Chapter 19: Life in the Universe
Chapter 1
How Science Works
- The Scientific Method
- Evidence
- Measurements
- Units and the Metric System
- Measurement Errors
- Estimation
- Dimensions
- Mass, Length, and Time
- Observations and Uncertainty
- Precision and Significant Figures
- Errors and Statistics
- Scientific Notation
- Ways of Representing Data
- Logic
- Mathematics
- Geometry
- Algebra
- Logarithms
- Testing a Hypothesis
- Case Study of Life on Mars
- Theories
- Systems of Knowledge
- The Culture of Science
- Computer Simulations
- Modern Scientific Research
- The Scope of Astronomy
- Astronomy as a Science
- A Scale Model of Space
- A Scale Model of Time
- Questions
Chapter 2
Early Astronomy
- The Night Sky
- Motions in the Sky
- Navigation
- Constellations and Seasons
- Cause of the Seasons
- The Magnitude System
- Angular Size and Linear Size
- Phases of the Moon
- Eclipses
- Auroras
- Dividing Time
- Solar and Lunar Calendars
- History of Astronomy
- Stonehenge
- Ancient Observatories
- Counting and Measurement
- Astrology
- Greek Astronomy
- Aristotle and Geocentric Cosmology
- Aristarchus and Heliocentric Cosmology
- The Dark Ages
- Arab Astronomy
- Indian Astronomy
- Chinese Astronomy
- Mayan Astronomy
- Questions
Chapter 3
The Copernican Revolution
- Ptolemy and the Geocentric Model
- The Renaissance
- Copernicus and the Heliocentric Model
- Tycho Brahe
- Johannes Kepler
- Elliptical Orbits
- Kepler's Laws
- Galileo Galilei
- The Trial of Galileo
- Isaac Newton
- Newton's Law of Gravity
- The Plurality of Worlds
- The Birth of Modern Science
- Layout of the Solar System
- Scale of the Solar System
- The Idea of Space Exploration
- Orbits
- History of Space Exploration
- Moon Landings
- International Space Station
- Manned versus Robotic Missions
- Commercial Space Flight
- Future of Space Exploration
- Living in Space
- Moon, Mars, and Beyond
- Societies in Space
- Questions
Chapter 4
Matter and Energy in the Universe
- Matter and Energy
- Rutherford and Atomic Structure
- Early Greek Physics
- Dalton and Atoms
- The Periodic Table
- Structure of the Atom
- Energy
- Heat and Temperature
- Potential and Kinetic Energy
- Conservation of Energy
- Velocity of Gas Particles
- States of Matter
- Thermodynamics
- Entropy
- Laws of Thermodynamics
- Heat Transfer
- Thermal Radiation
- Wien's Law
- Radiation from Planets and Stars
- Internal Heat in Planets and Stars
- Periodic Processes
- Random Processes
- Questions
Chapter 5
The Earth-Moon System
- Earth and Moon
- Early Estimates of Earth's Age
- How the Earth Cooled
- Ages Using Radioactivity
- Radioactive Half-Life
- Ages of the Earth and Moon
- Geological Activity
- Internal Structure of the Earth and Moon
- Basic Rock Types
- Layers of the Earth and Moon
- Origin of Water on Earth
- The Evolving Earth
- Plate Tectonics
- Volcanoes
- Geological Processes
- Impact Craters
- The Geological Timescale
- Mass Extinctions
- Evolution and the Cosmic Environment
- Earth's Atmosphere and Oceans
- Weather Circulation
- Environmental Change on Earth
- The Earth-Moon System
- Geological History of the Moon
- Tidal Forces
- Effects of Tidal Forces
- Historical Studies of the Moon
- Lunar Surface
- Ice on the Moon
- Origin of the Moon
- Humans on the Moon
- Questions
Chapter 6
The Terrestrial Planets
- Studying Other Planets
- The Planets
- The Terrestrial Planets
- Mercury
- Mercury's Orbit
- Mercury's Surface
- Venus
- Volcanism on Venus
- Venus and the Greenhouse Effect
- Tectonics on Venus
- Exploring Venus
- Mars in Myth and Legend
- Early Studies of Mars
- Mars Close-Up
- Modern Views of Mars
- Missions to Mars
- Geology of Mars
- Water on Mars
- Polar Caps of Mars
- Climate Change on Mars
- Terraforming Mars
- Life on Mars
- The Moons of Mars
- Martian Meteorites
- Comparative Planetology
- Incidence of Craters
- Counting Craters
- Counting Statistics
- Internal Heat and Geological Activity
- Magnetic Fields of the Terrestrial Planets
- Mountains and Rifts
- Radar Studies of Planetary Surfaces
- Laser Ranging and Altimetry
- Gravity and Atmospheres
- Normal Atmospheric Composition
- The Significance of Oxygen
- Questions
Chapter 7
The Giant Planets and Their Moons
- The Gas Giant Planets
- Atmospheres of the Gas Giant Planets
- Clouds and Weather on Gas Giant Planets
- Internal Structure of the Gas Giant Planets
- Thermal Radiation from Gas Giant Planets
- Life on Gas Giant Planets?
