Life in the Solar System ... and Beyond
• While Earth is the only confirmed body with life in the Solar System, we have found many other bodies which may support life
• Today, we will talk about some of these worlds and the life that may exist there
• Knowledge of life in our Solar System also lets us estimate what life may exist beyond
BestSolar Candidates System Inventory for Life?
1 star Sun
4 terrestrial planets Mercury, Venus, Earth, Mars
4 gas giant planets Jupiter, Saturn, Uranus, Neptune
7 large moons Moon, Io, Europa, Ganymede, Callisto, Titan, Triton
small moons, asteroids, Phobos, Ceres, Amalthea, comets, KBOs, dust Rhea, Enceladus, Titania, Proteus, Pluto, Charon
Mars
• No hard evidence for any lifeform on the surface, though only Viking landers looked
• Much more likely in the distant past when Mars was warmer and wetter
• May still be very small microbial life near water ice deposits around the poles or a few feet below the surface
1 Jupiter?
• The idea is a bit far fetched, but there are layers in Jupiter's atmosphere where temperatures and pressures would be right for liquid water
• Any life that developed in the atmosphere would float through the different layers, much like life in Earth's oceans
• Very unlikely, but possible
Europa and Enceledus • Of all the gas giant moons, Europa and Enceledus offer the best possibilities
• Each moon, especially Europa, shows evidence for a large amount of liquid water
• Life systems much like the deep sea vents on Earth may exist on both worlds
• It will take quite a bit of time and effort to know for sure
Titan
• Titan seems to have all the proper ingredients for life, but not high enough temperatures
• A large amount of hydrocarbons reside in the atmosphere and surface
• Some amino acids are also detected
• If Titan were warmer, it may be an ideal location for life
2 Beyond the Solar System The Habitable Zone
• When looking outside our Solar System, it is easiest to think about zones where life could survive
• The habitable zone usually corresponds to locations where liquid water can exist
• The habitable zone is 'not too hot' and 'not too cold', but should be just right for life
The Habitable Zone
• Since different stars have different brightness and temperatures, the size and location of the habitable zone can change
Problems with the Habitable Zone
• There has been some dispute of the accuracy of the habitable zone
• After all, there are locations in our Solar System well outside the HZ where liquid water may exist
• Despite the argument, the habitable zone is the most favorable location for life to occur in a star system
3 Finding the Right Star
• While it is easiest to think about life around a Sun-like star, that doesn't have to be the case
• However, there are some stars which make their surroundings quite inhabitable
• Some small, red M dwarfs have violent flaring activity
• Large, massive O/B stars have strong winds and put out more ultraviolet and X-ray Artist concept of a flaring M dwarf radiation
The Galactic Habitable Zone
• The HZ concept can be expanded to the galaxy as a whole
• The interior of the galaxy is a very energetic, very unstable place
• The extremities of the galaxy contain too few heavy elements for Earth-like planets to form
What About Intelligent Life?
• One of the most interesting and controversial branches of astronomy is the Search for Extra- Terrestrial Intelligence
• The SETI Institute is basically a large listening program, trying to find signals from aliens
• The search continues...
4 Can They Hear Us?
• Radio and television signals have been leaving Earth for over 50 years
• If anyone within 50 light-years is listening, then they could have detected us
• Odds of an intelligent civilization being that close is very slim
• Also, our signals are not concentrated - instead they spread out thinly in all directions
Can We Hear Them?
• Under current technology, it would be hard to hear them unless the signal was directed straight at us
• Unfortunately, they have no reason to send signals, since most of them would not know we exist
• Improvements in tech and facilities over the next decade will let us here the slightest whispers of civilization
SETI • The SETI Institute is a non- profit, privately funded organization
• They use radio data which comes primarily from the Arecibo dish in Puerto Rico and the 100m radio telescope in Greenbank, West Virginia
• A new array of telescopes is being built in California called the Allen Telescope Array, which will be fully devoted to SETI
5 Are They Even Out There?
• One of the ultimate scientific/philosophical/ religious questions is 'Are We Alone?'
• From a sheer numbers standpoint, the odds of only Earth possessing intelligent life are essentially impossible
• Each galaxy contains 100's of billions of stars and there are 100's of billions of galaxies out there
The Drake Equation
• One quick and easy way to estimate the number of civilizations in the galaxy is the Drake equation
• Frank Drake developed the equation in the 1960's
• Based on a set of astronomical and biological factors, the equation will tell you the number of intelligent, communicating civilizations in the galaxy
The Drake Equation
* N R f p ne f l fi f c L
N # civilizations with detectable electromagnetic emissions
R* rate of star formation (stars/year) fp fraction of stars that are stable with planetary systems ne # of worlds/system with environment suitable for life
fl fraction of planets on which life develops fi fraction of life-bearing planets on which intelligent life arises fc fraction of intelligent civilizations revealing existence L length of time civilizations release detectable signals (years)
6 Rate of Star Formation
• This factor we have a pretty good estimate for
• Stars are constantly being born in the galaxy, and the rate at which they are born will tell you how many are available
• The galaxy has between 100-200 billion stars, and is about 10 billion years old Dusty star formation regions • Therefore, the average rate (REAL PICTURE!) is about 10-20 stars per year
Fraction of Stable Stars with Planets
• Not all of the stars in the galaxy will be stable and not all of the stars will have planets
• We have only begun to find planets around stars, but the results are quite encouraging
• It is most likely a natural process for stars to have planets
• Earth-like planets are still waiting to be found
Number of Worlds with Conditions Suitable for Life
• This factor is the number of planets or moons in a solar system that may be able to support life
• Habitable zone considerations come into play with this factor
• As we already discussed, there may be five or six other places that could support life
7 Fraction of Worlds with Life
• Of all the planets or moons in the star system, this is the fraction which actually develop life
• We know of ONE planet with life in our Solar System
• If you include all the planets and large moons, then this number is something like 1/15
Fraction with Intelligent Life
• Of those planets which have developed life, this is the fraction which has developed 'intelligent' life
• For our Solar System, Earth is the only case with life and it developed intelligence
• The definition of 'intelligence' is somewhat hazy - some would argue the case for humanity
Fraction with Communicating Life
• Of the worlds with intelligence, what fraction will produce a signal that can travel through space?
• Our civilization has been around thousands of years, but we have only been communicating for ~50 years
• The development of technology will likely lead to some sort of signal into space
8 Length of Time Civilization is Communicating
• This is one of the hardest factors to estimate, since we have no good reference
• Our civilization has been communicating for ~50 years, but how much longer will we be sending signals?
• This factor depends on how long the civilization survives, and how long its signal leaks into space
So How Many?
* N R f p ne f l fi f c L
• Even conservative estimates for each of these factors produce a sizeable number of intelligent civilizations in the galaxy
• With advances in technology, it is likely that IF there is a signal to be found, it will be found in our lifetime
• We will do an exercise on Friday involving the Drake equation and the possibilities of alien life
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