Life in the Universe Astronomy 115
What is Life? • “Life is what dies when you stomp on it” -Dave Barry • Simple definitions all fail • Moves? Grows? Feeds? Reproduces? • Best definitions focus on: • Self-organizing, complexity, adaptation • Information coding • Feedback (homeostasis)
DNA • Deoxyribonucleic acid • Total length of human DNA in a single cell is about a meter • A human body contains about 20 trillion cells • The total length of DNA in a human body is thus 20 trillion meters, or twenty billion kilometers, the circumference of the orbit of Pluto.
Prebiotic Evolution • The basic molecules of organic chemistry are easily made • The first self-replicating molecule was almost certainly not DNA • DNA assembles from simpler materials all the time • Many likely candidates • Clay minerals as templates?
Plants and Animals • Carbon dioxide and water in the presence of an energy source can make sugars 6 CO2 + 6 H2O + energy C6H12O6 (glucose) + 6 O2 • O2 is actually toxic (even to us!) • Metabolism: Take the sugars and starches (from yourself or somebody else) combine it with the waste O2, and get the energy back to make different molecules
Planetary Habitable Zones • Primarily in the Liquid Water Zone • Can’t be too warm • Water Vapor in upper atmosphere broken down by solar UV and charged particles (photodissociation) • Hydrogen escapes to space • Too hot planets will quickly lose all their water • Freezing not so much of a problem
Reasons to be a "Carbon Chauvinist". • Carbon is abundant in the Universe • Carbon can bond to four neighboring atoms • Carbon can bond to other carbon atoms, sharing one, two, or three electrons • These properties make it possible to form a vast array of organic molecules • No other element has these properties
Reasons to be a ”Water Chauvinist". • Water stays liquid over a wide range of temperatures. • Water is very abundant in the Universe • Water is a polar or asymmetrical molecule. It attracts ions easily, making it a good transporter of nutrients • It does not dissolve organic molecules (so we do not dissolve in our own cell fluids)
The Role of Phosphorus DNA requires phosphorus to link itself Adenosine triphosphate (ATP) is an energy transport molecule in all organisms
Alternate Biologies? • Ingredients need to be abundant in Universe • Need solids to maintain structure • Need liquids or gases to transport nutrients and wastes • Need source of energy • High temperatures destroy complex molecules • Also solid state electronics • Why your computer has a fan
Alternate Biologies? • Silicon? Earth’s crust is 27% Si • If silicon biology is possible, why not here? • Silicon doesn’t bond to itself well • C + O = gas, Si + O = quartz • CO2 dissolves in water, SiO2 doesn’t • (SiH4 and SiF4 are gases, however) • Alternatives to water? • Ammonia? Polar like water • Methane? Not polar • Hydrogen peroxide?
Alternate Biologies? • Nitrogen or Phosphorus? • Can form complex molecules • Both are more rare in universe than carbon • Arsenic? • Alternative to phosphorus • Even more rare in universe • Chlorine • Alternative to oxygen? • More rare in universe than oxygen
Alternate Biologies? • Sulfur? • Not likely as alternative to carbon • Can be alternative to oxygen – even on earth • We cannot design a probe to look for “alternative biology” because we just don’t know what to look for.
Earth’s Alternative Biologies • Extremophiles: organisms that thrive under extreme temperature, pressure, or chemical conditions • Many of Earth’s simplest organisms are extremophiles • Extremophile domain is a bigger target • We’re more likely to find planets inhabitable by extremophiles than planets suitable for humans
Alternative Biology on Earth (GFAJ-1) Mono Lake, California, a highly saline lake
Arsenic-Eating GFAJ-1 • Not“arsenic based” life • Organic structures built of carbon • More than merely able to utilize arsenic • Can apparently replace P with As • Up till now, P considered absolutely essential • Seems to build As into DNA instead of P
Life on Early Earth • Oldest minerals: 4.4 b.y. • Oldest rocks: 4.2 b.y. • Probably no life until Late Heavy Bombardment over (3.8 b.y.) • Large impacts would sterilize Earth • Liquid water as far back as we can see • Life by 3 b.y.
Life on Early Earth • Faint Early Sun • Sun 4 b.y. ago was 70% as bright as today • We have liquid water throughout Earth history • Probably thick greenhouse atmosphere • How life originated? • Cold earth: improves chemical stability • Hot earth: speeds up chemical reactions • Use minerals as templates?
Planetary Habitable Zones Also known as the “Goldilocks zone”
The Ultimate Long-Term Forecast • Slow warming trend for the next billion years • Increasing humidity • CO2 decrease leads to extinction of plants? • Boiling and evaporation of the oceans
Surviving on a Warming Earth • Higher organisms stop reproducing around 50°C • There will be no place cool to hide • Possible evolutionary strategies • Ultra-sturdy heat resistant molecules • Concentrated solutions to raise boiling point • Retreat underground where pressure and boiling point higher • Pressurized cells • Ultimate limit 150-200° C?
