Percentage of planets that can have intelligent life depends on star formation rate, fraction of stars that have planets, percentage of planets that are suitable for life, fraction where life actually exists, intelligent-life probability, and average civilization longevity {Drake equation} (Frank Drake) [1961]. For planets to have life, they must be like Earth and have stars like Sun.
star
Sun is a yellow-orange class-G0 star. Only class F, G, and K stars can have suitable planets, because liquid-water zone is too small for smaller stars, and bigger stars have no rocky planets in that zone. Such stars have masses 0.7 to 1.5 times Sun mass. Lifetimes are long enough, and masses are big enough, for planets. Stars sufficiently like Sun are 1% of all stars.
Stars younger than Sun have time too short for life. Stars older have too few heavy elements. Time range is 3 to 7 billion years, one-third of all stars.
Multiple stars can have no planets. Single stars are one-fifth of all stars.
Stars with slow spins possibly indicate planets. Probably, one sixth of all stars qualify.
Stars must be in galactic arms. Galaxy centers have too much radiation. Edges have low metals and low star-formation rates. Galactic habitable zone is far from center and edge.
orbit
Circular orbits make temperature swings not too great. Probably 100% of planets at correct distance from star have circular orbits.
size
Earth size is big enough to retain oxygen and nitrogen and small enough to lose hydrogen and helium, so as not to have too much gas. Surface gravity is not too strong or too weak for living things. Diameter is 5000 km to 15000 km. Probably, one tenth of all planets have correct size. Therefore, 0.001% of all stars have Earth-like planets. Galaxy has 2.5 x 10^11 stars and so 2.5 x 10^6 habitable planets.
rotation
Planet rotation must not be too fast or slow. Probably 100% of planets at correct distance from star have Earth-like rotation speeds.
atmosphere
On early Earth, volcanoes gave off steam, nitrogen, methane, hydrogen cyanide, ammonia, carbon dioxide, and sulfur dioxide or hydrogen sulfide. If iron was already at core, atmosphere was carbon dioxide, nitrogen, and sulfur dioxide. Soon after Earth formed, atmosphere layered into decreasing-density gases. Ultraviolet light reaching Earth decreased, and temperature lowered. Crust cooled quickly, and lower temperature led to more atmosphere layering. Hydrogen, ammonia, and methane were no longer in oceans, so processes no longer formed organic molecules. Temperature became too low to make organic molecules. All gases can dissolve in oceans.
temperature
If planet surface temperature is hotter than 40 C, proteins denature and water evaporation is too high. If surface temperature is colder than ice, no water is available. Planets must be in circumstellar habitable zones. If planet forms too close to star, it has little water and large greenhouse effect, like Venus. If planet forms too far from star, surface is ice. Distance from star is 10^7 km for optimum temperature. Probably, one tenth of planetary systems have such planets. Composition, size, wind, rain, and sunlight cause tectonic and erosion processes.
minerals
If planet is at correct distance from star, mineral composition is similar to Earth mineral composition.
radiation
Cosmic radiation can react water and carbon dioxide to make organic acids.
energy
3,800,000,000 years ago, ultraviolet light, lightning, meteor impacts, thunder, volcanic heat, and hydrothermal vents provided energy.
meteors and comets
Perhaps, some organic molecules came to Earth in meteors and comets. Meteors have saturated hydrocarbons, porphyrin rings, and organic acids. Spores cannot come from space, because ultraviolet and ionizing radiation kill them.
ocean
First life probably arose in shallow seas or tidal areas. Oceans probably had water, gases, proteins, nucleic acids, carbohydrates, fats, and adenosine phosphate.
tides
Shallow seas with high evaporation rates allow molecule concentration. Tides add water. Large moons can cause tides. On Earth, tides were 30-meters high when Moon formed.
other factors
Probably, Earth life needs continental drift, orbital changes, star evolution, seasons, days and nights, major climactic changes, and magnetic fields.
catastrophe
Earth life needs no life-ending catastrophes, like too many comets or meteors, too much volcanism and earthquakes, too much erosion, or too much greenhouse effect.
life factors
Earth life needs brains, hands, vocal chords, speech centers, forebrains, vision, immune systems, and societies. Earth life must be large enough, be long-lived enough, be few in number, have slow reproduction cycles, and have long childhoods. Earth life must have no mass destruction, optimum competition, optimum population, enough energy and resources, few radioactive wastes, few chemical wastes, optimum ozone, and social cohesion.
number of times
On Earth-like planets, life has probability, possibly 10^-6. Perhaps, galaxies have 10^6 suitable planets. Therefore, galaxies have one planet with life. If other planets have intelligent life, they can send probes to Earth, but there is no evidence for this. Therefore, no other intelligent life forms are in Milky Way Galaxy yet.
first cells
Life began as non-photosynthetic one-celled bacteria-like organisms. First cells reproduced themselves, protected themselves, and found energy.
membrane
All cells have cell membranes. Cell membranes have lipids with embedded proteins. Cells can control membrane-molecule amounts and ratios. All cells have voltage differences across cell membranes, because inner and outer sodium, potassium, and chloride salt concentrations differ.
energy and entropy
Life can overcome dissipative forces and persist. Living systems have high order {negentropy} in small regions, surrounded by large energy sources that they can tap. Living systems must gather energy faster than surroundings can dissipate energy. Sunlight energy, planet interior heat, and lightning can make locally high temperature, for anabolic and catabolic chemical reactions.
Small cells have small fast energy changes. Fast heat and material exchanges among physical compartments allow rapidly removing order from surroundings. High combination and division rates make many new organic molecules and cells. Equilibrium conditions in oceans or tide pools allow reversible chemical reactions.
Biological Sciences>Biology>Life>Origin
Outline of Knowledge Database Home Page
Description of Outline of Knowledge Database
Date Modified: 2022.0224