiter-sized.
• One System Gliese 581 has a planet that is
  calculated to be about the mass of Neptune and 2
  planets called super-Earths (about 5-8 Earth
  masses).

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Spectral Signatures of Life
Venus
Earth

• oxygen/ozone

Mars
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Are Earth-like planets rare or common?
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Elements and Habitability

• Some scientists argue that proportions of heavy
  elements need to be just right for formation of
  habitable planets
• If so, then Earth-like planets are restricted to
  a galactic habitable zone

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Impacts and Habitability

• Some scientists argue that Jupiter-like planets
  are necessary to reduce rate of impacts
• If so, then Earth-like planets are restricted to
  star systems with Jupiter-like planets

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Climate and Habitability

• Some scientists argue that plate tectonics and/or
  a large Moon are necessary to keep the climate of
  an Earth-like planet stable enough for life

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The Bottom Line
We dont yet know how important or negligible
these concerns are.
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What have we learned?

• Are habitable planets likely?
• Billions stars have sizable habitable zones, but
  we dont yet know how many have terrestrial
  planets in those zones
• Finding Exo-planets
• Over 200 planets have been found by direct or
  indirect means. The smallest is Neptune-sized,
  no earth-sized worlds found yet.
• Are Earth-like planets rare or common?
• We dont yet know because we are still trying to
  understand all the factors that make Earth
  suitable for life

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24.4 The Search for Extraterrestrial Intelligence

• Our goals for learning
• How many civilizations are out there?
• How does SETI work?

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How many civilizations are out there?
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The Drake Equation

• Number of civilizations with whom we could
  potentially communicate NHP ? flife ? fciv ?
  fnow
• NHP total of habitable planets in galaxy
• flife fraction of habitable planets with life
• fciv fraction of life-bearing planets w/
  civilization at some time
• fnow fraction of civilizations around now.

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We do not know the values for the Drake Equation

• NHP probably billions.
• flife ??? Hard to say (near 0 or near 1)
• fciv ??? It took 4 billion years on Earth
• fnow ??? Can civilizations survive long-term?

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How does SETI work?
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SETI experiments look for deliberate signals from
E.T.
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Your computer can help! SETI _at_ Home a
screensaver with a purpose.
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What have we learned?

• How many civilizations are out there?
• We dont know, but the Drake equation gives us a
  framework for thinking about the question
• How does SETI work?
• Some telescopes are looking for deliberate
  communications from other worlds

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24.5 Interstellar Travel and Its Implications to
Civilization

• Our goals for learning
• How difficult is interstellar travel?
• Where are the aliens?

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How difficult is interstellar travel?
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Current Spacecraft

• Current spacecraft travel at lt1/10,000 c 100,000
  years to the nearest stars.

Pioneer plaque
Voyager record
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Difficulties of Interstellar Travel

• Far more efficient engines are needed
• Energy requirements are enormous
• Ordinary interstellar particles become like
  cosmic rays
• Social complications of time dilation

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Where are the aliens?
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Fermis Paradox

• Plausible arguments suggest that civilizations
  should be common, for example
• Even if only 1 in 1 million stars gets a
  civilization at some time ? 100,000 civilizations
• So why we havent we detected them?

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Possible solutions to the paradox

• We are alone life/civilizations much rarer than
  we might have guessed.
• Our own planet/civilization looks all the more
  precious

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Possible solutions to the paradox

• Civilizations are common but interstellar travel
  is not. Perhaps because
• Interstellar travel more difficult than we think.
• Desire to explore is rare.
• Civilizations destroy themselves before achieving
  interstellar travel

These are all possibilities, but not very
appealing
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Possible solutions to the paradox

• There IS a galactic civilization
• and some day well meet them

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What have we learned?

• How difficult is interstellar travel?
• Interstellar travel remains well beyond our
  current capabilities and poses enormous
  diffculties
• Where are the aliens?
• Plausible arguments suggest that if interstellar
  civilizations are common then at least one of
  them should have colonized the rest of the galaxy
• Are we alone? Has there been no colonization?
  Are the colonists hiding?