Chapter 18Life in the Universe

Chapter 18Life in the Universe

Galaxyrise Over Alien Planet by D. Berry

Cosmic evolution - phases in the history of the universe

Cosmic evolution - phases in the history of the universe• Particulate

• Galactic

• Stellar

• Planetary

• Chemical

• Biological

• Cultural

Figure 18.1Arrow of Time

Living organismsLiving organisms

• React to environment and can often heal themselves when damaged

• Grow

• Reproduce

• Genetic change and evolution to adapt to a changing environment

Assumption of mediocrityAssumption of mediocrity

• Life on earth depends on a few basic molecules

• Elements in these molecules are common to all stars

• Laws of science same everywhere

• Life must have originated elsewhere than on earth

Chemicals on earth before life formed

Chemicals on earth before life formed

• Outgassing in early earth produced hydrogen, nitrogen and carbon compounds

• Ammonia, methane, carbon dioxide and water formed

Energy inputEnergy input

• Energy from radioactivity, volcanism, lightning, UV radiation and meteoritic impacts modified chemicals

• Formed amino acids and nucleotide bases

• Organic (carbon-based) molecules that are the basis of life

Amino acids and nucleotides

Amino acids and nucleotides

• Amino acids build proteins, which control metabolism

• Sequences of nucleotide bases form genes, which are part of DNA molecules

• DNA controls synthesis of proteins and determines characteristics of living organisms

• Genes carry hereditary information to the next generation

Figure 18.2DNA Molecule

Miller-Urey experimentMiller-Urey experiment

• 1953 - took primordial chemicals, input energy, and generated amino acids

• A later experiment generated nucleotide bases

• Did not produce life, but did synthesize biological molecules

• Demonstrated chemical evolution

Figure 18.3Miller-Urey Experiment

Figure 18.4Chemical Evolution

Interstellar origin?Interstellar origin?

• Some scientists claim organic material came from interstellar space

• Experiment exposed icy mixture of water, methanol, ammonia and carbon monoxide to UV radiation

• Complex organic molecules formed

Figure 18.5Interstellar Globules

Diversity and cultureDiversity and culture

• Simple one-celled life appeared on earth about 3.5 billion years ago

• More complex one-celled life appeared about 2 billion years ago

• Multicelluar organisms appeared about 1 billion years ago

• Insects, reptiles, mammals, etc.• Biological evolution led to the strongly favored

trait of intelligence• Cultural evolution followed

Figure 18.6LLife on Earth

Figure 18.6RLife on Earth

Life as we know itLife as we know it

• Means carbon-based life originating in liquid water environment

• Did it happen elsewhere in the solar system?• No on Moon or Mercury - no protective

atmosphere• Not on Venus - atmosphere too dry and hot• Jovian planets have no solid surface, Pluto too

cold• Europa and Titan possibilities• Mars a possibility

Figure 18.7Murchison Meteorite

Alternative biochemistriesAlternative biochemistries

• Carbon is basis of life forms on earth• Maybe silicon could be basis of life

forms• Ammonia could be an alternative

instead of water

Intelligent life in our GalaxyIntelligent life in our Galaxy

• Distances in Galaxy too great to detect life with current technology

• Instead rely on estimating likelihood of intelligent life in Galaxy

Drake EquationDrake Equation

• Named after Frank Drake, who pioneered a probability type calculation

Number of technological civilizations in galaxy isNumber of technological civilizations in galaxy is

• Rate of star formation X• Fraction of stars having planetary systems X• Average number of habitable planets in those

planetary systems X• Fraction of those planets on which life arises X• Fraction of those planets on which intelligent life

evolves X• Fraction of those planets developing technology X• Average lifetime of a technological civilization

Figure 18.9Drake Equation

Rate of star formationRate of star formation

• Roughy 100 billion stars in Galaxy• Galaxy has been around 10 billion years• 10 stars per year forming

Fraction of stars having planetary systems

Fraction of stars having planetary systems

• Informed guess - fraction of 1 or so

Number of habitable planets per planetary system

Number of habitable planets per planetary system

• Determined by distance to star (too hot or cold)• Determined by spectral class of star• Affected by orbit of star and if star in orbit in a

binary star system• Depends on position in Galaxy - supernovae

damage and gravitational effects of close-encounter stars

• Estimate 1/10 (1 habitable planet per 10 planetary systems)

Figure 18.10Stellar Habitable Zones

Figure 18.11Galactic Habitable Zone

Figure 18.12Binary-Star Planets

Fraction of habitable planets on which life arises

Fraction of habitable planets on which life arises

• Can make guesstimates from chemical formation• Optimistically assign this 1

Fraction of life-bearing planets on which intelligence arises

Fraction of life-bearing planets on which intelligence arises

• Any hint of intelligence is highly favored by evolution

• Guess this fraction to be 1

Fraction of planets on which intelligence develops technology

Fraction of planets on which intelligence develops technology

• Technological societies developed independently at several locations on earth

• Give this factor a 1

Average lifetime of a technological civilization

Average lifetime of a technological civilization

• Modern civilization for only 100 years or so• Maybe 1000 years?• Multiply all of the factors together and end up with

there should be 1000 technological civilizations scattered throughout our Galaxy

• It is unlikely there is time enough for these civilizations to communicate

Search for Extraterrestrial Intelligence

Search for Extraterrestrial Intelligence

• If technological civilizations last 1 million years, there are 1 million in existence

• Average about 100 light-years apart• 200 years for two-way communication• In current fastest space ship, trip there and back

would take 1 million years

Figure 18.13Pioneer-10 Plaque

Radio searchRadio search

• Radio waves best for communication - not scattered by dusty interstellar space

• We would not broadcast to other stars• We would passively “listen” toward F, G and K

stars near the sun• Earth is currently stronger (man-made) radio

emitter than the sun

Figure 18.14Earth’s Radio Leakage

Most probable radio wavelengths

Most probable radio wavelengths

• Near 20 cm - H emission is so common• H radiates at 21 cm• OH radiates near 18 cm• They together make water, which our life form is

based on• Wavelengths between 18 and 21 cm are called a

“water hole”• Galactic background minimized

Figure 18.15Water Hole

SETISETI

• Search for Extraterrestrial Intelligence• Project Phoenix searched in 1-3 GHz range• Nothing resembling intelligence yet detected

Figure 18.16Project Phoenix

Consequences of two civilizations meeting

Consequences of two civilizations meeting

• Consider the history of earth• What happened whenever a more “advanced”

civilization met a less “advanced” civilization?