Download 4. How can we select stars whose planets are likely homes for life?

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LIFE ON OTHER WORLDS
KEY CONCEPTS
Life in our galaxy is highly probable; however, it will be very difficult to detect
and to communicate with.
The basics of life as we know it and how it evolved on our planet provide a
background for a look at where other life might be found and how we might
detect it.
Our solar system
Life in our solar system beyond Earth has not been positively ruled out, but the
odds are very small.
The presence of liquid water is important for the evolution and sustaining of life
because it provides the medium to transport nutrients and wastes within the
organism.
Chemical evolution is the build up of molecules of certain types because some
reactions are more likely to occur than others and some molecules are more stable
than others.
Biological evolution occurs when a molecule can reproduce a copy of its self.
Biological evolution is the splitting of a molecule to form two molecules the
same as the original.
DNA
Any DNA (deoxyribonucleic acid) mutation that leaves the organism with a clear
advantage over members of its species will have an easier time existing and
reproducing. This increases that organisms chance to pass the mutation,on to its
off-spring and for the mutation to slightly alter the species. Mutations in prey
animals, it would make it less likely to be killed by a predator, and in predators, it
would make it easier for them to get closer to and catch their prey. T
The oldest fossils known on Earth are of marine life. These fossils date back 500
to 600 millions years, while the earliest fossils of land creatures are 400 million
years old. Studies show that microscopic marine fossils may date back as far as
3.5 billion years.
The Miller experiment showed that complex organic compounds form easily
when basic chemicals and energy are present. Complex organic compounds such
as amino acids are necessary for life to begin and for life to be sustained.
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Intelligence increases our options for survival.. An intelligent organism can learn
from its. An organism that can learn is more likely to avoid fatal situations and
hence is more likely to reproduce. The intelligence is passed on, but NOT
NECESSARILY the specific knowledge.
Life on Mars?
The meteorite ALH84001 has caused reasonable excitement about the possibilities of
rudimentary life forms exiting in the distant past on Mars. PAHs (Polycyclic aromatic
hydrocarbons) were found in the meteorite. PAHs are fairly common, complex carbon-based
molecules. The particular PAHs found in the meteorite resemble those produced on
Earth by living organisms.
Other star systems
Our understanding of the lives of stars allows us to determine the types of
stars around which life has the best chances of evolving. This has helped us limit
our search for star systems that might have suitable planets and might harbor life
in some form.
Interstellar travel is really unfeasible and the only way to find and
communicate with an alien world will be through radio waves. While the radio
spectrum is large there are some frequencies that might provide a better place
to search for the existence of life around other stars and to transmit our
existence.
Choosing the stars in which to search for planets with life.
The star must be a solar-like star, spectral type G or K, and the planet must be
within the life zone of that star so that water can exist as a liquid.
For life to develop takes several billion years. A planet with life must be in a solar
s system which its star remains on the main sequence (H-R diagram).
Stars on the upper main sequence have lifetimes that are less than a billion
years. Therefore the energy output of these upper main sequence stars is not
stable for a long enough period of time for life to form.
For life as we know it, we must require the habitable zone. That is the presence of
liquid water and an atmosphere. These requirement set serious limits on the type
of star that such a planet would need to orbit and its orbital distance if life is to
evolve.
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Travel to distance stars and planets
Travel between stars is nearly impossible because the distances are too great
and nature has imposed a very real speed limit that we can not exceed.
Nothing can travel faster than the speed of light, and human travel can not be
expected to exceed even a small fraction of the speed of light.
Therefore, the time it would take to travel to the nearest star, Proxima Centuri,
which is about 4 ly away would take more than 4 years.
In fact, the actual travel time will be much larger. Even if we could travel at the
incredible speed of 3,000 km/sec, it would take 400 years to reach the nearest
star. The average distance between stars is about 4 Iy, so even if we can travel at
the incredible speed of 3,000 km/sec, traveling from one star to the next would
take generations of space travelers.
Communicating with life on other worlds
The technological stability of a life form is important in determining the length of
time over which it is capable of communicating. A life form that can
communicate but that possesses this ability for only a few years, has little chance
of being detected. The signal these life forms send out must reach another life
form technologically advanced enough to receive and process that signal. The
length of time that the signal is sent increases the odds that it will reach a
technologically advanced life form that can receive and process it.
The Water Hole
The water hole is a band of frequencies in the radio portion of the electromagnetic
spectrum that lie between the 21-cm line of neutral hydrogen and the 18-cm
line of OH. It is called the water hole because H and OH combined forms H2O,
water. It is a good place to look for radio signals from other civilizations because
it is not cluttered by natural radio emission from other sources and it is close to
the 21-cm radiation of neutral hydrogen. Additionally, water is very important
to the existence of life and this gap in the spectrum between H and OH could
easily be identified by
The water hole extends
from approximately 18 cm to 21 cm, which in frequencies
is from 1.43 x 109 Hz to 1.67 x 109 Hz. This means that the water hole is 2.47 x 108
Hz wide. If we were to look at chunks of the spectrum that were 1000 Hz wide at
one time, we would need to check nearly 240,000 frequencies!