- Why Giant Planets are Giant
- Gas Laws
- Ring Systems of the Giant Planets
- Structure Within Ring Systems
- The Origin of Ring Particles
- The Roche Limit
- Resonance and Harmonics
- Tidal Forces in the Solar System
- Moons of Gas Giant Planets
- Geology of Large Moons
- The Voyager Missions
- Jupiter
- Jupiter's Galilean Moons
- Jupiter's Ganymede
- Jupiter's Europa
- Jupiter's Callisto
- Jupiter's Io
- Volcanoes on Io
- Saturn
- Cassini Mission to Saturn
- Saturn's Titan
- Saturn's Enceladus
- Discovery of Uranus and Neptune
- Uranus
- Uranus' Miranda
- Neptune
- Neptune's Triton
- Pluto
- The Discovery of Pluto
- Pluto as a Dwarf Planet
- Dwarf Planets
- Questions
Chapter 8
Interplanetary Bodies
- Interplanetary Bodies
- Comets
- Early Observations of Comets
- Structure of the Comet Nucleus
- Comet Chemistry
- Oort Cloud and Kuiper Belt
- Kuiper Belt
- Comet Orbits
- Life Story of Comets
- The Largest Kuiper Belt Objects
- Meteors and Meteor Showers
- Gravitational Perturbations
- Asteroids
- Surveys for Earth Crossing Asteroids
- Asteroid Shapes
- Composition of Asteroids
- Introduction to Meteorites
- Origin of Meteorites
- Types of Meteorites
- The Tunguska Event
- The Threat from Space
- Probability and Impacts
- Impact on Jupiter
- Interplanetary Opportunity
- Questions
Chapter 9
Planet Formation and Exoplanets
- Formation of the Solar System
- Early History of the Solar System
- Conservation of Angular Momentum
- Angular Momentum in a Collapsing Cloud
- Helmholtz Contraction
- Safronov and Planet Formation
- Collapse of the Solar Nebula
- Why the Solar System Collapsed
- From Planetesimals to Planets
- Accretion and Solar System Bodies
- Differentiation
- Planetary Magnetic Fields
- The Origin of Satellites
- Solar System Debris and Formation
- Gradual Evolution and a Few Catastrophies
- Chaos and Determinism
- Extrasolar Planets
- Discoveries of Exoplanets
- Doppler Detection of Exoplanets
- Transit Detection of Exoplanets
- The Kepler Mission
- Direct Detection of Exoplanets
- Properties of Exoplanets
- Implications of Exoplanet Surveys
- Future Detection of Exoplanets
- Questions
Chapter 10
Detecting Radiation from Space
- Observing the Universe
- Radiation and the Universe
- The Nature of Light
- The Electromagnetic Spectrum
- Properties of Waves
- Waves and Particles
- How Radiation Travels
- Properties of Electromagnetic Radiation
- The Doppler Effect
- Invisible Radiation
- Thermal Spectra
- The Quantum Theory
- The Uncertainty Principle
- Spectral Lines
- Emission Lines and Bands
- Absorption and Emission Spectra
- Kirchoff's Laws
- Astronomical Detection of Radiation
- The Telescope
- Optical Telescopes
- Optical Detectors
- Adaptive Optics
- Image Processing
- Digital Information
- Radio Telescopes
- Telescopes in Space
- Hubble Space Telescope
- Interferometry
- Collecting Area and Resolution
- Frontier Observatories
- Questions
Chapter 11
Our Sun: The Nearest Star
- The Sun
- The Nearest Star
- Properties of the Sun
- Kelvin and the Sun's Age
- The Sun's Composition
- Energy From Atomic Nuclei
- Mass-Energy Conversion
- Examples of Mass-Energy Conversion
- Energy From Nuclear Fission
- Energy From Nuclear Fusion
- Nuclear Reactions in the Sun
- The Sun's Interior
- Energy Flow in the Sun
- Collisions and Opacity
- Solar Neutrinos
- Solar Oscillations
- The Sun's Atmosphere
- Solar Chromosphere and Corona
- Sunspots
- The Solar Cycle
- The Solar Wind
- Effects of the Sun on the Earth
- Cosmic Energy Sources
- Questions
Chapter 12
Properties of Stars
- Stars
- Star Names
- Star Properties
- The Distance to Stars
- Apparent Brightness
- Absolute Brightness
- Measuring Star Distances
- Stellar Parallax
- Spectra of Stars
- Spectral Classification
- Temperature and Spectral Class
- Stellar Composition
- Stellar Motion
- Stellar Luminosity
- The Size of Stars
- Stefan-Boltzmann Law
- Stellar Mass
- Hydrostatic Equilibrium
- Stellar Classification
- The Hertzsprung-Russell Diagram
- Volume and Brightness Selected Samples
- Stars of Different Sizes
- Understanding the Main Sequence
- Stellar Structure
- Stellar Evolution
- Questions
Chapter 13
Star Birth and Death
- Star Birth and Death
- Understanding Star Birth and Death
- Cosmic Abundance of Elements
- Star Formation
- Molecular Clouds
- Young Stars
- T Tauri Stars
- Mass Limits for Stars
- Brown Dwarfs
