Since those famous steps were taken, we’ve discovered what is approaching 200 additional moons of all shapes, sizes, and compositions. We’ve sent spaceships and robotic laboratories to photograph and study each of the planets, dozens of moons, and even the Sun. We’ve discovered ring systems around three additional planets; landed robotic explorers on Mars, on asteroids, and even on comets. We’ve also found thousands of exoplanets around other stars, with implications for our own origins. There has never been a more exciting time than today to explore and understand our solar system and beyond with A Field Guide to the Planets.

Your instructor, Professor Sabine Stanley, a Bloomberg Distinguished Professor at Johns Hopkins University, guides you on a thrilling ride of discovery, illustrated by the phenomenal images NASA has gathered from throughout the solar system. In 24 lectures, you will experience a journey that was never before possible as your professor makes these astronomical wonders accessible to anyone, allowing you to experience, via our robot explorers, what it is like to visit worlds that were previously unknown.

What Is Our Solar System?

When we think of the solar system, we tend to visualize it in two dimensions, generally as a map with planets orbiting in almost circular ellipses around the Sun. We also imagine some moons in that same plane, an asteroid belt, a few more planets and satellites, and maybe a comet coming in at a different angle. Our visual map tends to end with Neptune, the eighth and farthest planet from the Sun, and the Kuiper Belt objects, including Pluto.

And yet the solar system is also so much more. We now know that even Neptune’s orbital distance is less than one tenth of one percent of the distance from the Sun to the farthest objects bound by its gravity—the Oort Cloud, a spherical shell of small icy bodies orbiting the Sun 50,000 times farther out than the Earth. The solar system that began its formation 4.5 billion years ago is still a work in progress today—a three-dimensional, dynamic, ever-changing system of energy and matter all gravitationally bound to our star.

And if we had any doubts about the continuing forming and re-forming of the solar system, recent exploration has allowed us to:

Witness for the first time a collision between two bodies in the solar system—Jupiter’s gravity capturing comet Shoemaker–Levy 9, ripping the comet apart, and causing it to crash into the planet;
Monitor active volcanic eruptions on moons of Jupiter, Saturn, and Neptune; and
Discover propeller moonlets constantly shaping and reshaping the rings of Saturn.
These and other observations have helped fill out our knowledge of the solar system—and by doing so, has helped us better understand our own place in the universe, too.

A Grand Scale and Unique Features

Earth is home to spectacular features created by erosion, plate tectonics, and collision impacts over billions of years. But many of Earth’s features pale in scope compared to those on other planets and moons. As we’ve explored farther out into the solar system, we’ve encountered features whose magnitude we hadn’t anticipated or even imagined, such as:

Jupiter’s Auroras. Some of the most energetic auroras in the solar system, they are 1,000 times more powerful than those on Earth and are emitted not just as visible light, but as high-energy X-rays.
Verona Rupes. A cliff face on Uranus’ moon Miranda, measuring 20 kilometers high. With a gravitational acceleration 100 times smaller than Earth’s, a rock falling from the top would take almost 12 minutes to reach the bottom.
Olympus Mons. Located on Mars, it’s the solar system’s tallest mountain and largest known volcano, measuring an amazing 27 kilometers tall. But when it comes to volcanic activity, Jupiter’s moon Io is the winner with 400 active volcanoes mapped to date.
Diamond Rain. On Uranus and Neptune, it’s possible that carbon atoms could condense into crystals of diamonds that would rain out through the icy layer above. Uranus might even have an ocean of carbon under high pressure with floating chunks of solid “diamond-bergs.”
With Professor Stanley’s guidance, you’ll learn more about these and dozens of other unexpected features and objects—from the surprising prevalence of water throughout the solar system (even on blazing hot and dry Mercury); to puzzling shapes on the Moon; to the quantity of near-Earth objects we need to track for safety, now numbering upwards of 20,000.

Looking Outward to Understand Ourselves

One thing we’ve learned from our solar system exploration is precisely how the Earth is unique—and not just because our planet is teeming with life: Earth is the only planet or moon whose surface has been constantly reformed by the process of plate tectonics.

While all planets and moons have a hot core and experience the process of outward cooling— and some are even transformed by their own geological processes—the Earth is the only body whose outer layer is formed of rigid plates that “float” on top of the mantle. Across billions of years, these plates have ridden on top of and underneath each other, causing earthquakes and volcanoes. But this process, along with weathering and erosion, also means that the surface history of our planet has been almost completely erased.

The only way we can learn about the earliest history of Earth is by exploring the nearby terrestrial planets and moons. And we continue to make new discoveries using fieldwork from decades earlier. In fact, the oldest Earth rock ever found was discovered in 2019—when scientists re-examined Moon rocks Apollo 14 brought back almost 50 years ago. Embedded in this cache of Moon rocks was a 2-gram fragment whose chemistry indicated it came from the Earth almost 4 billion years ago, likely jettisoned onto the Moon by a collision with a large asteroid.

Did you know the Earth shares its orbit around the sun with an asteroid? We already knew other planets had so-called Trojans asteroids that share an orbit with a planet at a stable point either in front of or behind the planet—but we did not know Earth had a Trojan until it was discovered by NASA’s WISE mission in 2011. We’ve also been able to make amazing headway into understanding the building blocks of life and how they might be more common throughout the solar system than we had thought. In fact, we have discovered complex hydrocarbons on several bodies in the solar system. This suggests that we may be able to learn about the earliest development of life on Earth from the processes we study on these other moons and planets.

With A Field Guide to the Planets, you will experience a uniquely satisfying, vicarious journey—to every major destination in our solar system, and really understand a whole range of features with the excitement of a traveler who’s just returned from a truly eye-opening trip. You will look to humanity’s next space missions with new anticipation, and experience our own Earth with greater understanding and appreciation than ever before.