A Field Guide to the Planets

Since 1962, robots have been exploring our solar system to help answer this most important question: Who are we? With fascinating data and images now in hand, explore this family album overview of our planets, dwarf planets, moons, asteroids, Kuiper Belt objects, and long-period comets, and fly through some of our solar system’s most unique features!

Mercury is a planet of many solar system extremes. It's the smallest planet, the closest to the Sun, and it has the shortest year, most elliptical orbit, smallest axis tilt, and largest fraction of iron. Learn how these characteristics have resulted in a planet where the Sun sometimes moves backwards across the sky, and where water ice has been found at the poles.

While the Venusian carbon dioxide atmosphere has resulted in a runaway greenhouse effect and the hottest surface temperature in the solar system, the Earth and Venus actually contain about the same amount of carbon. Explore the forces that resulted in the extreme atmospheric differences between these two otherwise-similar planets.

Given the striking similarities between the four terrestrial planets, why is Earth the only one teeming with life? Proposed as a bold theory less than 70 years ago, could plate tectonics be a main driver of life on Earth? Explore the fascinating movement of our planet’s surface and the many ways in which a geologically active Earth has sustained our biologically active planet.

Compared to Venus or the giant planets, Earth has a relatively thin atmosphere. And yet, without this single, fragile layer, life would not have evolved and thrived. Discover the unique properties of each atmospheric layer, and encounter specific ways we’ve explored each layer as a springboard to exploring the rest of our solar system.

Our Moon, formed from the Earth about 4.5 billion years ago, is by far the largest moon in the solar system relative to its planet’s size. Explore the many ways in which this uniquely coupled system affects the tides on Earth and on the Moon, our rotation and revolution, the process of tidal locking, and even the planetary stability that has allowed for the development of life on Earth.

Even before the invention of telescopes, humans were familiar with the dark lunar highlands and bright maria on the Moon’s surface. But now, with knowledge gained from both robotic and crewed missions, you can also explore fascinating and complex lunar swirls, sinuous rilles, and the lava tubes that hold promise as ideal locations for future lunar bases.

Humanity’s fascination with Mars is never-ending, from the days when we posited a planet covered in straight-line canals and vegetation to NASA’s current Moon to Mars program. Learn how the intriguing similarities and differences between Earth and Mars have resulted in Mars’ planet-wide dust storms, migrating polar ice caps, and 3.9-billion-year-old impact craters.

Recent robotic exploration provides tantalizing evidence: Mars’ barren landscape could have been much more Earth-like in the past. With warmer temperatures, a thicker atmosphere, and the possibility of water oceans and tsunamis, could Mars have been an Earth sibling that supported life? Learn about recent discoveries that will guide future exploration and scientific inquiry on the Red Planet.

Fans of science fiction know that a collision with an asteroid has the potential to obliterate civilization as we know it. With 20,000 asteroids identified in near-Earth orbit, how can collision be avoided? Learn why these rocky bodies, and those in the Asteroid Belt between Mars and Jupiter, never accreted into planets and how we might harness their resources for future space travel.

Does Jupiter have a greater similarity to the Earth or to the Sun? It depends on which characteristics you consider. Explore the many ways in which Jupiter is unique among the planets and consider what our solar system would be like without it. This gas giant might seem too far away to make a difference in your daily life, but without Jupiter, life on Earth might never have had a chance.

Today we know of 79 Jovian moons: the spherical Io, Europa, Ganymede, and Callisto, and dozens of other smaller, odd-shaped satellites. Learn why Jupiter’s gravitational forces, plus the orbital resonance of the three interior moons, make these some of the most promising places to search for life, and why scientists believe the Jovian system once included generations of other moons, now gone.

With its exquisitely complex ring system, NASA describes Saturn as the “jewel of our solar system.” Learn what decades of exploration have revealed about the origin and morphology of these ever-changing icy rings and how they interact with Saturn’s closest moons. From the rings to propeller moonlets, a massive hexagonal polar storm, and the giant vortex, our fascination with Saturn never ends!

With a system of 62 moons located in and far beyond its ring system, Saturn has outer moons that are some of the most fascinating worlds in the solar system. Learn why Titan and Enceladus hold such promise in our search for extraterrestrial life, from global subsurface oceans of water on both moons to Titan’s Earth-like surface and organic molecules in its atmosphere.

What a fascinating world Voyager 2 revealed in 1986 during its short flyby of Uranus! Learn why Uranus seems to orbit “on its side” surrounded by a delicate system of 13 rings and 27 moons, how we discovered its multi-polar magnetic field, and why scientists think Uranus might contain an ocean made of liquid diamond, with floating chunks of solid “diamond-bergs!”

Neptune is the coldest, stormiest planet, and the only one that cannot be seen with the naked eye from Earth. Its moon, Triton, is the only spherical moon in the solar system that’s an irregular satellite that orbits opposite the direction of all the planets. Learn how tidal forces are not only changing that orbit, but also causing geologic activity on a surface that contains organic compounds.

Although Pluto is no longer categorized as a planet, Pluto and its “moon” Charon are considered the closest thing in the solar system to a binary planet system. Explore fascinating revelations from the New Horizons mission, including Pluto’s glacial flows, floating mountains, extreme seasons, unexpectedly complex atmosphere, and a surface that appears to be dusted in complex organic molecules.

Learn why scientists believe comets, the “leftovers” of planet formation in the outer solar system, could be partially responsible for the flourishing of life on Earth, bringing both water and organic material to the inner solar system. And explore the more distant Oort Cloud, where billions of cometary objects orbit at the outermost boundary of the solar system.

Fly through the corona of what is by far the largest, most massive, and most significant object in the solar system: the Sun. With its gravity, heat, light, magnetic fields, and plasma storms, learn how the Sun affects every object in the solar system, and follow the race to learn more about coronal mass ejections before one destroys trillions of dollars in electronics on Earth.

Today we see an orderly solar system with planets staying in their orbits around the sun, moons staying in their orbits around the planets, and comets coming and going in predictable fashion. But how did it all start? Learn how a molecular cloud gave rise to a proto-planetary disk in which our solar system developed step by step across time and space (and is developing still).

Planets orbiting other stars used to be purely in the realm of science fiction. How did we begin discovering them by the thousands? Learn about the methods scientists have used to discover so many exoplanets so quickly. From “hot Jupiters” to “mini-Neptunes” to planets whose clouds rain molten glass, these discoveries demonstrate that ours is not the only type of planetary system possible!

Beginning in 2009, the Kepler Space Telescope began staring intensively at a single patch of sky, about one quarter of one percent of the sky. Sift through the Kepler discoveries for planets with a variety of Earth-like features, including presence in a “habitable” zone, and learn why billions of Earth-like planets are estimated to exist in our galaxy.

The surprising detection of gas giant planets orbiting extremely close to other stars has led to the realization that planets can form in one part of a stellar system and then migrate to another part. Did that happen in our own solar system? Learn about the evidence for a “Late Heavy Bombardment” on the Moon, Mars, and Mercury.

What are the next big ideas that will help us ask and answer the next big questions? Consider the fascinating future technologies of centimeter-sized satellites propelled by laser photons, liquid mirror telescopes on the Moon, a magnetic shield large enough to help terraform Mars, and more. Nourish your imagination, and experience the inspiration of space exploration!