Search for Extra-Terrestrial Intelligence
Life in the Universe?
What is life? (as we know it) Auto-regulation (ex. : sweating) Organization (A cell is more organized than a bunch of atoms) Metabolism : Required energy to maintain it s internal processes Growth Adaptation Reproduction Let s test this : Are bacteria alive? Are viruses alive?
Can we use our current knowledge to estimate the number of civilizations in our galaxy?
The Drake equation Written in 1961 by Frank Drake Number of stars in the galaxy Earth-like planets Civilization lifetime N c = N f p n e f L f i F S Number of civilizations in the galaxy Fraction of stars with planets Fraction with Life Fraction with intelligent beings
Number of stars in our galaxy (N * ) From 1949 to 1958 with the 48in telescope at Palomar Observatory More than 2000 photographic plates covering about 75% of the total sky area Use a graded magnifying lens to count the number of stars and extrapolate to the whole galaxy
Fraction of sun-like stars with planets (fp) Primary methods : Direct imaging Transit Radial velocity
Direct imaging Planets are much fainter than their host star since they do not emit their own light By blocking the light from the star, it can be possible to reveal planets
Transit Looking at the planetary system from the side The passage of the planet create a very small dip in the luminosity of the star
Transit Looking at the planetary system from the side The passage of the planet create a very small dip in the luminosity of the star
Transit Looking at the planetary system from the side The passage of the planet create a very small dip in the luminosity of the star
Transit Looking at the planetary system from the side The passage of the planet create a very small dip in the luminosity of the star
Transit What other factor could influence the shape of the light curve? The size and period of the transiting planet
Radial velocity Gravity means that the star is pulling on the planet, but the planet is also pulling on the star This results in both going back and forth around their mutual centre of gravity This change in speed means the spectral lines from the star are going to vary due to Doppler shift if there s a planet The more massive the planet, the larger the effect
What is your best guess for the fraction of stars with planets?
Habitable planets in each Solar system (ne) This equation gives an estimate of the surface temperature for a planet around a sun-like star T 2 P [K] D p [AU] = 82944 What range would permit liquid water (273-373K)? Using the four systems listed on page 38, what is the average number of planets in the habitable zone around sun-like stars?
Fraction of planets with Life (fl) Simple life forms have existed on Earth since at least 3.9 Billion years ago Life has spread everywhere How could we estimate the fraction of planets in the habitable zone of their star that harbour life?
Fraction of planets with Life where intelligence develops? (fi) Intelligent life on Earth is very recent, proving it can develop, given enough time What do you think is a reasonable fraction of planets with Life that would harbour intelligent life at some point? Between 0 and 1
Lifetime fraction (Fs) If a civilization doesn t destroy itself, it s going to face danger coming from space (comet, asteroid, supernova, etc.) How long do you think a civilization will live at most? Divide that by the lifetime of a sun-like star to get Fs
Number of civilizations in our galaxy N c = N f p n e f L f i F S Combine your numbers to get an estimate of the number of civilization at any given time in the galaxy
Distance to the nearest civilization Sun is 30 000 light years from the centre of the galaxy, or 2/3 of the way out. What is the radius of the galaxy? What is the area of the galaxy (assuming a disc)? (Area = πr 2 ) Divide the area of the galaxy by the number of civilizations Take the square root to estimate the distance between two civilizations Light is the fastest thing in the Universe. How many years would it take light to go between two civilizations? How does that compare to the lifetime of a civilization?
Bonus! Detecting a message How far would the earliest radio signals from Earth be right now? Would they have made it to the nearest civilization? The SETI program looks for signals from advanced civilizations Based on your calculations, do you expect it to detect anything? Would communications between civilizations be possible?
Bonus! Decoding the message In 1976 NASA launched Pioneer 10 and 11 to photograph Jupiter and Saturn Both spaceships carried a plaque with various drawings on it What can you decipher from the plaque? (see Pioneer Plaque wiki page for answers)