Using Interferometry to Determine the Properties of Stars: Testing Stellar Evolution Theory Guillerm

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Understanding The Nature And Distribution of
Habitable Environments In The Universe
One World, Many Worlds Searching for Life on
Earth and on Other Planets Astrobiology
Series CfA, 22 November 2004
Guillermo Torres Harvard-Smithsonian Center for
Astrophysics
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What Is a Habitable Environment?

• Broad and restrictive definitions possible for
  habitable
• Appropriate for life (as we know it)
• What is life?
• Life is ubiquitous on Earth deserts, volcanoes
  and hot springs, high-altitude and polar lakes,
  deep-sea vents and salt ponds, toxic mines,
  nuclear reactors, etc.
• Suitable for humans (or similarly intelligent
  species)

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• Traditional definition
• A habitable planet is one in
  which liquid water is stable
  at the surface.
• Why water? Abundant, good solvent
  for carbon-based life-forms
• Implications for the search for life
• Surface water remote observations of planets
  around other stars (atmospheric signatures)
• Sub-surface water in situ exploration of places
  like Mars or Europa (no atmospheric signatures)

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Habitable Environments Around Stars (Habitable
Zones)

• The extent of the HZ is determined by temperature
  ?
• Dependence on the orbit of the planet ? distance
  to the star (ap)
• Dependence on the properties of the star ? size
  (R) and effective temperature (T)
• Dependence on the properties of the planet ?
  reflectivity, or albedo (Ap)

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• The actual temperature on the surface of a planet
  depends also on atmospheric conditions
• Formula gives 18º C for the Earth (0º F)
• Actual average temperature
  on the surface of the Earth is
  15º C (60º F)
• The 33º C difference comes
  from greenhouse effects,
  mainly due to CO2 and H2O
• Many other geological, climatic, and chemical
  phenomena come into play, including complex
  feedback mechanisms

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The Habitable Zone in the Solar System

• Difficult to determine accurately
• Inner edge photo-dissociation of water, and
  hydrogen escape
• Outer edge formation of CO2 clouds (increased
  albedo leads to cooling ? runaway glaciation)
• The Earth is inside the HZ (we are here!)
• Venus and Mars are outside the HZ, but near the
  inner and outer edges (0.951.37 AU). However,
  Mars may be inside if outer edge is 1.7 AU
  instead

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Possibilities for Life Elsewhere in the Solar
System

• Presence of water on the surface of a planet is a
  traditional requirement for life
• But we know that some forms of life can survive
  under the Earths surface
• Are there places in the solar system with
  sub-surface water?
• Evidence for water on Europa (satellite of
  Jupiter) and Mars

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Europa One of the Galilean Satellites of
Jupiter

• Slightly smaller than the Moon (and ¼ of the size
  of the Earth)
• Highly reflective surface (albedo 0.64)
• Icy crust? Could there be liquid water
  underneath?

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Europa a Surface Covered With Cracks

• Complex network of ridges and fractures
• Signs of ice-plate tectonics
• Little cratering (young surface)
• Slush or liquid water underneath?

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A Sub-Surface Ocean on Europa

• Surface temperature 160 C (260 F)
• Tidally locked P 3.55 days
  (day year)
• Internal source of heat provided by tidal forces
  between Europa, the other Galilean satellites,
  and Jupiter

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Mars Strong Visual Indications of Surface Water
in the Past
Evidence for channels, canyons, and ancient lake
shorelines
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Signs that water may have flowed in the past
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Polar Ice Caps on Mars
South pole
North pole
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Liquid Water on Mars

• Mars cooled down more rapidly than
  the Earth because of its lower mass, and
  lost its atmosphere
• Thin atmosphere on Mars does not currently
  support liquid water (it evaporated long ago)
• NASAs twin Mars Rovers have
  shown ample evidence for the presence
  of water on the surface in the
  past
• Conditions for life change with time as a planet
  evolves

