Imagining Other Earths


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.

What you will learn


This short lecture introduces Professor Spergel, the basic outline of the course and its goals.

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 visible universe.

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 discuss efforts to discuss life on Mars.

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 are likely to have liquid water.

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 variations in the Earth’s albedo (mostly due to snow) can produce “Snowball Earth” episodes, extended epochs during which the Earth was mostly covered with ice.

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.

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 Venus’s atmosphere and how a “runaway greenhouse effect” made its surface uninhabitable.


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. Seasons.

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 tides to see how it shapes the properties of planets around M stars.

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 have complex geologies and atmospheres. Some of the moons may be potentially habitable by life forms.

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 planets and determine their basic properties.

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 and how its observations have shaped our understanding of the properties of extrasolar planetary systems.

What’s included

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