- Young Star Clusters
- Cauldron of the Elements
- Main Sequence Stars
- Nuclear Reactions in Main Sequence Stars
- Main Sequence Lifetimes
- Evolved Stars
- Cycles of Star Life and Death
- The Creation of Heavy Elements
- Red Giants
- Horizontal Branch and Asymptotic Giant Branch Stars
- Variable Stars
- Magnetic Stars
- Stellar Mass Loss
- White Dwarfs
- Supernovae
- Seeing the Death of a Star
- Supernova 1987A
- Neutron Stars and Pulsars
- Special Theory of Relativity
- General Theory of Relativity
- Black Holes
- Properties of Black Holes
- Questions
Chapter 14
The Milky Way
- The Distribution of Stars in Space
- Stellar Companions
- Binary Star Systems
- Binary and Multiple Stars
- Mass Transfer in Binaries
- Binaries and Stellar Mass
- Nova and Supernova
- Exotic Binary Systems
- Gamma Ray Bursts
- How Multiple Stars Form
- Environments of Stars
- The Interstellar Medium
- Effects of Interstellar Material on Starlight
- Structure of the Interstellar Medium
- Dust Extinction and Reddening
- Groups of Stars
- Open Star Clusters
- Globular Star Clusters
- Distances to Groups of Stars
- Ages of Groups of Stars
- Layout of the Milky Way
- William Herschel
- Isotropy and Anisotropy
- Mapping the Milky Way
- Questions
Chapter 15
Galaxies
- The Milky Way Galaxy
- Mapping the Galaxy Disk
- Spiral Structure in Galaxies
- Mass of the Milky Way
- Dark Matter in the Milky Way
- Galaxy Mass
- The Galactic Center
- Black Hole in the Galactic Center
- Stellar Populations
- Formation of the Milky Way
- Galaxies
- The Shapley-Curtis Debate
- Edwin Hubble
- Distances to Galaxies
- Classifying Galaxies
- Spiral Galaxies
- Elliptical Galaxies
- Lenticular Galaxies
- Dwarf and Irregular Galaxies
- Overview of Galaxy Structures
- The Local Group
- Light Travel Time
- Galaxy Size and Luminosity
- Mass to Light Ratios
- Dark Matter in Galaxies
- Gravity of Many Bodies
- Galaxy Evolution
- Galaxy Interactions
- Galaxy Formation
- Questions
Chapter 16
The Expanding Universe
- Galaxy Redshifts
- The Expanding Universe
- Cosmological Redshifts
- The Hubble Relation
- Relating Redshift and Distance
- Galaxy Distance Indicators
- Size and Age of the Universe
- The Hubble Constant
- Large Scale Structure
- Galaxy Clustering
- Clusters of Galaxies
- Overview of Large Scale Structure
- Dark Matter on the Largest Scales
- The Most Distant Galaxies
- Black Holes in Nearby Galaxies
- Active Galaxies
- Radio Galaxies
- The Discovery of Quasars
- Quasars
- Types of Gravitational Lensing
- Properties of Quasars
- The Quasar Power Source
- Quasars as Probes of the Universe
- Star Formation History of the Universe
- Expansion History of the Universe
- Questions
Chapter 17
Cosmology
- Cosmology
- Early Cosmologies
- Relativity and Cosmology
- The Big Bang Model
- The Cosmological Principle
- Universal Expansion
- Cosmic Nucleosynthesis
- Cosmic Microwave Background Radiation
- Discovery of the Microwave Background Radiation
- Measuring Space Curvature
- Cosmic Evolution
- Evolution of Structure
- Mean Cosmic Density
- Critical Density
- Dark Matter and Dark Energy
- Age of the Universe
- Precision Cosmology
- The Future of the Contents of the Universe
- Fate of the Universe
- Alternatives to the Big Bang Model
- Space-Time
- Particles and Radiation
- The Very Early Universe
- Mass and Energy in the Early Universe
- Matter and Antimatter
- The Forces of Nature
- Fine-Tuning in Cosmology
- The Anthropic Principle in Cosmology
- String Theory and Cosmology
- The Multiverse
- The Limits of Knowledge
- Questions
Chapter 18
Life On Earth
- Nature of Life
- Chemistry of Life
- Molecules of Life
- The Origin of Life on Earth
- Origin of Complex Molecules
- Miller-Urey Experiment
- Pre-RNA World
- RNA World
- From Molecules to Cells
- Metabolism
- Anaerobes
- Extremophiles
- Thermophiles
- Psychrophiles
- Xerophiles
- Halophiles
- Barophiles
- Acidophiles
- Alkaliphiles
- Radiation Resistant Biology
- Importance of Water for Life
- Hydrothermal Systems
- Silicon Versus Carbon
- DNA and Heredity
- Life as Digital Information
- Synthetic Biology
- Life in a Computer
- Natural Selection
- Tree Of Life
- Evolution and Intelligence
- Culture and Technology
- The Gaia Hypothesis
- Life and the Cosmic Environment
Life Beyond Earth
Known exoplanets and their place within the habitable zone around their respective stars.
