The planets circling our Sun are as amazing as they are diverse. Come discover our planetary neighborhood—as if from interstellar space, moving in toward our Sun. This “outside-in” perspective is how we may someday explore other solar systems.
What is a Planet? Icy Worlds Giant Planets Rocky Planets
This artists concept shows the planets in our Solar System.
What is a planet?
Ages ago, people noticed that a few stars moved around the sky from night to night among all the countless nonmoving stars. They called them "planets," which means "wanderers."
We now know that these planets are not stars at all. They are fairly nearby objects that orbit our Sun. Many objects orbit our Sun but not all of them are planets! Defining which ones are planets is trickier than you might think—just ask Pluto!
Who decides what a planet is?
Scientists once agreed that nine planets orbited the Sun, from Mercury to Pluto. But then they discovered object orbiting beyond Pluto, some quite large. Which ones were planets? Scientists no longer agreed. The International Astronomical Union (IAU), comprised of scientists from nations around the world, debated the issue. In 2006 they voted on a new definition for a planet.
The IAU Prague General Assembly adopted the following definition on August 24, 2006:
A planet is a celestial body that
- is in orbit around the Sun,
- has enough mass for its gravity to make the objects have (nearly) a round shape, and
- has cleared other large objects from the region it crosses during its orbit. (Its gravity caused other orbiting objects to impact, or crash into, its surface or be ejected from our solar system.)
Unfortunately, Pluto does not meet the third requirement.
The IAU Prague General Assembly also approved a new term, dwarf planet.
A “dwarf planet” is a celestial body that
- is in orbit around the Sun,
- has enough mass for its gravity to make the object have a (nearly) round shape,
- has not cleared other large objects from the region it crosses during its orbit. (Its gravity is not great enough to cause other orbiting objects to impact, or crash into, its surface or be ejected from our solar system.)
- is not a satellite of another object.
Pluto meets all of these requirements. It is now the best example of a dwarf planet.
Pictured: Museum employees place symbol for "no" over the planet Pluto sign at the entrance to the Exploring the Planets gallery in 2006. Pluto had just been downgraded from full planet to dwarf planet status.
Our Solar System from the Outside In
Imagine entering our solar system from interstellar space. As you travel toward our Sun, you would move through three distinct regions. First you would pass countless icy worlds. Then you would enter the realm of the giant planets. Finally, you would reach the rocky planets closest to the Sun. Let's take a look at our solar system—from the outside in!
First Stop: Icy Worlds
Worlds in our outer solar system consist mostly of water ice, other ices, and some rock. Various processes have shaped their surfaces into strange landscapes. Because they are so far from Earth, we are just starting to learn about them, how they formed, and how they interact with the rest of our solar system. Scientists are especially interested in whether all this water in our outer solar system may contain life.
A spherical "cloud" of comets, known as the Oort Cloud, surrounds the outer reaches of our solar system. The Oort cloud is vast. It starts between 2,000 and 5,000 AU from the Sun and extends out to 50,000 AU. (One AU, or astronomical unit, is the average distance between the Earth and the Sun.) It may contain trillions of objects larger than two-thirds of a mile (one kilometer) in size.
Closer to the Sun but still beyond Neptune is a doughnut-shaped region, known as the Kuiper Belt, containing countless icy bodies. Some are quite large, however Pluto is the largest member of the Kuiper Belt.
Second Stop: Giant Planets
Our solar system has four giant planets: Neptune, Uranus, Saturn, and Jupiter. Giant planets are much larger than Earth—they are unimaginably huge, stunningly beautiful, and sometimes a little weird. They are made mostly of gases instead of solid materials, and a host of Moons orbits each one. Neptune and Uranus likely have regions of ices beneath their atmospheres. Saturn and Jupiter are our largest planets. They are mostly made of hydrogen and helium, the Sun's most abundant elements.
Third Stop: Rocky Planets
The planets closest to the sun—Mars, Earth, Venus, and Mercury—are made mostly of rock. Their geological history is preserved on their surfaces—their landscapes revealing the processes that shaped them. Gravity, temperature, air, and water all play leading roles in their geological stories. Earth is the largest of our rocky planets. It is the only planet we know of that has both abundant liquid water and living organisms.