Life in the Universe

 100 billion+ star systems in the Milky Way ◦ New exoplanets being discovered every day  100 billion+ galaxies in observable universe  Carbon can arrange in many, many ways… ◦ Given that and the amount of matter, nature can experiment practically an infinite number of times.

 We hope that we can: ◦ Find microbial life in our Solar System ◦ Find EM signals from extra-solar civilizations

 Earth was born 4.5 billion years ago  Heavy bombardment from 4.2 to 3.9 billion years ago  Life was thriving prior to 3.85 billion years ago

 So just as soon as the oceans quit getting vaporized by asteroids, life sprang into action – in an instant!

 “Mats” of bacteria build layered rocks  We see them today, we see similar ones from long ago  Dated to 3.5 billion years

 Oldest microbes (from DNA standpoint): ◦ Live near “black smokers” in the ocean ◦ Live in hot springs in Yellowstone

 Why did early life hang around extremes?  Earth may not have had an ozone layer yet; hide out from UV

 “Ice worms” live in glacial ice

 Lakes, rivers and thick atmosphere of basic organic molecules have been observed

 Chemists have recreated the pressure, temperature, and chemistry of Titan’s atmosphere  They found: ◦ Nucleotide bases ◦ Amino acids

 Source of nutrients to build living cells ◦ Organic molecules all over the Solar System  Energy for activity ◦ Sunlight, internal heat from a planet  Liquid water ◦ That’s the tough one.

 Liquid water requirement leaves us with: ◦ Mars ◦ Jovian moons, particularly Europa

 Evidence: Mars once thriving waterworld  Martian meteorite may show evidence for life

 Tidal heating  Evidence for water flow  Thick water-ice surfaces ◦ But not much energy from sunlight

EuropaGanymedeCallisto EnceladusTitan

 Life may only take ten thousand years to form  Intelligent life may take a few billion years ◦ Excludes blue stars; they only last 100 million years ◦ Sun-like stars and red dwarfs good candidates

 Habitable zone: Smaller and closer for smaller stars

 Outer galaxy: ◦ Too few heavy elements  Inner galaxy: ◦ More crowded, supernovas would nuke everything

 When an exoplanet transits a star, starlight passes through the planet’s atmosphere ◦ This is a cutting-edge technique, but does reveal chemical composition of the planet ◦ Look for carbon dioxide, ozone, methane, and water vapor ◦ Oxygen abundance on Earth result of biology

 It is entirely possible that extraterrestrial civilizations could be using this technique to monitor our progress. Perhaps at some point: ◦ We become cool enough to make friends ◦ We become advanced enough to pose a threat

 Jovian planets launched trillions of objects into the Oort Cloud  These objects now pose no danger to Earth  Might “comet launchers” be a prerequisite for intelligent civilizations?

Number of “Contactable” Civilizations = N HP x f life x f civ x f now N HP = Number of habitable planets in the galaxy f life = Fraction of planets that are habitable f civ = Fraction of planets that can communicate f now = Fraction of planets that have a civilization now

 EM waves travel forever – but get weaker  Look for signs of communication  SETI (Search for ExtraTerrestrial Intelligence) does that all day

 We have deliberately sent high-power signals into space to “make contact”  Used Arecibo in 1974 to send to M13 (21k LY away)

 Einstein’s Theory of Relativity limits all speeds to the speed of light  Out fastest spacecraft are travelling 1/10,000 the speed of light ◦ 100,000 years to Alpha Centauri (but not pointing there)  Time dilation and length contraction: ◦ At 99.9% the speed of light, 2 years pass for the astronauts while 50 years pass on Earth

 Detonate atomic bombs to propel the ship  Scoop up hydrogen and fuse it

 So we either build generational ships or we figure out how to warp space and time  We don’t know how to warp space and time, but current laws of physics don’t seem to prevent it

 Even our “slow” spaceships could populate stars within a few hundred LY ◦ Would take about 10,000 years – an instant in the overall scheme of the Universe ◦ In a few million years, we could colonize the galaxy  A few million years? This should have been done a long time ago.

 1 in 1 million chance a star will have life  100 billion stars in Milky Way: 100,000 civilizations  Suppose intelligent civilizations arise after 5 billion years ◦ Galaxy is 12 billion years old ◦ 100,000 civilizations should have arisen 7 billion years ago – billions of years ahead of us

So… where are the aliens?

 Galactic civilization should already exist! Three possibilities ◦ We are alone  … and therefore precious. The only part of the Universe that is self-aware. ◦ Civilizations are common but they haven’t bothered colonizing the galaxy  We explore every chance we get. Do they always end up destroying themselves? ◦ The galactic civilization hasn’t gotten around to us  Are we not yet invited?