Surviving on a Warming Earth • Plants need > 10 ppm CO2 • Animals and plants die around 50° C • Micro-organisms can tolerate 130° C + • But biology seems to like heat and we have a billion years to experiment • Pressurized cells? • Anti-boiling fluids? • Problem is breakdown of organic molecules • 150 – 200° C (300-380° F) max?
The Oreo Model of Life History Micro-organism Earth (0-3 billion years) Multicellular Earth (3-5 billion years) Micro-organism Earth (5-6 billion years) “The white creamy middle”
The Drake Equation • “A wonderful way to organize our ignorance” • - Jill Tarter
Life in the Universe • What are the odds of intelligent life elsewhere in the Universe? • How many communicating civilizations are there in the Milky Way? • How do you guess? • How do you guess the number of jellybeans in a jar? • Break the problem down into things you can guess.
The Drake Equation N = Number of communicating civilizations in our galaxy right now. And what about the rest?
Star Formation Rate: R* • There are 100 billion stars in the Milky Way. • The Milky Way is 10 billion years old. R* = 100 billion stars/10 billion years R* = 10 */year
Fraction with Planets: fp • What fraction of stars have planets? • Extrasolar planet research since 1995: • 539 confirmed planets • 414 planetary suystems • 55 multi-planet systems • 1 star has six fp = 1/20
ne = 1 Number of Earths: ne • How many habitable planets are there in each of these planetary systems? • Habitable zone: water should be a liquid • Depends on star. • No O or B • No M • No binaries
fl = ? Fraction with life: fl • On what fraction of habitable planets does life evolve? • Look at our Solar System. • 3 planets in habitable zone, life has evolved on 1 (or maybe 2).
fi = ? Fraction with intelligence: fi • What fraction of life bearing planets have life evolve to intelligence? • Is intelligence inevitable?
fc = ? Fraction that communicate: fc • What fraction of intelligent civilizations become technological enough that we could communicate? • Is technology inevitable? • Desirable?
L = ? years Lifetime: L • How long does a civilization last? • Do we have time to communicate with them? • For us: L = 75 years!
What’s the Answer? • R* = 10 */year • fp = 1/20 • ne = 1 • fl = ? • fi = ? • fc = ? • L = ? year • N = _____ # of technological civilizations in Milky Way, right now.
But space is vast • Milky Way is a giant cylinder (disk). • Radius = 15000 pc • Thickness = 1000 pc • Volume = p(Radius)2 x Thickness • V = 7 x 1011 pc3= 700 billion cubic parsecs!
How far to the neighbors? • If there are 100 civilizations in 700 billion cubic parsecs, then: • 1 civilization in every 15 billion cubic parsecs • Imagine every civilization surrounded by a bubble in which it is alone. • V = 15 billion cubic parsecs • R = 3 kpc • Distance between civilizations: 3 kpc = 9000 ly (optimistically)! R R
How Can We Know? • How can we tell if there are extraterrestrial civilizations? • Go visit. Is this practical? • Look for visitors? Is there evidence? • Look, or listen, for signals from E.T.
SETI • Search for Extraterrestrial Intelligence • Several organized searches over the last 30 years. • Use a radio telescope to try to detect signs of E.T. • Why? • The Earth floods space with radio waves. • TV • Radio • Cellular phones • etc
E.T. FM • Most natural radio sources are broadband. • Radio stations are narrowband.
Tuning the Dial • Nearly all searches have been looking for extremely narrow radio signals. • But there are millions and billions of frequencies to listen to. • And millions and billions of stars • Concentrate on nearby sun-like stars. • No luck. • Point in the sky randomly. • No luck yet. But you can help.
SETI@Home • Receiver piggybacks on Arecibo telescope. • Constantly searches as telescope observes. • Millions of channels (frequencies). • Thousands of hours. • Lots of data. • Needs lots of computer power. • Your power; http://setiathome.ssl.berkeley.edu/
“Is there anybody out there?” We have made three attempts to communicate. • The Arecibo broadcast to Hercules Cluster.
The Voyager Record Club Voyagers 1 and 2 contain a plaque and a record.
Voyagers 1 and 2 • Greetings 55 languages • 115 images • Music Sumerian"May all be well." Arabic"Greetings to our friends in the stars. We wish that we will meet you someday." Urdu"Peace on you. We the inhabitants of this earth send our greetings to you." Italian"Many greetings and wishes." Ila (Zambia)"We wish all of you well."
So Where Are They? • Populations expand exponentially • It would take an exponentially-growing civilization only a few million years to fill the Galaxy, even at sub-light speeds • 2 to the 40th power is over a trillion • If it takes 10,000 years for a colony to achieve interstellar travel, 40 doubling times is only 400,000 years. • So why aren’t they all around us?
Is There A Problem? • Alien psychology? We barely understand humans! • Why did it take us so long to develop technology? • Maybe we’re first? • Maybe we’re unique?