Life In the Universe 1

What Do You Need? Heavy Elements (> Li at least) Congenial Environment Not necessarily Earth Normal Remember that life exists at the bottom of the seas Time

Where are We Now?

Assumptions of Mediocrity because life on Earth depends on just a few basic molecules, and because the elements that make up these molecules are (to a greater or lesser extent) common to all stars, and if the laws of science we know apply to the entire universe then given sufficient time - life must have originated elsewhere in the cosmos

The Miller-Urey Experiment The idea that complex molecules could have evolved naturally from simpler ingredients found on the primitive Earth has been around since the 1920’s. The first experimental verification was provided in 1953 when Harold Urey and Stanley Miller, took a mixture of the materials thought to be present on Earth long ago ̶ a "primordial soup" of water, methane, carbon dioxide, and ammonia ̶ and energized it by passing an electrical discharge ("lightning") through the gas. After a few days they analyzed their mixture and found that it contained many of the same amino acids found today in all living things on Earth. About a decade later, nucleotide bases were constructed in a similar manner.

Experimental Design

A Dissenting View Some have argued that Earth's primitive atmosphere might not in fact have been a particularly suitable environment for the production of complex molecules. There may not have been sufficient energy available to power the chemical reactions, and the early atmosphere may not have contained enough raw material for the reactions to have become important in any case.

The Alternate Source Interstellar molecular clouds are known to contain very complex molecules, and large amounts of organic material were detected on comet Halley by space probes when Halley last visited the inner solar system. Similarly complex molecules were observed on comet Hale-Bopp. Thus, the idea that organic matter is constantly raining down on Earth from space in the form of interplanetary debris is quite plausible.

Complicated Molecules Ten or more atoms (15)[edit] Complicated Molecules Atoms Molecule Designation Mass Ions 10 (CH3)2CO Acetone[89][150] 58 — (CH2OH)2 Ethylene glycol[151][152] 62 CH3CH2CHO Propanal[115] CH3C5N Methyl-cyano-diacetylene[115] 89 11 HC8CN Cyanotetra-acetylene[28][147] 123 C2H5OCHO Ethyl formate[153] 74 CH3COOCH3 Methyl acetate[154] CH3C6H Methyltriacetylene[115][142] 88 12 C6H6 Benzene[129] 78 C3H7CN n-Propyl cyanide[153] 69 13 HC10CN Cyanodecapentayne[147] 147 HC11N Cyanopentaacetylene[147] 159 60 C60 Buckminsterfullerene (C60 fullerene)[155] 720 C60+[156][157] 70 C70 C70 fullerene[155] 840

Life In The Solar System The Earth Not Likely on Mercury or Venus Mars? Europa - There is probably water Gas giants - flying methane breathers anyone?

The Drake Equation N = NS * fs * (np fl fi ft) L/Lg N = current number of communicating civilizations NS = Number of stars in galaxy fs = fraction of stars that last long enough to be interesting L = lifetime of civilization Lg = lifetime of galaxy NS / Lg = rate of formation of stars (per year)

The Drake Equation N = R fp np fl fi ft L N = number of technological, intelligent civilizations now present in the Milky Way Galaxy R = rate of star formation, averaged over the lifetime of the Galaxy fp = fraction of those stars having planetary systems np = average number of planets within those planetary systems that are suitable for life fl = fraction of those habitable planets on which life actually arises

The Drake Equation N = R fp np fl fi ft L fi = fraction of those life-bearing planets on which intelligence evolves ft = fraction of those intelligent-life planets that develop technological society L = average lifetime of a technologically competent civilization.

The Rate of Star Formation We can estimate the average number of stars forming each year in the Galaxy simply by noting that at least 100 billion stars now shine in the Milky Way. Dividing this number by the 10-billion-year lifetime of the Galaxy, we obtain a formation rate of 10 stars per year. This may be an overestimate because we think that fewer stars are forming now than formed at earlier epochs of the Galaxy, when more interstellar gas was available. However, we do know that stars are forming today, and our estimate does not include stars that formed in the past and have since exploded, so our value of 10 stars per year is probably reasonable when averaged over the lifetime of the Milky Way.

Fraction of Stars with Planets Accepting the condensation theory and its consequences, and without being either too conservative or naively optimistic, we assign a value near 1 to this term - that is, we believe that essentially all stars have planetary systems.

The Number of Habitable Planets Per Star Eliminate the short lived stars immediately Most likely candidates are stars like the Sun Long lived Fairly large habitable zone Estimate is 1 star in 10 would have a habitable planet. This could be high as the best current estimate of planet frequency / star is 3%. Or it could be low – There are systems with several planets in the habitable zone. This means the number per star is 0.1 on the low side and could be as high as 1

Fraction on which Life arises If we accept the mediocrity principle then this fraction is 1. It could be as low as 0 if one believes life is rare.

Intelligent Life One school of thought maintains that, given enough time, intelligence is inevitable. In this view, assuming that natural selection is a universal phenomenon, at least one organism on a planet will always rise to the level of "intelligent life." If this is correct, then the fifth term in the Drake equation equals or nearly equals 1. Others argue that there is only one known case of intelligence, and that case is life on Earth. For 2.5 billion years ̶ from the start of life about 3.5 billion years ago to the first from the start of life about 3.5 billion years ago to the first appearance of multicellular organisms about 1 billion years ago ̶ life did not advance beyond the one-celled stage. This would means the fraction is - 0.

Technology The anthropomorphic view: if we do it every one else will so the fraction is 1. The view of the dolphins: show me the fish. The fraction is about 0.

Lifetime of Civilizations Guess We blow ourselves up: 75 years We do not blow ourselves up ̶ your guess is as good as mine: > 100 years 100000 years ?

Putting in the Numbers Unless one is pessimistic the fractions are all of order 1 so we get N = 1 * Lifetime So we get 10's to 1000's of civilizations

Communications Direct Travel: Interesting but not likely Radio: Could be - there are places to look TV Signals Radar Masers Von Neumann Machines A virus anyone.

Where Do these Guys Live? ExtraSolar Planets Of Course! How do we detect extrasolar planets? Direct Observation? Not Really at the moment Consider the Sun at a distance of 32.6 ly. The Earth would be at maximum 0.1 arcseconds from the Sun and 2(109) times fainter. Space Telescope resolution is about 0.1 seconds Wobble due to the motion of the planet about the star ̶ see the next slide. Very, very small Transits – Very small effect but very powerful

The Solar Motion

So How do we do it? The Doppler Effect The star moves around the center of mass with the same period as the planet. The velocity changes are small but detectable.

ExtraSolar Planets Kepler and TESS