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Transcript
Astronomy 110 Announcements:
• Extra Credit due tomorrow
• Review session tomorrow during class
• Final Exam this Friday
– No calculators, cell phones, etc.
– Mostly short answer, some multiple choice
• Sunday July 3rd, Deep Impact (Encounter with
Comet Temple 1)
– Events on all islands
– Viewing at Bishop Museum ($3 admission)
Three Methods for Detecting Extrasolar Planets
1. Doppler effect due to “wobble” of star
2. Transiting planets (eclipses)
3. Microlensing
Cannot currently detect extrasolar planets through
direct imaging. Planets are too faint compared to
their Sun. Improved technology like
interferometry may help in the future.
Deep Impact website:
http://astroday.net/DeepImpactHawaii.html
2. Transiting Planets
1. Doppler Effect
Doppler effect due to
“wobble” of star
•
•
•
Kepler’s 3rd law
gives us orbital
distance
Size of wobble gives
us mass of planet
Useful only for large
planets close to their
Sun
Transiting planets (eclipses)
•
•
Light from star dims as planet passes in front
Useful only for large planets
3. Microlensing
7.5 Earth as a Living Planet
Microlensing
•
•
•
Foreground star with
planets acts as a lens
against distant star.
As lens passes in
front, light from star
and is amplified.
Useful for low mass
planets (Earth-like)
But you only get one
shot at it
• Our Goals for Learning
• What unique features on Earth are
important for human life?
• How might human activity change our
planet?
• What makes a planet habitable?
What unique features of Earth
are important for life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
What unique features of Earth
are important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
Earth’s distance from the
Sun and moderate
greenhouse effect make
liquid water possible
What unique features of Earth
are important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
PHOTOSYNTHESIS
(plant life) is required to
make high concentrations
of O2, which produces the
protective layer of O3.
Plate Tectonics
• Pieces (plates) of the Earth’s surface move
around over geologic time (few centimeters/yr)
• Internal heat needed to maintain this process (heat
also drives volcanism)
What unique features of Earth
are important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
Plate tectonics are
an important step
in the carbon
dioxide cycle.
The Carbon Dioxide Cycle
• Atmospheric CO2
dissolves in the oceans.
• Rainfall erodes rocks and
sends minerals into the
ocean
• Minerals + CO2 combine
to form carbonate rocks
(e.g., limestone) – depends
on temperature
• Plate tectonics carries
rocks into subduction
zones
• Carbonate rock melts and
releases CO2 back into the
atmosphere.
What unique features of Earth
are important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
The CO2 cycle acts like a
thermostat for the Earth’s
temperature.
These unique features are intertwined:
• plate tectonics creates climate stability
• climate stability allows liquid water
• liquid water is necessary for life
• life is necessary for atmospheric oxygen
Earth’s ice ages end
as oceans freeze over
and volcanoes release
CO2 into the
atmosphere
How might human activity affect
Earth’s climate?
Chapter 18
Life in the Universe
Human activity is increasing the concentration of
greenhouse gases in the atmosphere, which may
strengthen the greenhouse effect and lead to global
warming.
We, this people, on a small and lonely planet
Travelling through casual space
Past aloof stars, across the way of indifferent suns
To a destination where all signs tell us
It is possible and imperative that we learn
A brave and startling truth.
— Maya Angelou
When did life arise on Earth?
• Probably before 3.85 billion years ago.
• Shortly after end of heavy bombardment,
4.2-3.9 billion years ago.
• Evidence from fossils, carbon isotopes.
2 billion years…
How did life arise on Earth?
• Life evolves through time.
• All life on Earth shares a common ancestry.
• We may never know exactly how the first
organism arose, but laboratory experiments
suggest plausible scenarios.
Laboratory experiments allow us to investigate
possible pathways to the origin of life.
Miller-Urey experiment (and more recent experiments):
• Building blocks of life form easily and
spontaneously under conditions of early Earth.
Chemicals to Life?
Microscopic, enclosed membranes or “pre-cells”
have been created in the lab.
Could life have migrated to Earth?
• Venus, Earth, Mars have exchanged tons
of rock (blasted into orbit by impacts)
• Some microbes can survive years in
space...
Maybe this is how it happened…
What makes the Earth habitable?
• Large size: retains
heat longer.
– Leads to plate
tectonics which drive
the CO2 cycle, which
regulates the climate
– Volcanism
(outgassing). Leads to
our oceans and
atmosphere.
• Orbital distance: just
the right distance to
retain liquid water.
Are habitable planets likely?
Definition:
A habitable world contains the basic
necessities for life as we know it, including
liquid water.
• It does not necessarily have life.
Caveat: Telescopically we can search only for
planets with habitable surfaces — not for worlds
with Europa-like subsurface oceans.
What are the necessities of life?
• Nutrient source
• Energy (sunlight, chemical reactions,
internal heat)
• Liquid water (or possibly some other liquid)
Hardest to find
on other planets
Constraints on star systems:
1) Old enough to allow time for evolution (rules
out high-mass stars - 1%)
2) Need to have stable orbits (might rule out
binary/multiple star systems - 50%)
3) 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.
Finding them will be hard
Recall our scale model solar system:
The more massive the star, the
larger the habitable zone — higher
probability of a planet in this zone.
• Kepler (2007 launch)
will monitor 100,000
stars for transit events for
4 years.
• 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.
• But new technologies should soon show the
way…
Spectral signatures of life
Venus
oxygen/ozone
Earth
Later: SIM (2009?), TPF
(2015?): interferometers
to obtain spectra and
crude images of Earthsize planets.
Mars
Are Earth-like planets rare or common?
• Galactic “habitable zone”: minimum limits
on heavy element abundance, distance from
galactic center?
• Jupiter protection from frequent impacts?
• Climate stabilized by a large Moon and
plate tectonics?
We don’t yet know how important
or negligible these concerns are.
How many civilizations are out there?
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.
Are we “off the chart” smart?
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?
How does SETI work?
Looking for deliberate signals from E.T.
Your computer can help! SETI @ Home: a screensaver with a
purpose.
We’ve even sent a few signals ourselves…
Earth to globular cluster M13: Hoping we’ll hear
back in about 42,000 years!