Download Lecture 13

Lecture 13
6/29/07
Astronomy 1001
Difficulties
• It is extremely difficult to detect extrasolar
planets
• Stars are a billion times brighter than the
reflected like would be
• How exactly should you define a planet
anyway?
Early Discoveries
• A planet around 51
Pegasi was discovered
in 1995
– Over 200 planets have
now been discovered
• We detect planets using
indirect methods now
– Will hopefully master
direct detection in the
next decades
Gravitational Tugs
• Jupiter (and all of the other
planets) exert a gravitational
tug on the planets
• We can use the Astrometric
Technique to simple watch the
position of the star changing
• We can use the Doppler
Technique to observe the
effects of the star moving
– Can figure out lots of properties
of the orbit using physical laws
Transits
• A third method is to
look for transits
– The planet will block
out some of the star’s
light, so we can tell its
size
• Can also do some
spectroscopy to figure
out the composition of
the atmosphere
Group Work
• Lets say that you wanted to detect a planet
with an Earth like radius (6500 km) around
HD209458. What percentage of the star’s
light would be blocked, and do you think
that its plausible that a telescope could
detect this transit?
Other Methods
• Direct detection
– Difficult to impossible
due to the glare of the
star
• Gravitational Lensing
• Disturbances in dust
disks
Properties of Extrasolar Planets
• The planets we can
detect are typically
large and close to their
host stars
– Unlike any planets in
our own solar system
– Not many Earth-like
planets discovered
• Gliese 581 appears to
have a terrestrial planet
in the habital zone
Solar System Formation
• The current model of planet formation
makes several predictions
– Jovian planets should only form far away from
their stars
– Planets should form more often around stars
with lots of metals and rock
• Jovian planets have probably migrated
inwards
The Future
• We want to detect Earthlike planets
• Kepler and COROT will
both star at the same patch
of sky and look for
brightness changes
– Kepler will look at ~100,000
stars for 4 years
– COROT already launched
Other Missions
• GAIA and SIM
– Extremely precise
astrometric method
devices
• TPF and Darwin
– Designed for direct
detection of planets
– Will block out light from
the star
– Scheduled to launch in
2015-2020
Life in the Universe
• 20th Century observations of many planets caused
the topic of life in the Universe to wane
• But there is renewed interest in thinking about life
in the Universe
– Life arose very early in Earth’s history
– Lab experiments indicate that you can form the building
blocks of life very easily
– We have found organisms that can survive in conditions
similar to those on other worlds
– We are beginning to find many planets outside of the
solar system
Early Life
• Heavy bombardment
ended about 3.9 billion
years ago
• We have strong evidence
of life at 3.5 billion years
ago
– Advanced stromolites
have been radiometrically
dated to that time
The Rise of Life
• Species have evolved over
time
• DNA was a key discovery
– Contains genetic information
• We have found organisms
that can survive in extreme
heat
• We have produced RNA in
labs, as well as the
necessarily amino acids for
DNA
A History of Life
• Panspermia is the theory that life evolved
on other planets and migrated here
• Life started off aquatic and single celled and
remained that way for 2 billion years
• Eventually the ozone layer was formed and
life moved to land
• Large life evolved rapidly beginning 540
million years ago
What is Necessary for Life?
• Life exists in such a wide
range of situations here on
Earth that the necessities
of life are fairly broad
– Need a source of nutrients
– An energy source
– Liquid water
Life on Mars
• Despite repeated
experiments, no evidence
for life on Mars has ever
been found
– Future missions will
explore more hospitable
locations
• Evidence from Martian
meteorites is
inconclusive
– ALH84001 created some
hype a few years back
Life Elsewhere in the Solar
System
• Europa represents the
most likely other
location for life
• Titan has been more
or less ruled out due
to its recent visit
from Huygens
• Other moons seem
less likely candidates
Life Around Other Stars
• Life could very well exist
around other stars if:
– The star is old enough
– Planets orbit probably has to
be stable
– Planet must be in the habitable
zone
• This means that there are
billions of stars that could
have planets or moons with
life
Detecting Life
• The best we can do with images is to
possibly spot oceans
• A better method of detecting life indirectly
is to look at the composition of atmospheres
– Recall that the oxygen in Earth’s atmosphere is
due to life
How Rare are Earth-like Planets?
• Location in galaxy could
potentially be important
• Jovian planets would have
to be in the right location
• Climate stability is
important
– Plate Tectonics
– Large moon
Extraterrestrial Intelligence
• It might be possible to
detect broadcasts from
other civilizations
– SETI
• The Drake Equation
estimates that there
should be thousands of
intelligent civilizations
out there
Group Work
• Any reasonable values for the Drake
Equation will estimate that there should be
thousands of intelligent alien civilizations
out there. But, none of them have ever
contacted us. Given the probable validity of
the Drake Equation, what might be the
reason why we have never been contacted?
How SETI Works
• SETI astronomers use
Radio telescopes to look
for alien signals
– Also use visible light
telescopes to look for
things like laser pulses
• Have sent out high
energy transmissions to
clusters of stars
Interstellar Travel
• Current probes will take
essentially forever (compared
to human time scales) to reach
distant stars
• It would take 2000x the whole
world’s energy output to
propel a spaceship to half of
the speed of light
• Any currently feasible method
of propulsion doesn’t improve
on the picture much