Extraterrestrial Life

Extraterrestrial Life The Day the Earth Stood Still (1951) Forbidden Planet (1956)

Extraterrestrial Life

Extraterrestrial Life

The Search for Extraterrestrial Intelligence (“SETI”) Is there simple, basic life elsewhere? Is there intelligent life elsewhere? Are there intelligent, communicative civilizations elsewhere? If there are, how do we find out about them?

Is there simple, basic life elsewhere? Nobody knows, but, to guess, almost surely yes, and maybe even quite close In the past 25 years, as we’ve explored Mars, Jupiter, and Saturn up close, the probability has increased that there may be simple extraterrestrial life even in our own Solar System!

Mars

Mars There is reasonably good evidence that there was once liquid water flowing on the Martian surface A global climate change event long ago evidently violently changed this Today’s surface Martian soil seems to have water ice mixed in: as much as 2% of the soil may be ice in some regions Could very simple life exist subsurface even today? Maybe….

Jupiter’s Moons One of the larger Moons, Europa, probably has a large subsurface ocean of liquid water

Saturn’s Moons Titan Enceladus

Saturn’s Moons The largest moon, Titan, has a dense atmosphere of nitrogen (98%), which is also the largest constituent of the Earth’s atmosphere (80%) Titan has a huge surface ocean of liquid methane Sunlight shining on methane can create complex organic molecules

Saturn’s Moons A smaller moon, Enceladus, almost definitely has a subsurface ocean of liquid water

Life in our Solar System? At least three other planets show strong evidence that they have, or had, liquid water on the planet or a satellite The moons of Jupiter and Saturn have a perpetual source of energy input due to tidal heating Similarly harsh environments on Earth do have thriving simple life

Hydrothermal Deep Sea Vents on Earth No sunlight Chemically toxic surroundings Biologically very active: bacteria, worms, clams, shrimp!

How about outside the Solar System? Can’t observe the chemistry of other Solar Systems directly (yet; JWST may help) Are the raw materials for life even present elsewhere? Even if they are, are there environments conducive to allowing those materials to organize into life?

Molecules outside the Solar System All life as we know it depends on organic molecules built into complex chemical structures Like atoms, molecules can be excited by light of certain specific wavelengths

Molecules outside the Solar System These wavelengths usually fall in the radio or infrared part of the spectrum Radio and IR observations of the interstellar medium, and also protostars and young stars, reveal hundreds of complex molecules

Building blocks of amino acids and DNA!

How do these complex molecules form in interstellar space How do these complex molecules form in interstellar space? Why are they not destroyed by UV light from stars? Hint: strong signals from molecular gas are almost always found in the same direction where the interstellar dust is particularly thick.

Interstellar Dust & Molecules go together Dust apparently “catalyzes” the formation of molecules: provides a working surface for them to assemble Dust shields the newly formed molecules from destructive UV starlight Very cold temperature of interstellar medium is ideal environment to form molecular bonds, which are fragile

So, the building blocks of life are everywhere So, the building blocks of life are everywhere. But are there environments conducive to allow those materials to organize into life? Occasionally? Commonly?

Are planets common around other stars? One hint: rotation of stars Our Sun rotates slowly (25 days), even though it should have spun up during protostellar collapse What happened to the excess spin? It went into the planetary orbits: the Sun has all the mass, but the planets have all the angular momentum

Measuring rotation in other stars The hottest 5% of all stars are found to have narrow lines, and thus rotate rapidly: they may not have shed planets during protostellar collapse The remaining 95% rotate slowly, like the Sun: they are good candidates for planets

Direct Observation of Extrasolar Planets: The Kepler satellite

Results from Kepler Thousands of extrasolar planets found; many stars have multiple planets like our Sun The easiest to find are massive planets like Jupiter, with short orbital periods; these hot planets with no solid surface are not suitable for life as we know it A few planets of mass comparable to Earth in the “Goldilocks zone” are starting to appear

The raw materials – complex organic molecules – are everywhere in the Galaxy Probably the majority of stars have planetary systems, and there are likely a large number of Earth-like planets in temperate orbits So: what fraction develop life?? And how about intelligent life?

The Drake Equation A means to estimate the number “N” of intelligent, communicative civilizations in our Galaxy

N = ns x fp x fs x fd x fi x fc x fl The Drake Equation N = ns x fp x fs x fd x fi x fc x fl ns the number of stars in our Galaxy fp the fraction of stars with planets fs fraction of planets suitable for life fd fraction of suitable planets where life does develop fi fraction of planets where life is intelligent fc fraction where intelligent life is communicative fi fraction of star’s lifetime that civilization remains

N = ns x fp x fs x fd x fi x fc x fl The Drake Equation N = ns x fp x fs x fd x fi x fc x fl ns the number of stars in our Galaxy = 2 x 109 fp the fraction of stars with planets = 0.5 fs fraction of planets suitable for life = 0.1 fd fraction of suitable planets where life does develop = 0.1 fi fraction of planets where life is intelligent = 0.5 fc fraction where intelligent life is communicative = 0.5 fi fraction of star’s lifetime that civilization remains = ?? N = 2.5 x 106 x fl

fl = fraction of a star’s lifetime that a civilization remains On Earth, our own civilization has had the ability to beam a radio message anywhere in our Galaxy for about 100 years; we “just” become communicative, compared to stellar lifetimes Our Sun’s total main sequence lifetime will be about 1010 years; the Sun is now about halfway through that lifetime: 5 x 109 years remain Assuming our civilization remains intelligent and communicative for the rest of the Sun’s main sequence lifetime, for us, fl = 5 x 109/1010 = 0.5

N = 2.5 x 106 x fl If fl = 0.5, then N = 1.25 x 106 !! There are more than a million technical, communicative civilizations active today, scattered throughout our Galaxy On average, the closest one to us could be about 100 pc (= 300 lt-years) away

fl = fraction of a star’s lifetime that a civilization remains However, a nuclear war could in just a few months easily end all life on earth permanently Then fl = 100 years / 1010 years = 10-8 Then N = 2.5 x 106 x fl = 0.025 << 1 In that case, we expect no other technical, communicative civilizations other than our own anywhere in our Galaxy today

By far the largest uncertainty in estimating the number of intelligent, communicative civilizations elsewhere in our Galaxy today is a social and diplomatic issue, not an astronomical one!

If we are optimists, how do we communicate? Our current technology allows us to send and receive radio messages anywhere in the Galaxy Other wavelengths are also possible, but not as cheap or easy Should we listen? Or send?

If we are optimists, how do we communicate? It’s certainly easiest and cheapest to listen But, if everyone listens, and no one sends…..