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Worlds Unnumbered
Lecture Twenty-Nine, Apr. 14, 2003
Projects due April 25
• Background papers have been graded (out
of 15 points).
• Happy to offer suggestions or answer
questions for your project.
The search for planets around
other stars
Buy Land!
• Because they are not making a whole lot more
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of it.
What if they were making new land by the
planet full!
It’s time we start searching for these “worlds
unnumbered”. Unnumbered because there are
so many of them in the galaxy. Unnumbered
because we have not yet discovered most of
them.
Search for Extrasolar Planets
• To find extraterrestrial life!
• To find good places to colonize?
• To understand the possible places for life
in the universe.
• To see how unique/ rare is earth. To
understand better planet formation.
• It will be very exciting to find “another
earth”.
Starships
• Are very very hard to build
because the distances are so
great.
• It might cost 100 billion $ to
send people to Mars. This is
comparable to the space
station or the Apollo moon
landings.
• A starship might cost our full
gross national product???
• Only for a far future time when
worlds economy is larger.
Justifying the Cost
• Maybe starships are too expensive just to
explore other stars.
• Colonization of new worlds could justify
great cost. Or avoiding disaster on earth.
• For now work hard to explore other stars
with sensitive instruments from within our
solar system.
Extrasolar Planets
• Golden age of exploration starts with Columbus
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discovering the new world in 1492.
New worlds in our solar system? Mars is most
Earth like but it is still very hostile. Can we do
better in another solar system?
Now we are looking to discover lots more new
worlds around other stars.
Can we find another Earth? For now we are
only sensitive to more massive planets.
Where should we look in
another star system?
Habitable Zone
• Region correct distance from a star for temp. for
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liquid water.
Where in a planetary system to look for life.
For Sun habitable zone from beyond Venus to
inside Mars: about 0.75 to 1.5 Astronomical
units.
– Depends somewhat on planets atmosphere.
– For brighter star habitable zone farther out, for
dimmer star, zone closer in.
• Assumes heat from star.
Limitations on habitable zones
• If star is too large.
– It is bright with a large habitable zone far from star. But:
– It burns its fuel quickly and will only last millions instead of
billions of years.
– There may not be enough time for complex life to evolve.
• If star is much smaller then Sun.
– Habitable zone is close in.
– For a planet this close in tides will lock its rotation to always
keep one side facing the star. Example, near side of Moon
always faces Earth.
– This lack of day and night is probably not good for life.
• Therefore look for stars about the same mass as the Sun
and look for planets orbiting at about 1 AU.
Double Stars
• Many stars are in double or multiple star
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systems.
Gravity from additional stars can hurl planets
from system.
Most planetary orbits in multiples star systems
are unstable over billions of years.
Multiple star systems are probably not good
places to look for planets and life.
Look near single stars.
Worlds Unnumbered
• To find inhabited worlds we must first find
worlds.
• Milkyway galaxy contains 1011 stars and
we think a significant fraction have planets
• Very many possible places for life!
• We are just starting to be able to detect
planets around other stars.
Objects in our search
• Planets: “low mass” objects including rocky
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planets like Earth and gas giants such as Jupiter.
Question when is a “giant” gas giant big enough
to be a “star”?
Brown dwarfs: in between objects neither fish
nor fowl. More massive then planets but less
massive then stars.
Stars: objects more massive then 0.08 the mass
of the Sun. Cores get hot enough for nuclear
reactions.
Finding Extrasolar Planets is Hard
• Stars are far away (light years).
• Planets at these distances are very faint
and appear very close to the star.
• Faint planet lost in glare of star.
• Atmospheric blurring mixes star and
planet light.
• Space dust and Earth’s atmosphere can
scatter bright star light obscuring planet.
A Light Year
• Light travels 186,000 miles per second.
• In one year light travels 6 £ 1012 miles.
• Thus a light year is 6 trillion miles!
• The nearest star is 4.3 light years away.
• Interstellar journeys need to cover many
trillions of miles!
• Need sensitive means to detect planets
10s of trillions of miles away!
Direct Detection of Planets
• Direct detection: observe light from planet.
• Has never been done.
• Try in infrared. Planets absorb visible light from
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star and reradiate it in infrared. Planet is not as
overpowered by star’s glare in infrared.
Technology should improve to allow direct
detection of some extrasolar planets soon.
The sun is 1010 times as bright as the earth in visible light!
Doppler shift of star light
• Instead of seeing light from planet, observe
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indirect effect of planet on star.
Planet and star orbit a common center of mass.
Star orbits in a small circle, planet a large circle
about the center of mass
Star
Massive Planet
Center of Mass
Planet and star move around
common center of mass
Star Wobble
• As the star orbits in a small circle it moves first
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slowly towards and then slowly away from Earth.
This wobble Doppler shifts the star light ever so
slightly.
We can detect the effect of the planet in this
Doppler shift of the star light without ever being
able to detect light reflected from the planet.
Because stars are so massive compared to
planets they move in very small circles at very
slow speeds (of order a few m/s).
Doppler shift of 51 Pegasi
First extrasolar planet
51 Peg
Solar system
Indirect detection of planets
• No direct information on planets composition,
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atmosphere …
Instead magnitude of star wobble gives planets
mass, period of wobble tells length of planets
“year”.
From length of “year” we can determine radius
of orbit (how far it is from the star).
We don’t know what the planet looks like, all we
know are its mass and orbit.
Hot Jupiters
• We were surprised to find massive planets orbiting so
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close to their stars.
This is not like the solar system. Perhaps solar system
had a close in “Vulcan” that fell into the Sun at some
point.
Note searches based on stellar wobble are most
sensitive to massive planets orbiting close in. We don’t
yet have the sensitivity to detect Earth mass planets.
Therefore results suggest a variety of solar systems. We
have not yet ruled out systems like our own.
Greatly magnified motion of center of sun from Jupiter and other planets
Amateur Planet Searches
• Finding extrasolar planets is extraordinarily hard
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and has involved large professional
observatories and experienced astronomers. For
example the Keck 10 meter telescopes
(presently the largest optical telescopes) spend
considerable time searching for planets.
However finding new worlds is exciting.
Can amateurs get involved?
Amateur Planet Search
16” telescope sends star
light through optical fiber.
Tom Kaye
Business owner, paintball gun
manufacturing company. Age 41
Extrasolar Planets
• We have now found about 100 planets around
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other sun like stars by observing small Doppler
shifts in the star light.
Planets not seen directly. Only have information
on planets masses and orbits.
Doppler method most sensitive to close in
massive planets.
New methods and instruments will allow the
discovery of many more planets.