Imagining Other Earths
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assignment Level : Introductory
chat_bubble_outline Language : English
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credit_card Free access
verified_user Fee-based Certificate
timer 160 hours in total

About the content

Are we alone? This course introduces core concepts in astronomy, biology, and planetary science that enable the student to speculate scientifically about this profound question and invent their own solar systems. All the features of this course are available for free. It does not offer a certificate upon completion.

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  • Week 1 - Introduction
    This short lecture introduces the Professor, the basic outline of the course and its goals.
  • Week 1 - The Universe is Big!
    One of the most striking features of the universe is its enormous size. The lecture discusses the structure of our Solar System, the new dwarf planets, and the distance to the nearest stars. The lecture then moves outwards to the scales of our galaxy and the ...
  • Week 2 - What is Life?
    This lecture introduces different approaches to defining life, a necessary step if we are going to search for life. We then introduce the basic building blocks of biochemistry. One potential way of detecting life is through its byproducts like Methane. We disc...
  • Week 3 - Why is Mercury Hot?
    This lecture uses energy balance to determine the effective temperature of planets. The lecture introduces the idea of temperature, black body spectrum and luminosity. We then calculate the location of the habitable zone, the range of distances where planets a...
  • Week 4 - Snowball Earth
    Planet temperatures depend on the albedo of the planet (its reflectivity) and the transparency of its atmosphere. This lecture introduces the basic physics behind global warming, discusses how non-linear feedbacks can exacerbate its effects, and describes how ...
  • Week 5 - Planetary Atmospheres
    This lecture begins by introducing the basic physics of gases, liquid and solids. The lecture then describes how the balance between gas pressure and gravity shapes the structure of planetary atmospheres.
  • Week 6 - Mercury and Venus
    This lecture discusses some of the remarkable properties of the two innermost planets, Mercury and Venus. We discuss how Mercury, a very hot planet, can have ice at its poles. We describe its surprisingly strong magnetic field. We discuss the structure of Venu...
  • Week 7 - Mars
    Mars may be the other planet in the Solar System that hosts life. This lecture introduces the basic properties of Mars, Mars’ atmosphere and its seasons. We discuss Martian exploration, the search for water and methane on Mars and potential signature of life. ...
  • Week 8 - The Moon
    By studying the properties of the Moon, we learn not only about its history but about the formation history of the Earth. This lecture discusses the physics of craters and the tidal interactions between the Earth and the Moon. We then apply the physics of tide...
  • Week 9 - Small Bodies of the Solar System
    This lecture begins by discussing comets and asteroids, remnants of the formation of our Solar System. We discuss how collisions of comets and asteroids with Earth have shaped its history. The lecture then discusses the moons of Jupiter and Saturn—these moons ...
  • Week 10 - Kepler’s Law and Search for Extrasolar Planets
    This lecture shows how Kepler’s Law, the relation between a planet’s Period and the radius of its orbit, can be understood in terms of the physics of gravity. We then see how we can use observations of star’s motions (and Kepler’s Law) to detect extrasolar pla...
  • Week 11 - Kepler and Transits
    Transits have been an important event for astronomy for over 400 years. This lecture describes the transit of Venus and how it was used to measure the size of our Solar System. This lecture then discusses how NASA’s Kepler mission observes planetary transits a...
  • Week 12 - Einstein and the Search for Planets
    Einstein’s theory of General Relativity tells us that mass curves space and deflects light. Thus, observations of light deflection can tell us about the distribution of mass. Microlensing observations use this effect to detect planets around distant stars. We ...
  • Week 13 - Properties of Stars
    Stars are remarkably simple objects: their mass and age determine the basic properties. This lecture discusses how we determine the distance and luminosity of stars and how this relates to the Star’s size and temperature.
  • Week 14 - Stellar Evolution
    Like planets, the structure of stars are shaped by the balance between gravity and pressure. Nuclear fusion, the energy source at the star’s interior, converts Hydrogen to heavier elements. This lecture introduces these concepts and shows how nuclear burning d...
  • Week 15 - Quantum Mechanics and Fingerprinting Planets
    This lecture introduces the Pauli exclusion principle, which requires that only one electron can be in any state. We use this principle to understand the properties of materials and the atomic lines seen in planetary and stellar structure.
  • Week 16 - Optics and Imaging Planets
    This lecture introduces the basic of optics, explains how lenses and telescopes work, and then discusses the challenges of imaging planets around bright stars.
  • Week 17 - Formation of the Earth and the Origin of Water
    This lecture discusses the physics of star and planet formation. Because of the conservation of angular momentum, collapsing gas clouds form disks that then fragment to eventually form planets. We observe a tremendous diversity of planetary systems which impli...
  • Week 18 - Biochemistry of Life
    This lecture discusses the building blocks of life. We focus first on the importance of water as a medium for life and then discuss how amino acids are the building blocks of complex proteins. We then discuss the role of RNA and DNA in reproduction and protein...
  • Week 19 - Is Earth Special? Origin of Life
    This lecture describes our efforts to understand the origin of life on Earth. While it is relatively easy to synthesize amino acids, we do not yet understand how this building blocks are assembled into complex cells. The lecture ends with an exploration of the...
  • Week 20 - Evolution of Life
    The basic principles of evolution (selection, mutation, and heritability) do not depend on the details of biochemistry. Thus, we can expect evolutionary principles to apply in extrasolar life. We discuss the role of sex in evolution and discuss how we can use ...
  • Week 21 - Extremophiles
    Extremophiles are life forms that flourish in extreme environments—regions of very high (or low) temperatures, acidities, and even intense radioactivity. The lecture describes some of these fascinating life forms including the bacterial mats of Yellowstone and...
  • Week 22 - Extinctions
    There have been at least 5 mass extinctions in the Earth’s history. These extinctions were events that wiped out most of life on the planet and drove a large fraction of all life forms into extinction in a very short period of time. The most famous of these ex...
  • Week 23 - What makes a Habitable Planet?
    This lecture discusses some of the requirements for habitability. We discuss the faint Sun problem—the ancient Sun was much colder yet the Earth was still habitable and discuss the range of planetary systems that might be able to host life. We conclude by desc...
  • Week 24 - Habitable Zone, Fermi Paradox & SETI
    The final lecture discusses the search for technologically advanced life in the Galaxy. We describe the “Fermi Paradox”: if technologically advanced life is common, then it would have long ago spread through the Galaxy. Thus, we might have expected to have alr...


David Spergel
Charles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics


Content Designer

Princeton University
Princeton University is a private research university located in Princeton, New Jersey, United States. It is one of the eight universities of the Ivy League, and one of the nine Colonial Colleges founded before the American Revolution.



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