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Finding Earths
Lecture 31, April 18, 2003
Transits in our solar system
• Mercury transits about a dozen times a
century but it is hard to observe the small
planet against the glare of the sun.
• Note, next transit of Mercury is May 7,
2003.
• Venus transits about twice per century.
Next transit June 2004.
Mercury Transits are hard to see
SOHO spacecraft view of 1999 Transit
It takes a telescope at high magnification to see Mercury.
Furthermore the planet may be invisible (lost in the sun’s
glare) until it starts to transit.
Warning
• Never look directly at the sun.
• To observe the sun safely with a telescope
requires a special solar filter that keeps
the vast majority of the sun’s light from
entering the telescope.
• Your eyes can be damaged much faster
then you can blink!
Visibility of 2003 Mercury Transit
Planet finding methods
• Doppler shift needs large mass planet close in.
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Otherwise change in velocity of star is too small
to measure. Lost in turbulence of star surface.
Transit searches most sensitive to large planets
orbiting close in. Otherwise unlikely to be
exactly edge on and drop in brightness of star
during transit will be very small.
We can and have found Jupiters. We can’t yet
find earths.
Planets we have found
• About 10% of sun like stars have Jupiter like
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planets inside of 5 AU.
Stars with planets have a higher concentration
of “metals” (chemical elements heavier then H
and He such as C, O, Fe…).
We think these elements helped form the first
solids as the gas cloud cooled and these solids
acted as nucleation sites for additional material
to condense to form rocky cores of planets.
Can we find lower mass
planets?
“Jupiters” are interesting but
finding “earths” would be exciting!
More sensitive searches from space
• A dedicated satellite can very accurately monitor
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the brightness of thousands of stars looking for
small transits. Without atmospheric fluctuations
one can measure the brightness very accurately
and can observer all the time.
A large optical telescope with a coronagraph or
an infrared interferometer could actually see
earth like planets around nearby stars!
Chronograph
• Disk that blocks most of the light from the
star but still allows one to detect the
nearby planet.
• Adaptive optics (deformable mirrors) allow
chronograph to cancel out almost all light
from star.
• Planet is only 1 billionth as bright as star!
Interferometer
• Way to very carefully combine the light
from two or more telescopes to form an
image with the resolution of a single giant
telescope equal in size to the distance
between the two telescopes.
• Need to keep the two telescopes very very
stable.
Terrestrial Planet Finder
• NASA research program to built a
spacecraft that can detect earth like
planets around nearby stars.
• One reference design would be a single
elliptical shaped mirror 8 to 10 meters
long. As long as the present largest
ground based telescope the two Keck 10
meter telescopes.
40 meter Infrared Interferometer
Finding earths
• NASA terrestrial planet finder. Europeans
are working on a simillar project they call
Darwin.
• What would we like to find in a planet?
• What information can we detect from
100+ trillion miles away?
What makes an earth an earth?
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Correct distance from correct star.
Correct mass to have correct gravity.
Correct atmosphere.
Magnetic field.
Geologic activity / plate tectonics?
Correct temperature.
Presence of H2O and chemical elements for life such as
C, N, O, …
Stable orbit.
Protection from many impacts, radiation …
Perhaps the planet we are looking for is very special.
Galactic Habitable Zone
• How many earth like planets are in the galaxy?
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How many places with life? How many
civilizations?
Not all places in the galaxy may be equivalent.
The chance for life may be highest in a
particular region of the galaxy known as the
galactic habitable zone.
Where do you want to life in the galaxy?
Chance for life may be high in only some places in the
Andromeda Galaxy
Galactic Core is Dangerous
• High density of stars and radiation in galactic
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core is dangerous for life.
Lots of chances for a nearby supernova.
Oort cloud of comets constantly being disturbed
by passing stars.
Radiation from nearby neutron stars and black
holes.
The downtown area may have too much “crime”
and violence. Not a good place to raise kids.
Halo of Galaxy is Undeveloped
• Gas in fringes of galaxy has not been
processed through very many stars and
does not have very many chemical
elements aside from H and He.
• Therefore few rocks to start planet
formation so planets may be rare.
• Few chemical elements needed for life.
Best to Live in the Suburbs
• Want to be far enough away from the core to be
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safe.
Yet close enough in so that past generations of
stars have made elements.
We are lucky that the sun is out between spiral
arms where the density of stars is low. Yet not
way out in the halo.
Focus on intermediate band in our and other
spiral galaxies. This is the galactic habitable
zone.
Next Time
• Project due April 25, 2003.
• SETI