Furthermore, experiments conducted in the laboratory as well as evidence from the early history of life on Earth have significant implications for the formation of life on other planets. It has been suggested that life will probably begin on another planet if energy, liquid water, and the right chemicals are simultaneously present. The four main elements that are necessary for life as we know it (hydrogen, oxygen, carbon, and nitrogen) are acknowledged to be everywhere throughout the universe. However, through no lack of trying, scientists so far have not been able to reproduce the progression from organic molecules to simple cells in the laboratory. Despite our inability to recreate the origin of life on Earth, we do know that life originated very quickly, within the first 10% of the planet's entire history. Is it likely, then, that life exists elsewhere?
Mammoth Hot Springs is a large complex of colorful hot springs containing microbes, called extremophiles, located on a hill of in Yellowstone National Park
The beginning of life on Earth as single-celled organisms and its subsequent evolution to complex life forms was neither smooth nor predictable. Nor is it clear that the beginning of life on Earth was a single, isolated incident. Throughout the geologic history of Earth, life was likely to have been eliminated or severely set back by external events such as meteor or comet impacts. Consequently, life may have arisen several times independently, in a number of forms. In any case, biological evidence shows that life is highly adaptable and species can evolve to thrive in different environments, from the extreme temperatures and pressures of the ocean depths to highly acidic environments in sulfur hot springs. Once life was established permanently on the planet, it began its evolution toward greater complexity, even though complexity is not necessarily required for an organism to endure or adapt to environments. Although most scientists agree that the origin of single-celled organisms on planets and moons is likely to be a commonplace occurrence, there is a debate amongst astrobiologists about the probability of complex (and thus intelligent) life beyond Earth. Peter Ward and Donald Brownlee wrote what is known as "The Rare Earth Hypothesis", a thesis that argues against the ubiquity of complex life in the universe. However, many of their arguments have a counter-argument, and other astrobiologists argue that complex life could be common.
Graphical Timeline of Geological Periods
If we examine the existence of complex life by observing our own planet, we know that life eventually evolved intelligence in a few species and the ability to manipulate the global environment in one. However, intelligence may not be the inevitable consequence of evolution. On Earth, it took nearly four billion years to evolve and is only evident in a handful of the millions of species on the planet. What if the development of intelligent life is not a probable end result for life in general?
Despite the enormous uncertainty about the existence of intelligent life in the galaxy, astronomers have nevertheless made tentative efforts to communicate across interstellar space. Currently, there is no evidence that extraterrestrial life forms have made a visit to our planet or otherwise tried to establish contact with us. Humanity can only guess the reasons why. Perhaps the development of life in the universe is not commonplace and we are indeed alone. Perhaps the closest intelligent civilization is just too far away. Perhaps it is in a civilization's natural progression to destroy itself before successfully navigating the universe. Perhaps we are the only civilization that finds merit in communication across space.
The Arecibo message as sent 1974 from the Arecibo Observatory.
Perhaps they do exist, but we have not been able to recognize them. After all, there is no scientific reason to believe that aliens would resemble us physically or culturally. Arthur C. Clarke has remarked that any technology much advanced beyond our own would look like magic. Physicist Enrico Fermi proposed what has now become known as the Fermi Paradox. Perhaps we are too limited by our own concept of civilization. According to Fermi, we have reason to believe that Earth is in no way special or unique, so other Earth-like planets and civilizations should be in existence. However, if this statement is true, then our galaxy should have already been colonized. Yet there is no evidence of any galactic or universal civilization. Many scientists and philosophers have proposed their own solutions to this paradoxical situation. Despite hundreds of years of speculation, no one has arrived at any concrete answers about life beyond Earth. As in all scientific debates, the only way forward is to gather data.