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Multiple Choice Questions
1. A cells DNA molecules are constructed of
a.
amino acids.
b.
proteins.
c.
sugars, phosphates, and bases.
d.
all of the above
e.
none of the above.
2. DNA molecules contain patterns for the production of
a.
proteins.
b.
amino acids.
c.
phosphates and sugars.
d.
fours kinds of bases.
e.
all of the above
3. The Miller experiment created
a.
life.
b.
artificial proteins.
c.
DNA.
d.
simple molds.
e . amino acids.
4. The habitable (life) zone is
a.
the region on a planet where conditions are acceptable for life to exist.
b.
a region around a star where a planet's temperature would permit the
existence of liquid water.
c.
very small for a massive star.
d.
only found around G and K stars.
e.
a region on a young planet where amino acids can begin to form DNA
molecules.
5. Life on Earth is based on
a.
. carbon chemistry.
b.
helium chemistry
c.
silicon chemistry.
d.
oxygen chemistry.
e.
nitrogen chemistry.
f.
6. The number of communicative civilizations in our galaxy is limited by
a. the size of the average planet.
b. the survival of technological societies.
c. 21-cm radiation.
d. their distance from Earth.
e. the number of high mass stars in the galaxy.
7. Current searches for extraterrestrial intelligence use radio telescopes to
a. listen to many stars at the same time.
b. listen to many frequencies at the same time.
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c. transmit signals to many stars at the same time.
d.transmit many frequencies at the same time.
e.look for planets like Earth in other galaxies
8. The most likely region of the radio spectrum for communication with other
civilizations is the water hole, the part of the spectrum
a.
near the radio emission of water.
b.
near the radio emission of oxygen molecules,
c.
between the radio emission of hydrogen and oxygen.
d. between the radio emissions of hydrogen and OH.
e. near the radio band where water in the Earth's atmosphere strongly absorbs
energy.
9. An offspring born with altered DNA due to radioactivity, cosmic rays, or errors
in reproduction
a.
is called a mutant.
b.
is called a new species.
c.
is more likely to survive than if its DNA were not altered.
d.
is no different than if its DNA were unchanged
e.
will never he able to reproduce itself.
10. What limitation(s) make it impossible to travel between stars?
I. Fuel requirements
II. The tremendous distances between stars
III. The finite speed at which objects can travel
a.
b.
c.
d.
E
I
II
III
II & III
. I, II, & III
11. DNA is important to a cell because
a.
DNA is the cell membrane that allows nutrients to enter the cell and waste
to leave.
b.
DNA contains the information necessary to make all of the molecules
required by the organism.
c.
DNA contains the information necessary for the cell to reproduce itself.
d.
all of the above
e.
b and c above
12. ALH 84001
a. is a meteorite that shows evidence for life evolving on Mars.
b.
is the compound used by Miller to begin his experiment to produce amino
acids.
c.
is the amino acid produced in the Miller experiment.
d.
is the molecule that controls the reproduction of DNA within a cell
e.
is the molecule that acts as DNAs messenger that assembles proteins from
amino acids.
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13. Why is it believed that life can not evolve on a planet that orbits a spectral
type A, B or 0 star?
a.
These stars produce too much infrared radiation which would destroy
carbon-based organisms.
b.
These stars have very small life zones.
c.. These stars are not capable of producing planets.
d. These stars do not remain on the main sequence long enough for life to evolve.
e. These stars take too long to form.
Fill in the Blank Questions
1. The portion of the electromagnetic spectrum between the 21 cm Tine of neutral
hydrogen and the 18 cm line of OH is known as the _____________
2. The Miller experiment succeeded in creating _
3. The most important quantity necessary for life to form appears to be the
existence of _____________________________ on the surface of the planet.
True-False Questions
1. All life on Earth is based on carbon chemistry.
2. DNA molecules cannot he altered by chance errors in reproduction.
3. The Miller experiment created synthetic DNA.
4. If life is to develop on a planet, the planet must lie inside the inner edge of its
star's life zone.
5.The wavelengths that are most likely for communication with other civilizations
are those in the water hole.
6. Even if societies survive at a technological level for only 10 years, that is long
enough to communicate with them.
Essay Questions
1. What are the three properties of living things?
2. Why must the information in a species' DNA be changeable?
3. What is the significance of the Miller experiment?
4. How can we select stars whose planets are likely homes for life?
5. Describe three things that limit travel between stars.
6. What part of the radio spectrum is thought most likely to be successful for
communication with other civilizations? Why?
7. How does the likelihood of the survival of technological societies restrict the
number of civilizations with which we might communicate? THE END