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Changes in the Location of the HZ With Time

• Stars change their luminosity as they age the
  Sun is now 30 brighter than when it formed, 4.6
  billion years ago
• The HZ moves outward with time

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Continuously Habitable Zone (CHZ)
The CHZ is the region around a star where
conditions for habitability are maintained for
long periods of time.
The CHZ over the 4.6 billion year lifetime of the
solar system is between 0.95 and 1.15 AU. Thats
why were still here!
The HZ moves outward with time
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Habitable Zones Around Other Stars

• Discovery of Jupiter-size planets around other
  stars (current tally more than 130)
• Planets are too faint to be observed directly
• Indirect methods must be used (wobble, transits)
• Initial surprises
• Small orbit sizes ? Bad news for HZs
• Orbit shapes range of eccentricities
• Discovery of Neptune-size planets (14 MEarth)

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Relative planet sizes
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An Interesting Extrasolar Planet HD 209458

• Found to undergo transits every 3.52 days
• Distance from star 0.047 AU
• Temperature ? 1000º C
• Mass 0.69 MJup (219 MEarth)
• Size 1.35 RJup (15 REarth)

• Density 0.35 g cm-3 (gaseous)
• Possibly tidally locked

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The Atmosphere of HD 209458 Gone With the Wind

• First planet outside the solar system in which an
  atmosphere has been discovered, using HST
• Sodium (2002)
• Hydrogen (2003)
• Carbon and oxygen (2004)
• Calculations indicate that
  hydrogen is leaking out the
  star is losing its outer atmosphere

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Discovery of Hydrogen in HD 209458 The cloud of
gas evaporating away blocks some of the light of
the star as it passes in front of it, and this
can be measured. This discovery was made possible
by the fact that the planet transits across the
star.
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HD 209458 is estimated to be losing more than
10000 tons of hydrogen per second. The planet is
believed to have lost 17 of its total mass by
evaporation. This planet is not habitable!
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So, Are There Any Habitable Extrasolar Planets?

• Many of the planets discovered are too close to
  the star (periods of a few days)
• This is an observational bias
• As observations continue, we discover planets in
  larger and larger orbits
• We already know of many planets that are far
  enough from their stars to be habitable

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Other Factors Affecting Habitability

• Most stars in the Galaxy are multiple (binary
  stars, triple stars, etc.)
• Planets around binary stars may find it more
  difficult to become habitable
• Stability of the orbit
• Radiation from two stars
• Massive stars do not live very long, and emit
  strong ultraviolet radiation (harmful for life)

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• Is the Moon necessary for the
  evolution of life (tides)?
• Inclination of the rotation axis
  are seasons necessary for life?
• Eccentricity of the orbit (variable
  illumination)
• Size of the planet (cooling time)

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Habitable Zones in the Galaxy

• Not all regions of our Galaxy are favorable for
  the appearance of life
• What are the requirements for the GHZ?

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Requirements for the GHZ

• Must have enough heavy elements (metals) to form
  terrestrial planets
• Metals are formed in stars by nucleosynthesis ?
  need enough time for stars to evolve
• Must be a sufficiently calm and safe environment
  to allow the biological evolution of complex
  multicellular life
• Supernovae produce lethal bursts of radiation
• Inner regions of the Galaxy are crowded ?
  danger of close encounters with passing stars

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Evolution of the GHZ With Time

• Intense star formation toward the crowded inner
  Galaxy provides heavy elements
• Supernovae rate is higher early on
• Eventually the regions with the right amount of
  metals spread outward

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Why Are We Interested in Habitable Environments?

• Not just science fiction anymore we know now
  there are lots of planets out there
• We hope to be able to discover
  Earth-type planets within the next
  decade or so (e.g., NASAs Kepler
  mission, to be launched in 2007)
• Future space missions will attempt
  to observe Earth-size planets directly
  (e.g., NASAs Terrestrial Planet Finder mission)