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March 21, 2006 Astronomy 2010 1
Chapter 29: Life in the Universe
Does life exist beyond Earth?
In our own solar system?
Does intelligent life exist?
How can we search for life?
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The Case for Life Beyond Earth
There are billions of galaxies in the universe, and the typical galaxy contains billions of stars: 1018 stars!
Planets have been found around many nearby stars; planet formation seems to be a natural result of star formation.
Even if life is rare, there is a strong possibility that it occurred more than once.
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The Copernican Principle“There is nothing special about our place in the universe”Copernicus proposed a model for the solar system with planets orbiting the Sun.Later we discover that the Sun is not the center of the Milky Way, nor is the Milky Way the center of the universe.The atoms of which we’re made comprise just a fraction of the stuff of the universe.
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Astrobiology
“The study of the origin, distribution, and ultimate fate of life in the universe.”
Combines astronomers, biologists, chemists, geologists, …
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How to Search for Life
A basic search for life should include the simplest form of single celled life.
Can we find some requirements for life that will narrow a search?
Water (can something take its place?)
Energy source: light or chemical
Chemistry: organic compounds or alternate
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Organic MoleculesLife on Earth is based on organic molecules: carbon-based molecules with hydrogen, nitrogen, oxygen, or a few other elements.Organic molecules have been found in clouds of interstellar gas and dust!Starting from these organic molecules, chemists formed amino acids in a simple apparatus that mimics the conditions of Earth 4 billion years ago.
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Extremophiles
Biologists have discovered that life can exist in rather extreme environments:
Hot springs where the water temperature is close to 100ºC.
Frozen for thousands of years in Antarctic ice, and able to reanimate when thawed.
Living without light near deep sea vents called “black smokers”.
Living in high acidity environments like the Rio Tinto.
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Biomarkers
To search for life around other stars, we should look for something distinctive of life: biomarkers.
The simultaneous presence of oxygen and methane is a clear biomarker.
Unfortunately the Earth didn’t have much free oxygen for much of the time that life existed.
An indisputable signal is still being sought.
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Habitable ZoneThe Habitable zone is the region around a star where a planet with liquid water could exist.
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Could There Be Life on Mars?Or Europa?Or Encelades?
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Intelligent Life
Of course, electromagnetic signals from extra-terrestrial life would be a very clear signature.
SETI: Search for Extra-Terrestrial Intelligence
Scientists scan many radio frequencies searching for non-random signals.
The latest idea is to search for brief signals in visible light.
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Listening for Signals from Intelligent Life
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The Drake Equation“Predicts” number of civilizations we could detect signals from in galaxy, N:
R* = rate of star formation (about 10/year)
fp = fraction of stars with planets
ne = number of Earth-like planets / system
fl = fraction of them where life develops
fi = probability that intelligence evolves
fc = prob. that advanced civilization evolvesL = lifetime of advanced technology
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The Drake EquationIf we take:
R* = 10/year
fp = 1.0
ne = 1.0
fl = 0.1
fi = 1.0
fc = 1.0
then N = L, or the number of detectable intelligent civilizations equals the length of time that such civilizations survive.Our civilization has been at this stage since about 1900, or 100 years.
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Sending Signals
We have sent several space probes on voyages out of our solar system.
These probes carry plaques with information about us, in case they are intercepted by other intelligent life.
Kind of like a note in a bottle.
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Discussion Question
Should we actively try to send signals to any extra-terrestrial life that may be out there? Please consider what the consequences could be.
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